Introduction to Programming in Java - TU/emarko/iq/part1-handouts.pdf · ... Fundamentals of programming 1.Introduction to Java ... write a program that computes the area of a circle,

Block IQ

Marko Boon ([email protected])Jacques Resing ([email protected])

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Department of Mathematics and Computer Science Block IQ 1

Schedule (programming part):• Wednesday June 18, 13h30 – 16h30, MF 13

• Friday June 18, 9h00 – 12h00, MF 15 Will be rescheduled

• Friday June 27, 9h00 – 12h00, MF 11

• Wednesday July 2, 9h00 – 12h00, Zwarte Doos 1.03 (?)

• Thursday July 2, 9h00 – 12h00, Zwarte Doos 1.03 (?)

Block IQ 2014

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Part I: Fundamentals of programming

1. Introduction to Java

2. Primitive Data Types and Operations

3. Control Statements

4. Methods

5. Arrays

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The History of Java

• developed at Sun Microsystems

• originally called Oak, designed in 1991 for embedded systems

• redesigned in 1995 for Internet applications and renamed to Java

• nowadays used in many mobile phones

Java is not limited to web applications, but it has full programming featuresand it can be used to create stand alone applications. It was designed to beobject-oriented from the start.

Introduction to Java

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Characteristics of Java

• simple

• object-oriented

• platform independent

• robust, reliable (exception handling)

• secure

• multithreaded

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Installing Java

The Java Development Kit and Runtime Environment can be downloaded from:

http://www.oracle.com/technetwork/java/javase/

javase stands for Java 2 Platform, Standard Edition. It contains the javainterpreter and compiler, as well as many other tools.

Many graphical text editors have been written for development of Java pro-grams. We will use Netbeans, which can be downloaded from:

http://www.netbeans.org

Alternative:

• Eclipse

http://www.oracle.com/technetwork/java/javase/

http://www.netbeans.org

http://www.eclipse.org/

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Creating, compiling and executing a Java program

R e s u l t

C o m p i l e S o u r c e C o d e

R u n B y t e c o d e

C r e a t e / M o d i f y S o u r c e C o d e

i f c o m p i l a t i o n e r r o r s

i f r u n t i m e e r r o r so r i n c o r r e c t r e s u l t

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Create source code

Create a file Welcome.java:

// This program prints Welcome to Java!

public class Welcome {

public static void main(String[] args) {System.out.println("Welcome to Java!");

}}

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Compile source code

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Compile source code

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Compile source code

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Run bytecode

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Run bytecode

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Anatomy of the Application Program

Components:

1. comments

2. reserved words

3. modifiers

4. statements

5. blocks

6. classes

7. methods

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Comments

Comments are lines of text that are not interpreted by the compiler. They arenot programming statements. Comments help programmers and users com-municate and understand the program.


/* This is commentspread over morethan just one line

*/

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Reserved words (keywords)

Reserved words, or keywords, are words that have a specific meaning to thecompiler and cannot be used for other purposes in the program.When the compiler sees the word class it understands that the word afterclass is the name for the class.


public class Welcome {

public static void main(String[] args) {System.out.println("Welcome to Java!");

}}

All reserved words are printed in blue.

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Modifiers

Modifiers are reserved words that specify the properties of the data, methods,classes and how they can be used. Examples: public, static, private, final,abstract and protected.Modifiers will be discussed in the part Object Oriented Programming.

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Statements

A statement represents an action or a sequence of actions. The statement

System.out.println("Welcome to Java!");

is to display the welcome text. Every statement in Java ends with a semicolon( ; ).

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Blocks

Blocks group the components of the program. Each block bgegins with anopen brace ( { ) and ends with a closing brace ( } ). Every class has a classblock that groups the data and methods of the class. Every method has amethod block that groups the statements in the method.Blocks can be nested, meaning that one block can be placed within another./ / T h i s p r o g r a m p r i n t s W e l c o m e t o J a v a !

p u b l i c c l a s s W e l c o m e {

p u b l i c s t a t i c v o i d m a i n ( S t r i n g [ ] a r g s ) { S y s t e m . o u t . p r i n t l n ( " W e l c o m e t o J a v a ! " ) ; }}

c l a s s b l o c k

m e t h o d b l o c k

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Classes

The class is the essential Java construct. In Object Oriented Programming theywill be discussed in more detail. Every Java program has (at least) one class,and programs are contained inside a class definition enclosed in blocks.A class contains data and method declarations.The file name has to be equal to the class name (case sensitive!).

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Methods

A method is a collection of statements that are grouped together to perform anoperation. Methods are part of classes. Predefined classes have predefinedmethods (e.g. System.out.println). If you are writing your own class, you canwrite your own methods (e.g. main).Methods may have one or more arguments (e.g. "Welcome to Java").In certain other languages methods are called procedures and functions.

The main method

One method is "special":

public static void main(String[])

This method defines where the program begins. The Java interpreter executesthe application by invoking this main method.The array of Strings contains the command-line arguments.

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Example: write a program that computes the area of a circle, given the radius.

1. specify the value of the radius,

2. compute the area using the formula a = πr2,

3. display the area.

Primitive Data Types

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Write the class declaration

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public class AreaComputer {

}

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/*** This class computes the area of a circle

* given a specified radius.

*/


}

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Create the main method



*/


}

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*/


public static void main(String[] args) {

}}

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*/


// main method: the program starts herepublic static void main(String[] args) {

}}

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*/


// main method: the program starts herepublic static void main(String[] args) {// Step 1: declare all variables that we need

}}

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*/


// main method: the program starts herepublic static void main(String[] args) {// Step 1: declare all variables that we need// Step 2 specify the radius and the constant PI

}}

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*/


// main method: the program starts herepublic static void main(String[] args) {// Step 1: declare all variables that we need// Step 2 specify the radius and the constant PI// Step 3: compute area

}}

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*/


// main method: the program starts herepublic static void main(String[] args) {// Step 1: declare all variables that we need// Step 2 specify the radius and the constant PI// Step 3: compute area// Step 4: display area

}}

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Step 1: declare all variables that we need


// main method: the program starts herepublic static void main(String[] args) {

}}

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Step 1: declare all variables that we need


// main method: the program starts herepublic static void main(String[] args) {double radius, area, PI;

}}

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Step 2: specify the radius and the constant PI



}}

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radius = 5;

}}

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radius = 5;PI = 3.1415926536;

}}

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Step 3: compute the area



radius = 5;PI = 3.1415926536;

}}

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Step 3: compute the area



radius = 5;PI = 3.1415926536;

area = PI * radius * radius;

}}

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Step 4: display the area



radius = 5;PI = 3.1415926536;


}}

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Step 4: display the area



radius = 5;PI = 3.1415926536;


System.out.println("The area for the circle");System.out.println("of radius " + radius);System.out.println("is " + area);

}}

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Compile and run AreaComputer

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IdentifiersJust as every entity in the real world has a name, so you need to choose namesfor the things you will refer to in your programs.Programming languages use special symbols called identifiers to name suchprogramming entities as variables, constants, methods, classes, and pack-ages.

Some rules for naming identifiers:

• an identifier is a sequence of characters that consists of letters, digits,underscores (_), and dollar signs ($).

• an identifier cannot start with a digit.

• an identifier cannot be a reserved word.

• an identifier cannot be true, false, or null.

Java is case-sensitive, so X and x are different identifiers.

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VariablesVariables are used to store data input, data output, or intermediate data. Inthe example, radius and area are variables of double-precision, floating-pointtype (called double). The values of variables can be reassigned.

Declaring Variables

To use a variable, you declare it by telling the compiler the name of the variableas well as what type of data it represents.Syntax: datatype variableName;

int n;double radius;char c;String firstName;

You can declare multiple variables of the same data type in one line:

double radius, area, PI;

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Assigning a value to a variable

Syntax: variable = expression;

An expression represents a computation involving values, variables, and op-erators that evaluates to a value.

n = 10;double x = 3.5;double y = x * 3;int m = x*2; // WRONG: possible loss of precisiondouble z = n * 3 + 2;String firstName = "Marko";String lastName = "Boon";String fullName = firstName + " " + lastName;n = n + 1;

As you can see, it is possible to declare and initialise a variable in one step.

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Constants

The value of a variable may be changed during the execution of the program.It is possible to create permanent data that never change: constants.

final double PI = 3.14159;

By convention, constants are named in uppercase.

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Numeric data typesName Rangebyte −128 to 127short −32768 to 32767int −231 (2147483647) to 231

− 1long −263 to 263

− 1

float −3.41038 to 3.41038 (6 to 7 significant digits of accuracy)double −1.710308 to 1.710308 (14 to 15 significant digits of accuracy)

For integers the most common type is int.For reals the most common type is double.

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Numeric operators

Addition (+), subtraction (–), multiplication (*), division (/) and remainder (%).Examples:

int i1 = 34 + 1; // i1 becomes 35double d1 = 34.0 - 0.1; // d1 becomes 33.9long i2 = 300 * 30; // i2 becomes 9000double d2 = 1.0 / 2.0; // d2 becomes 0.5;int i3 = 7/3; // i3 becomes 2 (!)byte i4 = 7%3; // i4 becomes 1

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Shortcut operatorsOperator Name Example Equivalent

+= addition assignment i+= 8 i = i + 8-= subtraction assignment f-= 8.0 f = f - 8.0

*= multiplication assignment i*= 8 i = i * 8/= division assignment i/= 8 i = i / 8%= remainder assignment i%= 8 i = i % 8

Two more shortcut operators:

i++; // i = i + 1i--; // i = i - 1

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Accuracy

Calculations involving floating-point numbers are approximated becausethese numbers are not stored with complete accuracy. Example:

System.out.println(1 - 0.1 - 0.1 - 0.1 - 0.1 - 0.1);

Result: 0.5000000000000001

System.out.println(1 - 0.9);

Result: 0.09999999999999998

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Numeric type conversions

You can always assign a value to a numeric variable whose type supports alarger range of values:

byte b = 120;int i = b;long l = i;

float f = 100.7f;double d = f;

The other way around is not allowed, even if the numeric value is within therange of the variable:

long l = 110;int i = l; // Possible loss of precision

double d = 100.7;float f = d; // Possible loss of precision

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Numeric type conversions

You can cast a variable to another variable of a different data type, even witha smaller range:

long l = 110;int i = (int) l;

double d = 100.7;float f = (float) d;

Warning – this is allowed, but might not give the expected result:

double d = 100.7;int i = (int) d; // i = 100

int j = 270;byte b = (byte) j; // j = 14

Please note that casting does not change the variable being cast!

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Character data type

char letter = ’A’;char numChar = ’4’;

Sequences of letters are stored in a String:

String message = "Welcome to Java!";

Please note that String is not a primitive data type, but a class. This meansthat is has got methods and properties.

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String concatenation

You can concatenate Strings using the + operator. If you concatenate otherdata types to a String, they will be converted to a String automatically.

String message = "Welcome" + " to " + "Java!";int age = 62;System.out.println("I am " + age + " years old.");

You cannot concatenate characters using the + operator. The + operator castsa char to an int:

char b = ’B’;char c = ’C’;char d = (char) (c+1);int i = b + c + d; // i = 201char e = (char) i; // e = ÉString s = ’a’ + "b"; // s = "ab" !!!!

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Converting Strings to numbers

A number can easily be converted to a String:

int n = 12;String nStr = ""+n;

Converting a String to a number is slightly more work:

String iStr = "-122";String dStr = "34.6";String qStr = "3+5";int i = Integer.parseInt(iStr);double d = Double.parseDouble(dStr);double q = Double.parseDouble(qStr); // ERROR

Integer and Double are classes, just like String. They will be discussed in moredetail later.

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Special CharactersDescription Character Escape SequenceBackspace \bTab \tLinefeed \nCarriage return \rBackslash \\Single quote \’Double quote \"

System.out.println("Hello!\nMy name is \"Marko\".");

Hello!My name is "Marko".

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Unicode Characters

Java characters use Unicode, a 16-bit encoding scheme established by theUnicode Consortium to support the interchange, processing, and display ofwritten texts in the world’s diverse languages (see www.unicode.org). Uni-code takes two bytes, preceded by \u, expressed in four hexadecimal num-bers that run from \u0000 to \uFFFF.

System.out.println("\u00A9 2004"); // CopyrightString alphaBeta = "\u03b1 \u03b2"; // Greek

http://www.unicode.org

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The boolean data type

The boolean data type can have two values: true and false. Usually a booleanis the result of a logical comparison. The comparison operators are:

Operator Name Example Answer< less than 1 < 2 true<= less than or equal to 1 <= 2 true> greater than 1 > 2 false>= greater than or equal to 1 >= 2 false== equal to 1 == 2 false! = not equal to 1! = 2 true

Caution: do not confuse the equality comparison operator (==) to the assign-ment operator (=)!

Please note that booleans cannot be cast into a value of other types, like int.

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Boolean operatorsOperator Name Description! not logical negation&& and logical conjunction|| or logical disjunctionˆ exclusive or logical exclusion

boolean a = true;boolean b = false;boolean c = true;System.out.println(!a); // falseSystem.out.println(a || b); // trueSystem.out.println(a || c); // trueSystem.out.println(a ^ c); // falseSystem.out.println(a && b); // falseSystem.out.println(a && !b); // true

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Message and input dialogs

The class JOptionPane contains methods to show message and input dialogs.This class is part of the package javax.swing, so it needs to be imported man-ually. Packages, classes and especially user interfaces will be discussed later.Nevertheless dialogs are so convenient that we will show how to create easydialogs.

Message dialog:

JOptionPane.showMessageDialog(null, "Message","Title", JOptionPane.PLAIN_MESSAGE);

Input dialog:

String input = JOptionPane.showInputDialog(null,"Question", "Title", JOptionPane.QUESTION_MESSAGE);

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Message and input dialogs – Exampleimport javax.swing.JOptionPane;

public class DialogsExample {

public static void main(String[] arg) {String radiusStr =JOptionPane.showInputDialog(null,"This program computes the area of a circle.\n"+ "Please specify the radius:", "Area Computer",JOptionPane.QUESTION_MESSAGE);double radius = Double.parseDouble(radiusStr);double area = 3.14159 * radius * radius;JOptionPane.showMessageDialog(null,"The area equals: "+area,"Area Computer", JOptionPane.PLAIN_MESSAGE);

System.exit(0);}

}

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Message and input dialogs – Example

Two remarks:

1. the Java code to import the class JOptionPane from the packagejavax.swing is:

import javax.swing.JOptionPane;

2. Programs containing UI components have to be stopped manually usingthe following command:

System.exit(0);

The reason is that a new thread is started for the UI components. Threadswill be discussed later during the course.

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Computing example

Problem: write a program that classifies a given amount of money into smallermonetary Euro units.

Solution:

1. prompt the user to enter the amount as a decimal number

2. convert the amount into cents

3. divide the cents by 200 to find the number of €2 coins. Obtain the remain-ing cents using the cents remainder 200.

4. divide the remaining cents by 100 to find the number of €1 coins. Obtainthe remaining cents using the cents remainder 100.

5. divide the remaining cents by 50 to find the number of €0.50 coins. Obtainthe remaining cents using the cents remainder 50.

6. repeat this for 20, 10, 5 and 2 cents.

7. the remaining cents are the Eurocent coins. Display the result.

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Computing example: ChangeComputer.java

import javax.swing.JOptionPane;

/*** This class breaks down an amount.

*/

public class ChangeComputer {

/*** Main method.

*/

public static void main(String[] arg) {

}}

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// Receive the amount entered from the keyboardString amountStr = JOptionPane.showInputDialog(

null, "Enter an amount:", "Example 2.4 input",JOptionPane.QUESTION_MESSAGE);

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// Convert String to doubledouble amount = Double.parseDouble(amountStr);

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// Convert String to doubledouble amount = Double.parseDouble(amountStr);

// multiply by 100 to obtain centsint remainingAmount = (int) (100*amount);

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// Find the remaining amount of two euro coinsint twoEuroCoins = remainingAmount / 200;

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// Find the remaining amount of two euro coinsint twoEuroCoins = remainingAmount / 200;remainingAmount = remainingAmount % 200;

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// Find the remaining amount of two euro coinsint twoEuroCoins = remainingAmount / 200;remainingAmount = remainingAmount % 200;

// Find the remaining amount of one euro coinsint oneEuroCoins = remainingAmount / 100;remainingAmount = remainingAmount % 100;

...

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String output = "Your amount " + amount+ " consists of:\n"+ twoEuroCoins + " two euro coins\n"+ oneEuroCoins + " one euro coins\n"+ fiftyCentCoins + " fifty cent coins\n"+ twentyCentCoins + " twenty cent coins\n"+ tenCentCoins + " ten cent coins\n"+ fiveCentCoins + " five cent coins\n"+ twoCentCoins + " two cent coins\n"+ oneCentCoins + " one cent coins";

}

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JOptionPane.showMessageDialog(null, output,"Example 2.4 output",JOptionPane.PLAIN_MESSAGE);

}

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JOptionPane.showMessageDialog(null, output,"Example 2.4 output",JOptionPane.PLAIN_MESSAGE);

System.exit(0);}

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Programming style and documentation

It is important to have readable, well documented, structured source code.Otherwise it is not possible for other people to read, understand and/or use(parts of) your code.

1. document your classes and methods

2. use understandable names for identifiers (names of classes, variables andmethods)

3. make sure indentation and spacing in your source is consistent

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Javadoc comments

Javadoc comments have a special format:

/** ... */

These comments should be provided:

• at the beginning of the program (or class) to explain what the programdoes.

• for every public property of a class (this will be discussed later)

• for every public method of a class

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Javadoc comments

Javadoc comments may contain special commands (starting with the @character) to provide special information about author, version, functionparameters and return values.

Example:

/*** This program computes the area of a circle

* given the radius.

* Company: Technische Universiteit Eindhoven

* @author Marko Boon

* @version 1.0

*/

The javadoc tool uses these comments and these special commands to gen-erate documentation about all classes in HTML format.

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Naming conventions

Use descriptive names with straightforward meanings for the variables, con-stants, classes and methods in your program. Names are case-sensitive.

Some conventions for Java:

• always use lowercase for variables and Methods. If a name consists ofseveral words, concatenate them into one, making the first word lower-case and capatilising the first letter of each subsequent word.

double amount = 3.0;int remainingAmount = (int) (100*amount);JOptionPane.showMessageDialog( ... );

• capitalise the first letter of each word in a class name.

public class AreaComputerJOptionPane

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Naming conventions

• capitalise every letter in a constant, and use underscores between words.

final double PI = 3.141592;final int MAX_VALUE;JOptionPane.QUESTION_MESSAGE

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Proper indentation and spacing

A consistent indentation style makes programs clear and easy to read. Inden-tation is used to illustrate the structural relationships between a program’scomponents or statements.

public class IndentationExample {

public static void main(String[] arg) {int age = 65;System.out.println("I’m " + age + " years old.");

}}

A single space should be added on both sides of a binary operator:

int b = 3 + 4 * (5 + (2 / a));

You should use single space lines to separate segments of the code to makeit easier to read.

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Block styles

End-of-line style:

public class IndentationExample {

public static void main(String[] arg) {System.out.println("Hello World!");

}}

Next-line style:

public class IndentationExample{

public static void main(String[] arg){System.out.println("Hello World!");

}}

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Programming errors

1. Syntax errors (or: compilation errors)

public class ShowSyntaxError {

public static void main(String[] args) {i = 30;System.out.println(i / 4);

}}

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Programming errors

2. Runtime errors

public class ShowRuntimeError {

public static void main(String[] args) {System.out.println(30 / 0);

}}

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Programming errors

3. Logic errors

public static void main(String[] args) {String tempCStr = JOptionPane.showInputDialog(

null,"Enter the temperature in degrees Celsius:","Celsius to Fahrenheit converter",JOptionPane.QUESTION_MESSAGE);

double tempC = Double.parseDouble(tempCStr);double tempF = tempC * (9 / 5) + 32;JOptionPane.showMessageDialog(null,

tempC + " \u00B0C is " + tempF + " \u00B0F","Celcius to Fahrenheit converter",JOptionPane.PLAIN_MESSAGE);

System.exit(0);}

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Program control specifies the order in which statements are executed in a pro-gram. Until now all statements were executed in sequential order.

We will discuss:

• selection statements

• loop statements

Control statements

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Selection statements

The simple if statement:

if (booleanExpression) {// this code will be executed if condition is true

}

The if . . . else statement:

if (booleanExpression) {// this code will be executed if condition is true

}else {// this code will be executed if condition is false

}

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Simple if statement - examplepublic static void main(String[] arg) {String ageStr = JOptionPane.showInputDialog(

null, "What is your age?", "Question",JOptionPane.QUESTION_MESSAGE);

int age = Integer.parseInt(ageStr);String comment = "Perfect!";if (age < 18)

comment = "You are young.";if (age < 0)

comment = "This is an invalid age: "+age;if (age > 50)

comment = "You are old!";JOptionPane.showMessageDialog(null,

comment, "Answer", JOptionPane.PLAIN_MESSAGE);System.exit(0);

}}

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Simple if statement - example

Multiple statements within the if-block:

if (age < 0) {age = -age;comment = "You entered a wrong number.\n" +

"I assume you meant: " + age;}

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if ... else statement - example

int age = Integer.parseInt(ageStr);String comment;if (age < 0)

comment = "This is an invalid age: "+age;else if (age < 18)

comment = "You are young.";else if (age > 50)

comment = "You are old!";else

comment = "Perfect!";JOptionPane.showMessageDialog(null,

comment, "Answer", JOptionPane.PLAIN_MESSAGE);System.exit(0);

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Nested if statements - example

String catsStr = JOptionPane.showInputDialog(null, "How many cats do you have?", "Question",JOptionPane.QUESTION_MESSAGE);

int nrOfCats = Integer.parseInt(catsStr);String answer;if (nrOfCats > 0) {String plantStr = JOptionPane.showInputDialog(null, "How many plants have they destroyed?","Question",JOptionPane.QUESTION_MESSAGE);

int nrOfPlants = Integer.parseInt(plantStr);if (nrOfPlants > 5) answer = "Aren’t they cute?";else answer = "Just wait...";

}elseanswer = "Too bad...";

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More complex conditions - examples

if (nrOfCats > 0 && nrOfDogs == 0) {answer = "I like you!";

}else {answer = "Too bad...";

}

if (denom != 0 && num/denom > 1)...

This statement will not throw a DivisionByZeroException.

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Loop statements

Loops are structures that control repeated executions of a block of state-ments. The statements that should be repeated are called loop body.The loop is being executed as long as a boolean expression called theloop-continuation-condition is true.

Example 1: calculate S = 1+ 2+ . . .+ 19+ 20.

// initialisationint S = 0;S = S + 1;S = S + 2;S = S + 3;...S = S + 20;System.out.println("S = " + S);

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The while loop

while (loop-continuation-condition) {// loop body;

}

n e x t s t a t e m e n t ( s )

s t a t e m e n t ( s )

l o o pc o n t i n u a t i o nc o n d i t i o n ?

T r u e

F a l s e

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The while loop


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The while loop


// initialisationint S = 0;

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The while loop


// initialisationint S = 0;// create a counterint counter = 1;

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The while loop


// initialisationint S = 0;// create a counterint counter = 1;while (counter <= 20) {

}

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The while loop


// initialisationint S = 0;// create a counterint counter = 1;while (counter <= 20) {S = S + counter;

}

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The while loop


// initialisationint S = 0;// create a counterint counter = 1;while (counter <= 20) {S = S + counter;counter = counter + 1;

}

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The while loop



}System.out.println("S = "+S);

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The while loop




Warning: please note that S and counter are initialised outside the loop. Seethe section on The scope of Local Variables for more information.

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The do-while loop

do {// loop body;

}while (loop-continuation-condition);

N e x t s t a t e m e n t ( s )

T r u e

F a l s e


S t a t e m e n t ( s )

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The do-while loop


// initialisationint S = 0;// create a counterint counter = 1;do {S = S + counter;counter = counter + 1;

}while (counter <= 20);System.out.println("S = "+S);

Difference: the statement(s) in the loop body will be executed at least once!

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The for loop

for (initialisation; loop-continuation-condition;action-after-each-iteration) {

// loop body;}

n e x t s t a t e m e n t ( s )

s t a t e m e n t ( s )l o o p b o d y


T r u e

F a l s e

a c t i o n a f t e re a c h i t e r a t i o n

i n i t i a l a c t i o n

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The for loop


// initialisationint S = 0;for (int counter = 1; counter <= 20; counter++) {S = S + counter;


If there is only one statement in the loop body, the braces can be omitted.

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Example 1: calculate∑N

k=0 αk given numbers α and N .

public class LoopExample1 {


}}

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public static void main(String[] arg) {double alpha = 0.6;int N = 20;

}}

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public static void main(String[] arg) {double alpha = 0.6;int N = 20;double sum = 0.0;

}}

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public static void main(String[] arg) {double alpha = 0.6;int N = 20;double sum = 0.0;// initialisation: powerAlphaK = alpha^0double powerAlphaK = 1.0;

}}

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public static void main(String[] arg) {double alpha = 0.6;int N = 20;double sum = 0.0;// initialisation: powerAlphaK = alpha^0double powerAlphaK = 1.0;for (int k = 0; k <= N; k++) {

}

}}

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// precondition: powerAlphaK = alpha^ksum += powerAlphaK;

}

}}

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// precondition: powerAlphaK = alpha^ksum += powerAlphaK;powerAlphaK *= alpha;

}

}}

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// precondition: powerAlphaK = alpha^ksum += powerAlphaK;powerAlphaK *= alpha;

}System.out.println("Sum = "+sum);

}}

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Example 2: calculate∑∞

k=0 αk with absolute precision 10−5, relative precision

10−4 and a maximum of 102 iterations.

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Step 1: initialisation

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final double ABSOLUTE_PRECISION = 1E-5;final double RELATIVE_PRECISION = 1E-4;final double MAX_ITERATIONS = 1E2;

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double alpha = 0.6;double sum = 0.0;double powerAlphaK = 1.0;

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double alpha = 0.6;double sum = 0.0;double powerAlphaK = 1.0;

double oldSum = 0.0;double absPrec = 0.0;double relPrec = 0.0;int n = 0;

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Step 2: the main loop

do {

}while (

);

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do {oldSum = sum;

}while (

);

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do {oldSum = sum;sum += powerAlphaK;

}while (

);

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do {oldSum = sum;sum += powerAlphaK;powerAlphaK *= alpha;

}while (

);

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do {oldSum = sum;sum += powerAlphaK;powerAlphaK *= alpha;absPrec = sum - oldSum;

}while (

);

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do {oldSum = sum;sum += powerAlphaK;powerAlphaK *= alpha;absPrec = sum - oldSum;relPrec = absPrec / sum;

}while (

);

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do {oldSum = sum;sum += powerAlphaK;powerAlphaK *= alpha;absPrec = sum - oldSum;relPrec = absPrec / sum;n++;

}while (

);

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}while (absPrec > ABSOLUTE_PRECISION

);

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&& relPrec > RELATIVE_PRECISION);

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&& relPrec > RELATIVE_PRECISION&& n < MAX_ITERATIONS);

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Nested for loops – Example: multiplication table

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String msg = " Multiplication Table\n\n";

System.out.println(msg);

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String msg = " Multiplication Table\n\n";// create the title row


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String msg = " Multiplication Table\n\n";// create the title rowmsg += " ";for (int i = 1; i <= 9; i++)

msg += " "+i;


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msg += " "+i;msg += "\n------------------------------\n";


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msg += " "+i;msg += "\n------------------------------\n";

// create the table

....


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// create the table

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// create the tablefor (int row = 1; row <= 9; row++) {

}

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// create the title columnmsg += row+" |";

}

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// create the title columnmsg += row+" |";// create the columnsfor (int col = 1; col <= 9; col++) {

}

}

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// create the title columnmsg += row+" |";// create the columnsfor (int col = 1; col <= 9; col++) {int product = row * col;

}

}

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// create the title columnmsg += row+" |";// create the columnsfor (int col = 1; col <= 9; col++) {int product = row * col;

msg += " " + product;

}

}

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// create the title columnmsg += row+" |";// create the columnsfor (int col = 1; col <= 9; col++) {int product = row * col;if (product < 10)

msg += " " + product;

}

}

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msg += " " + product;else

msg += " " + product;}

}

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msg += " " + product;else

msg += " " + product;}msg += "\n";

}

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A method is a collection of statements that are grouped together to performan operation. Methods that have a return value are called functions. Methodswithout a return value are called procedures. Methods may have parameters.These parameters will be treated as local variables in the method body.

Methods

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Methods

Example: method without arguments and return type.

public class MethodTest1 {


}}

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Methods



public static void printName() {

}


}}

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Methods



public static void printName() {System.out.println("My name is Marko Boon.");System.out.println("I am 65 years old.");

}


}}

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Methods



public static void printName() {System.out.println("My name is Marko Boon.");System.out.println("I am 65 years old.");

}

public static void main(String[] arg) {printName();

}}

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Methods

Example: method with arguments, but no return type.



}}

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Methods



public static void printName(String name, int age) {

}


}}

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Methods



public static void printName(String name, int age) {System.out.println("My name is " + name);System.out.println("I am " + age + " years old.");

}


}}

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Methods




}

public static void main(String[] arg) {printName("Marko Boon", 65);

}}

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Methods




}

public static void main(String[] arg) {printName("Marko Boon", 65);printName("Mick Jagger", 98);

}}

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MethodsExample: method with arguments and return type.



}}

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public static boolean isPrime(int number) {...

}


}}

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}

public static void main(String[] arg) {int numberOfPrimesFound = 0;

}}

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}

public static void main(String[] arg) {int numberOfPrimesFound = 0;for (int i = 2; numberOfPrimesFound < 50; i++) {

}}

}

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}


if (isPrime(i)) {

}}

}}

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}


if (isPrime(i)) {System.out.println(i+" ");

}}

}}

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}


if (isPrime(i)) {System.out.println(i+" ");numberOfPrimesFound++;

}}

}}

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Methods

Example: method with arguments and return type.

public static boolean isPrime(int number) {

}

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Methods


public static boolean isPrime(int number) {boolean prime = true;

}

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Methods


public static boolean isPrime(int number) {boolean prime = true;for (int i = 2; i < number; i++) {

}

}

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Methods



if (number % i == 0) prime = false;}

}

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Methods



if (number % i == 0) prime = false;}return prime;

}

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Methods – passing parameters

Java uses pass by value when you invoke a method with parameters. Thismeans that a copy of the value of the actual parameter is passed to themethod. The actual variable outside the method is not affected.

public class PassByValue {

public static void changeValue(int n) {n = 5;System.out.println("inside method: "+n);

}public static void main(String[] arg) {int n = 3;System.out.println("Before calling method: "+n);changeValue(n);System.out.println("After calling method: "+n);

}}

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Method overloading

You can create different methods with the same name, but with different pa-rameters.


}

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Method overloading


public static int max(int n1, int n2) {...

}


}

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Method overloading



}

public static int max(int n1, int n2, int n3) {...

}


}

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Method overloading



}


}

public static void main(String[] arg) {int n = max(4, 6);

}

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Method overloading



}


}

public static void main(String[] arg) {int n = max(4, 6);int m = max(7, -3, 3);

}

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Method overloading



}


}

public static void main(String[] arg) {int n = max(4, 6);int m = max(7, -3, 3);System.out.println("n = "+n+", m = "+m);

}

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Method overloading

public static int max(int n1, int n2) {

}

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Method overloading

public static int max(int n1, int n2) {if (n1 > n2) return n1;

}

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Method overloading

public static int max(int n1, int n2) {if (n1 > n2) return n1;else return n2;

}

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Method overloading


}

public static int max(int n1, int n2, int n3) {

}

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Method overloading


}

public static int max(int n1, int n2, int n3) {if (n1 > n2) {

}

}

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Method overloading


}


if (n1 > n3) return n1;else return n3;

}

}

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Method overloading


}



}else {

}}

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Method overloading


}



}else {


}}

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Method overloading

Smarter implementation:


}

public static int max(int n1, int n2, int n3) {return max(n1, max(n2, n3));

}

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Recursion

Recursion is the powerful mathematical concept of a function calling itself.Example: factorial.

0! = 1n! = n × (n − 1)! (n > 0)

public static int factorial(int n) {if (n < 0) return 0;else if (n == 0) return 1;else return n * factorial(n - 1);

}

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Recursion

Of course n! can be computed without using recursion:

int factorial = 1;for (int i = 2; i <= n; i++) {factorial *= i;

}

This is always more efficient!But sometimes the solution using recursion is much easier.

Example: Fibonacci numbers (0, 1, 1, 2, 3, 5, 8, 13, 21, 34, . . .)

f (0) = 0f (1) = 1f (n) = f (n − 2)+ f (n − 1) (n ≥ 2)

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Recursion

public class RecursionExample2 {

public static int fibonacci(int n) {if (n < 0) return 0;else if (n == 0 || n == 1) return n;else return fibonacci(n - 2) + fibonacci(n - 1);

}

public static void main(String[] arg) {System.out.println("Fibonacci numbers:");for (int i = 0; i <= 25; i++) {

System.out.print(fibonacci(i)+" ");}

}}

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Recursion – Example: The Tower of Hanoi

Problem: move a specified number of disks of a distinct size from one towerto another while observing the following rules:

• there are n disks labelled 1, 2, . . . , n and three towers labelled A, B and C

• no disk can be on top of a smaller disk at any time

• all the disks are initially placed on tower A and should be moved to B

• only one disk can be moved at a time, and it must be the top disk of onetower.

A B C

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A B C

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A B C

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A B C

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A B C

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A B C

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A B C

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A B C

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Recursion:

• suppose you solved the problem for n − 1 disks

• then move the first n − 1 disks to C

• then move disk n to B

• finally move the n − 1 disks back to B

Now we have to find one n for which the problem can be solved easily. This isn = 1, because one disk can simply be moved to the desired tower.

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}

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public static String moveDisks(int n, char fromTower,char toTower, char auxTower) {

...}public static void main(String[] arg) {

}

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...}public static void main(String[] arg) {String nStr = JOptionPane.showInputDialog(null,"Please enter the number of disks:","The Tower of Hanoi",JOptionPane.QUESTION_MESSAGE);

}

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int n = Integer.parseInt(nStr);

}

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int n = Integer.parseInt(nStr);String solution = moveDisks(n, ’A’, ’B’, ’C’);

}

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int n = Integer.parseInt(nStr);String solution = moveDisks(n, ’A’, ’B’, ’C’);JOptionPane.showMessageDialog(null,"Solution:\n" + solution,"The Tower of Hanoi", JOptionPane.PLAIN_MESSAGE);

}

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int n = Integer.parseInt(nStr);String solution = moveDisks(n, ’A’, ’B’, ’C’);JOptionPane.showMessageDialog(null,"Solution:\n" + solution,"The Tower of Hanoi", JOptionPane.PLAIN_MESSAGE);

System.exit(0);}

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}

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if (n <= 0) return "No disks to move.\n";

}

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if (n <= 0) return "No disks to move.\n";else if (n == 1) return "Move disk 1 from "

+ fromTower + " to " + toTower + "\n";

}

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+ fromTower + " to " + toTower + "\n";else {

}}

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+ fromTower + " to " + toTower + "\n";else {String s =

moveDisks(n - 1, fromTower, auxTower, toTower);

}}

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moveDisks(n - 1, fromTower, auxTower, toTower);s += "Move disk " + n + " from " + fromTower

+ " to " + toTower + "\n";

}}

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+ " to " + toTower + "\n";s += moveDisks(n - 1, auxTower, toTower,

fromTower);

}}

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+ " to " + toTower + "\n";s += moveDisks(n - 1, auxTower, toTower,

fromTower);return s;

}}

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A B C

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A B C

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A B C

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A B C

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A B C

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A B C

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A B C

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A B C

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A B C

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A B C

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A B C

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A B C

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A B C

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A B C

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A B C

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A B C

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An array is an object which stores a collection of data of the same type. Youcan retrieve data from this collection by specifying the position (or index)where the item is stored. The first index is 0!

As an example we are going to create an array called a that can hold 5 integervalues. Then we are going to store the following numbers in it: 3, 4, 5, 6 and7.

Arrays

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Step 1: declare the array

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Step 1: declare the array

int[] a;

a

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Step 2: initialise (construct) the array

int[] a;

a

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Step 2: initialise (construct) the array

int[] a;a = new int[5];

a0 1 2 3 4

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Step 3: fill the array

int[] a;a = new int[5];

a0 1 2 3 4

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int[] a;a = new int[5];a[0] = 3;

a

30 1 2 3 4

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int[] a;a = new int[5];a[0] = 3;a[1] = 4;a[2] = 5;a[3] = 6;a[4] = 7;

a

3 4 5 60 1 2 3 4

7

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Arrays

• an array is an Object, no primitive type

• this means that an array that is declared but not constructed has the valuenull

• this also means that an array variable does not actually contain that array,but a reference to the array

• arrays have a property length that returns the capacity (or size) of the array

• you cannot store items in an array before it is constructed (to specify thesize)

• an alternative array declaration, compatible with C++, is:

dataType arrayName[];

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Another way to fill the array is to use the alternative constructor.

int[] a = {3, 4, 5, 6, 7};

Most popular approach is to use the for-loop when possible:

int[] a = new int[5];for (int i = 0; i < a.length; i++)a[i] = 3 + i;

It is easy to compute the sum of the elements:

int sum = 0;for (int i = 0; i < a.length; i++)sum += a[i];

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Array example

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Array example

String size = JOptionPane.showInputDialog(null,"How many numbers do you want to enter?","Array Example", JOptionPane.QUESTION_MESSAGE);

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Array example


int n = Integer.parseInt(size);

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Array example


int n = Integer.parseInt(size);double[] numbers = new double[n];

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Array example


int n = Integer.parseInt(size);double[] numbers = new double[n];for (int i = 0; i < numbers.length; i++) {

}

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Array example


int n = Integer.parseInt(size);double[] numbers = new double[n];for (int i = 0; i < numbers.length; i++) {String nr = JOptionPane.showInputDialog(null,

"Please enter number "+(i+1),"Array Example", JOptionPane.QUESTION_MESSAGE);

}

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Array example


int n = Integer.parseInt(size);double[] numbers = new double[n];for (int i = 0; i < numbers.length; i++) {String nr = JOptionPane.showInputDialog(null,

"Please enter number "+(i+1),"Array Example", JOptionPane.QUESTION_MESSAGE);

numbers[i] = Double.parseDouble(nr);}

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Array example

// find largest number

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Array example

// find largest numberdouble max = -99999;

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Array example

// find largest numberdouble max = -99999;String output = "The numbers you entered: ";

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Array example

// find largest numberdouble max = -99999;String output = "The numbers you entered: ";for (int i = 0; i < numbers.length; i++) {

}

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Array example


if (numbers[i] > max) max = numbers[i];

}

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Array example


if (numbers[i] > max) max = numbers[i];output += numbers[i] + " ";

}

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Array example



}output += "\nThe largest number is: "+max;

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Array example




JOptionPane.showMessageDialog(null,output, "Array Example",JOptionPane.PLAIN_MESSAGE);

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Array example




JOptionPane.showMessageDialog(null,output, "Array Example",JOptionPane.PLAIN_MESSAGE);

System.exit(0);

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Array example 3 – Sorting an array

public static double[] sortArray(double[] numbers) {

}

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Step 1: Create a copy of the original array.

public static double[] sortArray(double[] numbers) {double[] sorted = new double[numbers.length];

}

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Step 1: Create a copy of the original array.

public static double[] sortArray(double[] numbers) {double[] sorted = new double[numbers.length];for (int i = 0; i < sorted.length; i++)sorted[i] = numbers[i];

}

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Step 2: create a loop that runs through the elements


for (int i = 0; i < sorted.length; i++) {

}

}

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Step 3: create another loop that find the smallest number behind ipublic static double[] sortArray(double[] numbers) {double[] sorted = new double[numbers.length];for (int i = 0; i < sorted.length; i++)sorted[i] = numbers[i];

for (int i = 0; i < sorted.length; i++) {for (int j = i + 1; j < sorted.length; j++) {

}}

}

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Step 4: if element at position j is smaller, swap with i .



if (sorted[j] < sorted[i]) {

}}

}

}

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if (sorted[j] < sorted[i]) {double tmp = sorted[j];

}}

}

}

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if (sorted[j] < sorted[i]) {double tmp = sorted[j];sorted[j] = sorted[i];

}}

}

}

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if (sorted[j] < sorted[i]) {double tmp = sorted[j];sorted[j] = sorted[i];sorted[i] = tmp;

}}

}

}

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Step 5: return the sorted array.



if (sorted[j] < sorted[i]) {double tmp = sorted[j];sorted[j] = sorted[i];sorted[i] = tmp;

}}

}return sorted;

}

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3 45 60 1 2 3 47

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3 45 60 1 2 3 47

i

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3 45 60 1 2 3 47

i j

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3 45 60 1 2 3 4

7

i j

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3 45 60 1 2 3 4

7

i j

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3 45 60 1 2 3 4

7

i j

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3 45 60 1 2 3 4

7

i j

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3 45 60 1 2 3 4

7

i j

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3 45 60 1 2 3 4

7

i j

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3 45 60 1 2 3 4

7

i j

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3 45 60 1 2 3 4

7

i j

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3 4 560 1 2 3 4

7

i j

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3 4 560 1 2 3 4

7

i j

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3 4 560 1 2 3 4

7

i j

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3 4 560 1 2 3 4

7

i j

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3 4 5 60 1 2 3 4

7

i j

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3 4 5 60 1 2 3 4

7

i j

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3 4 5 60 1 2 3 4

7

i j

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References to arrays

Creating an array actually creates a reference to an array. This means thatmethods actually can modify array entries. So be careful when you think youare using Reference by value in a method. Suppose we have the following tomethods:

public static void changeArray1(double[] numbers) {numbers = new double[] {1, 2, 4};

}

public static void changeArray2(double[] numbers) {numbers[0] = 1;numbers[1] = 2;numbers[2] = 4;

}

Do these methods change the original array?

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public static void printArray(double[] array) {for (int i = 0; i < array.length; i++)

System.out.print(array[i]+" ");System.out.println();

}

public static void main(String[] arg) {double[] array = {5, 6, 7};printArray(array);changeArray1(array);printArray(array);changeArray2(array);printArray(array);

}

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}

a r r a y

75 60 1 2

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}

a r r a y

75 60 1 2

n u m b e r s

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}

a r r a y

75 60 1 2

n u m b e r s

41 20 1 2

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}

a r r a y

75 60 1 2

n u m b e r s

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}

a r r a y

71 60 1 2

n u m b e r s

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}

a r r a y

71 20 1 2

n u m b e r s

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}

a r r a y

41 20 1 2

n u m b e r s

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• since an array variable is actually only a reference to an array, “pass byvalue” only creates a new local array variable that references to the samearray. This means that you can actually replace the items in an array (butnot the array size!)

• to prevent mistakes, always create a copy of the array at the beginning ofa method:

public static void changeArray2(double[] numbers){double numbersCopy = new double[numbers.length];for (int i = 0; i < numbersCopy.length; i++)numbersCopy[i] = numbers[i];

...}

Whatever you do to numbersCopy will not affect the original array num-bers.

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Multidimensional arrays

Until now we only created one-dimensional arrays. It is also possible to createmultidimensional arrays.

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Multidimensional arrays – Example

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int[][] matrix;

Declaration: matrix is a two-dimensional array of integers. At this point matrixequals null, which is the value of an Object that has been declared, but hasnot been initialised (constructed) yet.

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int[][] matrix;matrix = new int[3][5];

m a t r i x0 1 2 3 4

0

1

2

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int[][] matrix;matrix = new int[3][5];matrix[2][0] = 7;

m a t r i x0 1 2 3 4

7

0

1

2

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int[][] matrix;matrix = new int[3][5];matrix[2][0] = 7;for (int i = 0; i < matrix.length; i++)for (int j = 0; j < matrix[i].length; j++)matrix[i][j] = i * matrix[i].length + j;

m a t r i x

0 1 2 30 1 2 3 4

45 6 7 8 91 0 1 1 1 2 1 3 1 4

0

1

2

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In Java a multidimensional array is in fact an array of arrays. To obtain thelengths of multidimensional arrays we have to determine the size of each di-mension:

double[][][] a = new double[7][10][14];int d1 = a.length; // d1 = 7int d2 = a[0].length; // d2 = 10int d3 = a[1].length; // d3 = 10int d4 = a[0][1].length; // d3 = 14

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This makes it possible to create ragged arrays: an array with rows of unequallengths:

double[][] triangle = {{1, 2, 3, 4},{5, 6, 7},{8, 9},{10}};

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This makes it possible to create ragged arrays: an array with rows of unequallengths:

double[][] triangle = {{1, 2, 3, 4},{5, 6, 7},{8, 9},{10}};

int d1 = triangle.length; // d1 = 4int d2 = triangle[0].length; // d2 = 4int d3 = triangle[1].length; // d3 = 3int d4 = triangle[2].length; // d4 = 2int d5 = triangle[3].length; // d5 = 1

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Alternative constructor for ragged arrays:

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double[][] triangle = new double[4][];

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double[][] triangle = new double[4][];triangle[0] = new double[4];triangle[1] = new double[3];triangle[2] = new double[2];triangle[3] = new double[1];

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double[][] triangle = new double[4][];triangle[0] = new double[4];triangle[1] = new double[3];triangle[2] = new double[2];triangle[3] = new double[1];int n = 1;

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double[][] triangle = new double[4][];triangle[0] = new double[4];triangle[1] = new double[3];triangle[2] = new double[2];triangle[3] = new double[1];int n = 1;for (int i = 0; i < triangle.length; i++) {

}

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double[][] triangle = new double[4][];triangle[0] = new double[4];triangle[1] = new double[3];triangle[2] = new double[2];triangle[3] = new double[1];int n = 1;for (int i = 0; i < triangle.length; i++) {for (int j = 0; j < triangle[i].length; j++) {

}}

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double[][] triangle = new double[4][];triangle[0] = new double[4];triangle[1] = new double[3];triangle[2] = new double[2];triangle[3] = new double[1];int n = 1;for (int i = 0; i < triangle.length; i++) {for (int j = 0; j < triangle[i].length; j++) {triangle[i][j] = n;

}}

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double[][] triangle = new double[4][];triangle[0] = new double[4];triangle[1] = new double[3];triangle[2] = new double[2];triangle[3] = new double[1];int n = 1;for (int i = 0; i < triangle.length; i++) {for (int j = 0; j < triangle[i].length; j++) {triangle[i][j] = n;n++;

}}

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double[][] triangle = new double[4][];triangle[0] = new double[4];triangle[1] = new double[3];triangle[2] = new double[2];triangle[3] = new double[1];int n = 1;for (int i = 0; i < triangle.length; i++) {for (int j = 0; j < triangle[i].length; j++) {triangle[i][j] = n;n++;

}}printMatrix(triangle);

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Introduction to Programming in Java - TU/emarko/iq/part1-handouts.pdf · ... Fundamentals of programming 1.Introduction to Java ... write a program that computes the area of a circle,