Java I Refresher

Jonathan F. Gemmell

Saturday, January 7th / Friday, January 13th

Room 819 CTI

Before we begin- Sign up sheet- Survey- Does everyone’s computer work?- These slide are new (let me know if you spot a typo)- This is meant to be a hands-on review. We will

spend about 50% of the time working through problems. Make sure I have your email address, so I can send you the solutions to the problems.

- Please feel free to raise your hand, shout out questions, or throw rotten fruit during the review.

Outline- Basic structure of an application

- Demo: HelloWorld- Primitive data types and operations

- Demo: MathProblems- Using predefined objects

- Demo: StringDemo, ScannerDemo- Branching structures (if, if-else, switch)

- Demo: ZipCode- Loops (for, while, do-while)

- Demo: Factor- Demo: MultiplicationTable

Outline continued- Arrays

- Demo: CalculateAverage

- More on the structure of an application- Demo: CircleCalculations

- Method Overloading- Demo: PrintNames

- Objects- Demo: Rectangle- Demo: Country and CalculatePopulations

Basic structure of an application A Java application consists of a definition of a class. The name of the class (ClassName) and the name of

the file (ClassName.java) must be identical. The class definition has a header (public class

ClassName) followed by a body inside braces. The body of the class may contain a method called

“main”. The main method always has the modifiers public, static and void in front of it.

Demo: HelloWorld Write the following program on your

computer public class HelloWorld

{

public static void main (String[] args)

{

System.out.println("Hello World!");

}

}

Primitive data types and operations There are eight java primitives

boolean true or false byte signed 8-bit integer char 16-bit Unicode 2.0 character short signed 16-bit integer int signed 32-bit integer long signed 64-bit integer float signed 32-bit floating-point double signed 64-bit floating-point

Primitive data types and operations Most often you will use….

booleans for true/false variables true, false

chars for characters a, b, c …

ints for integers 1, 2, 3 … (not 1.00)

doubles for decimals 1.0, 3.14, 9.9999999 …

Primitive data types and operations Java is a strongly typed language. An explicit type must be assigned to

every data value. Ex:

int x = 5;int y;y = 7;int z = x + y;System.out.println(z);

Output: 12 Notice that

x, y and z are all declared as ints You can only declare a variable once!!!

int x = 5;int x = 6; Can’t re-declare x!!

Primitive data types and operations Common operations on primitives

Addition, Multiplication, etc double z = x + y, double z = x * y

Modulus int z = x % y

Numerical comparison and equality boolean z = ( x < y ) boolean a = (x = = y)

Recall that one equals sign ( = ) is an assignment Two equals signs ( = = ) is a comparison

Primitive data types and operations Casting

Will this compile?double x = 3;

double y = 2;int z = x / y;

Notice that a double has 64 bits, while an int has only 32. Trying to assign the value of x/y to z is like trying to pour a gallon of water into a 2-liter bottle. Try this instead….

double x = 3;double y = 2;int z = (double)(x / y);

This is called casting. But be careful! You could lose precision. In the above example z will equal 1, not 1.5

Primitive data types and operations What is the value of z? Why?

int x = 3;

int y = 2;

int z = x / y;

In “integer math” the decimal gets thrown out. z equals 1, not 1.5

Primitive data types and operations Modulus is a useful operation. It results in

the remainder of the division operation. 10 % 3 = 1 6 % 2 = 0 5 % 3 = 2 14 % 5 = 4

What is the value of z? int z = 8 % 3;

Demo: MathProblems (5 minutes) Write an application called MathProblems.

Declare variables where appropriate. Try some of the examples in the previous slides. Be sure to try at least one example of primitive operations, casting, “integer math”, and modulus.

Using predefined objects One of the strengths of Java is the ability to create

objects in your program from a large variety of pre-defined classes.

REMEMBER THIS WEBSITE!!!!! http://java.sun.com/j2se/1.5.0/docs/api/index.html

Notice that primitive data types create a variable, whereas a class produces an object.

Each pre-defined class has a collection of public methods defined for it.

Using predefined objects The String class

One of the first classes you used in Java 211 was the String class. A String represents a sequence of characters.

To create a String object: String s = new String(“A string of characters”);

There is also a short-hand way to create a String: String s = “Another string of characters”;

Using predefined objects Methods of the String class

int length(): returns the number of characters in a StringString s = "this is a String";int i = s.length();

char charAt(int index): returns the character at the index specified

String s = "this is a String";char c = s.charAt(3);

int indexOf(char target): returns the index of the first occurrence of the target. -1 otherwise.

String s = "this is a String"; int i = s.indexOf('S');

Using predefined objects String substring(int start, int end): returns a String that

represents the characters from positions “start” to “end”String s = "this is a String";String t = s.substring(5,10);

Many more! Concatenation

You can concatenate two Strings together with the ‘+’ operator. You can also concatenate a String and a primitive.

String s = "this is a String.";String t = "this is also a

String";String u = s + t;

StringDemo Write an application that declares some

Strings. Use the methods described in the previous slide. Concatenate two Strings together. Try concatenating a String and a primitive.

Using predefined objects The Scanner class

The Scanner class allows a program to read input taken from standard input. It can be found in the java.util package so you must put “import java.util.*;” in the program in order to use it.

Declare and initialize a Scanner object: Scanner s = new Scanner(System.in);

Use the Scanner object to obtain an integer value: int x = s.nextInt();

Use the Scanner object to obtain a decimal value: double y = s.nextDouble();

Using predefined objects The Scanner class cont.

In order to use the Scanner class to input a String: 1. Declare and initialize the Scanner object 2. Change the delimiter from whitespace to the line

separator 3. Use the Scanner object to input a String

Scanner s = new Scanner(System.in);

s.useDelimiter( System.getProperty("line.separator") );

String str = s.next();

ScannerDemo Write an application that declares and

initializes a Scanner object. Use it to input integer values, decimal values, and Strings. Print the input values to the screen.

Branching structures This is the most basic if-statement

if (boolean expression)

{

//do this block of

}

If the boolean expression is true, the program will execute the next block of code.

Branching structures Compound if–else statements

if (boolean expression){

//do this} else if (boolean expression){

//do something else}else{

//do yet something else}

Branching structures A switch statement evaluates the given integer and execute the

appropriate block of code.switch (x){

case 1: //this block of code will be executed if x equals 1{

System.out.println("first case");break;

}case 2: //this block of code will be executed if x equals 2{

System.out.println("second case");break;

}default: //this block of code will be executed as a default{

System.out.println("third case");break;

}}

Demo: ZipCode Write an application that takes a three digit area code

from a user and then tells a user where s/he lives. 312 Chicago 212 New York 510 San Francisco 972 Dallas

Write one version that uses a switch statement and a second version that uses a compound if-else statement.

Loops The purpose of a loops is to repeat a block of

code many times. for loop while loop do- while loop

Loops for loop

Use a for loop when you know exactly how many times you want to repeat the loop. You may know the particular value, or know a variable that contains the value.

The for loop has three crucial parts The initialization of the counter The boolean statement that tests the continuation condition Incrementing the counter

For loops are useful when working with arrays (as we will see) EX: print hello 5 times.

for(int i=0; i<5; i++){

System.out.println("hello");}

Loops while loop

Use a while loop when you want to repeat a block of code an unknown number of times (maybe not at all)

A while loop will repeat the block of code until the boolean statement is no longer true

while (boolean expression)

{

//do this block of code

}

Loops do-while

Use a do-while loop when you want to repeat a block of code an unknown number of times, but at least once.

Perfect for user inputs. EX: ask a user for a number between 1 and 10, repeat

until the user enters a valid number.Scanner s = new Scanner(System.in);int input;do{

System.out.print("Enter a number between 1 and 10: ");input = s.nextInt();

}while(input < 1 || input > 10);

Demo: Factor Write an application that prompts the user for

a positive integer and prints to the screen the factors of the input. The program should not accept negative numbers as an input, and should continue to ask for a value until the user enters a proper input. For example, if the user enters 7, the output

should be 1 and 7. If the user enters 12, the output should be 1, 2, 3, 4, 6, and 12.

Demo: MultiplicationTable It is crucial for you to understand and be able

to implement nested loops. A nested loop is a loop inside another. Using nested loops, write an application that prints to the screen a five by five multiplication table.

Arrays In Java, arrays are objects You can create an array of any primitive or object

class by using “[]” The declaration of an integer array:

int[] x; The declaration of a String array:

String[] s; Declaring an array does not create the object or

allocate memory. It merely creates the variable.

Arrays Initializing arrays:

Use the key word “new” to initialize an array and provide the type of the array, and the size of the array within the brackets. int[] x = new int[100];

Creates an int array x, 100 elements long

String[] s = new String[10]; Creates a String array s, 10 elements long

Arrays Modifying elements

int[] x = new int[10];x[0] = 5;x[2] = 8;

In the above example, a new array is declared and initialized. The 0 th element is set to 5, while the 2nd element is set to 8. Recall that a 10 element array is numbered 0 through 9.

Accessing elementsint i = x[2];

In the above example, the program will access the 2nd element of the array and assign its value to the integer variable i.

Array Length In Java, an array knows how long it is.

int[] x = new int[20];int l = x.length;

Demo: CalculateAverage Write an application that creates an int array

of size 20, fills it with random numbers in the range 0 through 99, prints the array to the screen, and then calculates and prints the average of the numbers. (hint: use for loops) To generate a random number in the range 0

through 99:int x = (int)(Math.random() * 100);

More on the structure of an application So far the example programs have had only

one method (the main method) and none of the examples have had any class variables. In practice, most programs will have many class variables and many methods.

More on the structure of an application Class variables

A class variable is declared at the class level, rather than inside the body of a method. Consequently, the scope of variable extends throughout the entire class.

public class Test{

static int i;public static void main (String[] args){

i = 5;}

} Notice that even while the variable i is declared outside the main

method, it can still be accessed inside the main method. Also notice the variable i has the modifier “static.” More on this

later.

More on the structure of an application Scopepublic class Scope{

int x = 5;public static void main (String[] args){

int y = 6;if (y == 6){

int z = 7;}

}} What is the scope of the variables x, y and z? Notice how proper

indentation makes determining the scope easier.

More on the structure of an application Methods – A method has two parts

Header – Describes the method in general terms, including its name, visibility, inputs and outputs. Visibility modifier Return type Method Name List of parameters

Body – Describes the specific actions the method will perform The body consists of a sequence of program statements to be

executed. If the method returns a value, there must be a return statement.

More on the structure of an application Visibility modifiers

public Visible outside the class Called a service method

private Not visible outside the class It can only be used by methods inside the class Called a support method

More on the structure of an application Return type

Can be any primitive, a class, or void. It tells us what kind of result comes back from the

method when it is evaluated. If a method is supposed to return a value, there

must be a return statement.

More on the structure of an application Method Name

Gives the method a name, so that it can be called from elsewhere in the program.

The convention is to use lower case letters except for the first letter of subsequent words. No white spaces. EX: calculateSum EX: getLastName EX: SetPhoneNumber not convention

More on the structure of an application List of parameters

A list of the type and name of the expected inputs Type gives what kind of data is expected for that input Name gives the name by which it will be referred to

during the method’s execution Parameters are given in parentheses, separated by

commas. There does not need to be any parameters at all if the

method does not require them.

More on the structure of an application An example of a method

public static int squareNumber(int n)

{

int result = n * n;

return result;

} Visibility modifier - public Return type - int Method Name - squareNumber List of parameters – an int named ‘n’

More on the structure of an application Another Example

private static void printSum(double x, double y)

{

double result = x + y;

System.out.println(result);

} Visibility modifier - private Return type - void Method Name - printSum List of parameters – a double named ‘x’ and a double named ‘y’

Demo: CircleCalculations Write a program with a class variable named pi equal to 3.14. Write a method (don’t put it in main!) that asks the user to

enter a radius (double) and returns this value. Write a method that accepts a radius (double) as a parameter

and calculates the area of the circle with that radius. It should return the area. ( a = pi * r2 )

Write a method that accepts a radius (double) as input and prints to the screen the circumference of a circle given the radius. The return type should be void. ( c = 2 * r * pi )

In the main method call these three methods appropriately.

Method Overloading Methods expect the proper input. The

compiler will object if the wrong type or number of variables are supplied as arguments.

An overloaded method has several different definitions, all with the same method name, but different types and/or number of inputs.

Method Overloading Signature

The ordered list of types of parameters in the header of a method is called a signature.

As long as they have different signatures, we can define as many methods as we want with the same name. The compiler will choose the correct method based on the method’s signature.

Demo: PrintNames Write a method called printName that accepts a String called

lastName as a parameter. It should print to the screen the name provided. EX: if the input is “Smith”, it should print out “Mr. Smith.”

Write a method with the same method name. This method should take two Strings as parameters, firstName and lastName. EX: if the inputs are “John” and “Smith,” it should print out “John

Smith” Write a third method with the same method name. This

method should take two Strings as arguments and one char. EX: if the inputs are “John”, ‘A’, and “Smith,” the method should

print out “John A. Smith.” Write a main method to test the overloaded methods.

Objects You have already used built-in classes

String and Scanner for example You must now learn to write your own. A class should contain two distinct pieces of

information Properties: The state of the object

Given by the data members or instance variables Behaviors: The capabilities of the object

Given by the methods or instance methods

Objects Anatomy of a class

public class ClassName

{

//instance variables

//constructor(s)

//methods(s)

}

Objects Instance Variables

Typically, instance variables are not directly accessible. All your instance variables should be declared private. public – any program can see (and perhaps modify!) this data private – only visible to methods of the class

EX:public class Rectangle{

private int width;private int height;

…. Any object you create from the Rectangle class will have

its own width and height.

Objects Constructor

Constructors are a special type of method. They are often used to initialize the data members of a class when you create objects. They are automatically called upon declaration of an object. They should be declared public They should have exactly the same name of the class Like any other method, they may or may not have

parameters

Objects Constructor cont

…public Rectangle(int w, int h){

width = w;height = h;

}…

In this example the constructor takes two parameters, both ints, and assigns their values to the instance variables of the class.

Objects Constructor cont..

To call the constructor and create an object of the Rectangle class: Rectangle r = new Rectangle(2,3);

Assuming our Rectangle class is complete, this will create Rectangle object with a width of 2 and a height of 3.

Objects Instance methods represent the operations that can

be performed by objects in the class The methods can also be public or private When deciding which to use, consider whether or not you

want the operation available outside the class. Consider the methods on the following slides

Notice the getter and setter methods Notice that the calculateArea method will be called on a

specific Rectangle object and will return the area of a that specific Rectangle object.

Objectspublic void setWidth(int w){

width = w;}

public void setHeight(int h){

height = h;}

public int getWidth(){

return width;}

Objectspublic int getHeight(){

return height;}

public int calculateArea(){

int area = width * height;return area;

}

Objects Static

Notice that neither of the methods or instance variables in the Rectangle class are static.

Static variables or methods belong to the class as a whole rather than to a particular object of the class. You do not need to declare an object from a class in order to use

static methods Conversely, you must declare an object from a class in order to

use non-static methods

Non-static variables belong to a specific instance of the class, not the class as a whole.

Demo: Rectangle Implement the Rectangle class as described in

the notes. Create a second program called

RectangleDriver that creates many Rectangle objects and thoroughly tests all the methods.

Add to the rectangle class a static int that keeps count of how many Rectangle objects have been created.

Country and CalculatePopulations We have gone over many small examples

together. Now I would like you to go over a larger example on your own.

I will be happy to speak with you individually if you require help.

Don’t forget that tutors are available at CTI seven days a week and are happy to help you with your studies.

Country and CalculatePopulations Write a Class called Country

It should contain the following instance variables A String for the name of the country An int for the population of the country (in millions) A double for the growth rate of the country

It should contain the follows methods A constructor to initialize all the member variables Appropriate getter and setter methods. A method called calculatePopulationForYear that takes as a

parameter a year (int) and using the population and growth rate of the country calculates the population for that year. I’ll give you this formula.

Country and CalculatePopulations Write a class called CalculatePopulations

It should create an array of 5 countries and instantiate the Country objects using the constructor of the Country class. See http://www.cia.gov/cia/publications/factbook/ for a list of

countries and the populations, etc.

It should print to the screen the five countries (hint: use a for loop) and label them 0 through 4.

It should then ask the user to input an int, 0 through 4, and repeat until the user enters an appropriate number.

Country and CalculatePopulations CalculatePopulations cont..

It should then ask the user to input a year greater than 2000. Repeat if necessary.

Finally, it should print to the screen the population of the selected country in the selected year.