Chapter 2

An Intro to Java and

Your First ProgramWhat is Java?

Advantages of Java

Four Steps for Creating and Running a Java Program

Framework of a Simple Java Program

What is a Java class?

Three Types of Java Output Programs

The Methods print and prinln

Syntax and Compile-Time Errors

What is Java?A simple,

object-oriented,

network-savvy,

interpreted,

robust,

secure,

architecture neutral,

portable,

high-performance,

multithreaded,

dynamic language.

What is Java?• Java is Object-Oriented. It is the world’s fastest growing

programming language for building software components that can be maintained and re-used.

• Java is network-savvy. It has extensive library routines for coping easily with TCP/IP protocols like HTTP and FTP so it runs more efficiently over a network than other languages.

• Java is interpreted. Java bytecodes are translated on the fly to native machine instructions (interpreted) and not stored anywhere.

• Java is robust. That means that Java puts a lot of emphasis on checking for possible problems and trying to eliminate situations that are error prone. This helps programs run efficiently and smoothly.

• Java is secure. It provides for the creation of virtually virus-free systems that are hard to break into.

What is Java?• Java is architecture neutral. Java was designed to support

applications on networks not a particular computer platform. So it doesn’t matter what your platform is.

• Java is portable. Programs can be run on different types of computers without having to change any of the code in a program.

• Java is high-performance. The performance of bytecodes converted to machine code is almost indistinguishable from native C or C++.

• Java supports threads. Instead of performing only one operation at a time and not starting another operation until one is completed, Java can run numerous operations at the same time if each operation is a thread.

• Java is dynamic. Java avoids the problems other languages have and makes use of the object-oriented paradigm in a much more straightforward manner. Libraries can freely add new methods and instance variables without any effect on their clients.

• To learn more about Java’s Features in detail Click here

The Three Steps for Creating & Running a

Java Program

Three Steps for Creating & Running a Program1. Edit (write) the source code in a .java file

2. Compile the source code into byte code for a .class file

3. JVM interprets the byte code into machine language 1s and 0s and the computer then executes the instructions

Source Code is Written in a .java File

Step1. When a programmer writes Java code, it is stored in a source code file that has a .java extension like RobotMan.java.

The source code file is what the programmer compiles and runs to see the output of a program.

Source code can be typed in any simple text editor, but we will use Eclipse, which is both a text editor and a compiler. A compiler lets you run the program.

Eclipse is a nice Java editor and compiler because it will point out syntax errors when java code is typed incorrectly. A program will not run until all syntax errors are removed.

Source Code is Compiled to Byte Code

Step 2. An IDE (Integrated Development Environment) like Eclipse, has a built in compiler that translates the Java source code into Java byte code.

When source code is compiled into byte code, the byte code is stored in a file that has the same name as the source code file but with a .class extension. It is not a text file and you cannot open it! For example, the source code file RobotMan.java will be compiled into a byte code file named RobotMan.class.

The file is created during the compiling process, however, Eclipse sometimes translates .java files into .class files behind the scenes before you ever compile them. You can see these files in your package folders if you open your workspace folder on your hard drive.

Byte Code is Interpreted to Machine CodeStep 3. Before a program can be displayed, the JVM (Java

Virtual Machine) must interpret the byte code into machine language. The JVM is software that acts like hardware. That is why it is called a virtual machine. Java bytecodes are translated on the fly to 1s and 0s instead of being stored in an executable file, which some languages do. The main advantage of Java is that there are JVMs for every kind of computer and this makes the code portable from one platform to another. (You’ll understand this more as we go.)

Interpreting can be slow, but software improvements are always being made to speed up this process.

Every kind of computer, whether it is an Apple, Windows, or Unix machine, has an operating system that contains a JVM software component. The JVM was automatically installed when your computer’s system software was installed and updates to the JVM may be contained in any system software updates you install now or in the future.

Just-in-Time Compilation by the JVMSome JVMs support just-in-time compilation (JIT).

JITs will translate byte code instructions the first time they are encountered into machine language code and that code is saved so that the next time that code is encountered it can be executed without being re-interpreted. This can speed up the interpreting of the byte code of a program so it will run faster.

Machine Language Code is ExecutedStep 4. Finally, the machine language code (1s and 0s), those

instructions that the computer understands at the lowest level, are executed. This allows the program to appear in its runnable form, either in a console window, an applet window, or a GUI (Graphical User Interface) window. Again the overall process is ….

Framework of a Simple Java Program

1. package declaration

2. import statements

3. class declaration line

4. main method

Framework of a Simple Java ProgramThe Framework of a simple console or standalone program:1. package declaration for the file 2. import statements (if any)3. class declaration line4. main method

package unit1;

import java.util.Scanner;

public class DistanceCalculator{ // opening curly brace for the DistanceCalculator class

public static void main(String args[ ]){ // opening curly brace for main method

// code for program goes inside these curly braces

} // ending curly brace for main method

} // ending curly brace for the DistanceCalculator class

All Java console and JFrame (standalone) programs have this framework. The framework for applets is different and you will become familiar with it later.

Note: curly braces always appear in pairs.

Classes in JavaMost of the time, every .java source code file contains 1 class.

It is possible to have more than one class in a .java file, but this is usually only done in advanced programming projects where the code is more complicated and there is a reason for doing it. So don’t worry about that for now.

So assume that every .java file contains only one class.

However, any Java program can contain more than one class file. In fact, it can contain many class files.

A class is a module of code that can stand alone by itself and if made “public” other files can “see it” and “access it”. Eclipse is a nice IDE for organizing Java files so that classes are accessible to each other if they are in the same package folder or if an appropriate import statement is used. Eclipse manages this behind the scenes automatically and lets you know if something is “unorganized” so you can fix it.

Declaring Classes in a FileA class is declared with a line like:

public class RobotMan

and has a set of curly braces that go with it { and }.In Java, a class like RobotMan must be stored in a file

named RobotMan.java. If they don’t match, then Eclipse or any other IDE will flag it with an error and it must be corrected before the file is used in a program. Eclipse has short cuts to renaming the file (compilation unit). You’ll learn about that soon.

Java programs can be designed so that they contain just one class. We will now look at some examples.

Three Types ofJava Output Programs

Three Types of Java Output Programs

There are basically three types of Java output programs that a programmer can make:

1. A console text output program.

2. A standalone graphics or GUI program that can be

displayed in a normal window frame.

3. An applet graphics or GUI program that can be displayed

in a browser window.

Console Text Output ProgramsSome Java programs, like RobotMan, are displayed only in a console window, because they show

only text output. You will learn how to do more than draw things with text characters in a console window. Some programs may make mathematical calculations and display them.

When there is only one file in a Java Program, we sometimes refer to that file as a “Driver file”, because it is the file that makes the program run. If a program has more than one file, then one of the files is a driver file.

Graphics & GUI ProgramsBesides console text programs, you can have applet or

standalone programs that contain graphics or GUI components:

1. GUI components are elements in a program like text fields, buttons, labels, text areas, or menus.

2. Graphics are lines, and curves, and pretty much anything that can be drawn.

So you can have:1. An applet program with GUI components.2. An applet program with graphics.3. A standalone program with GUI components.4. A standalone program with graphics.5. An applet or standalone program that has both GUI

components and graphics.

Applet GUI Output ProgramsJava applets are Java programs that run in a Web browser. A JVM is

incorporated into every browser, so the program can be interpreted, executed and displayed inside the browser. Any game you have played online is more than likely a Java applet program. Both applet and standalone programs can have GUI components as seen here.

Because Java programs run inside a Virtual Machine, it is possible to limit their capabilities. What this means is that some other programmer can’t inject a virus into the code you have written for an applet program. Therefore, everybody doesn’t have to worry about a Java applet infecting their computers with a virus that will erase files on their hard drive or steal sensitive information.A GUI program with fields, buttons, labels, and a text area.

Applet or Standalone Graphics Output ProgramsApplet Graphics program can contain lines, arcs, ovals, rounded-rectangles,

etc. GUI or graphics programs pretty much look the same whether they are in an applet or standalone program.

Source Code

The code a programmer writes!

A Program has Source Code

• A program is a sequence of instructions that perform a task.

• Source code: The code written by a programmer in a programming language and stored in a text file with a .java extension. Each line of code is an instruction written by the programmer that tells the computer what to do. Java programs may have more than one file, but one of the files is always a “Driver File” that is the one that you run to get the program running.

Java Is Hot Source Code

Mr. Conrey changed the class name of this file to JavaIsHot, even though the authors originally called it HelloWorld. In this case, the name of the class doesn’t matter.

Method Calls with ParametersIn the line of code:System.out.println(“Hello World!”);• System is a class and out is an object of that

class that knows how to display or print characters in a console or terminal window.– println is the message (operation) being sent to the

System.out object. It is the name of a method. (Another way to say it is we are “calling the method println”.)

– The item inside the parentheses is the parameter. Here the parameter is a string (string of characters that make up the words “Hello World”) that appear in quotation marks and contains characters to be printed to the screen. However, the parameter could be a variable instead of a string. More on that later.

General Rule of Method Calls• A message may require zero, one, or multiple

parameters:• System.out.println(); // calling println with no parameters• System.out.println(“Hello World!”); // one parameter• calculateArea(length, width); // calling some method

named calculateArea with two parameters length and width

• The general form for sending messages to objects:

<name of object> . <name of message> (<parameters>)

OR

<name of object> . <name of method> (<parameters>)

The Method Selector is the Period• The Method selector operator is the period . and is

always placed between the object’s name and the method’s name

• System.out.println(“Hello World!”);• num2 = reader.nextInt();• num4 = reader.nextDouble();

In the last two examples, reader is the variable that represents the object and nextInt and nextDouble are the names of the messages or methods. The variable reader is also referred to specifically as an object variable much of the time.

Semicolons End Most Java Statements

• A Semicolon (;) marks the end of most Java statements. A statement is a “sentence” in a program. Examples:– System.out.println(“Hello World!”);– num1 = reader.nextInt();– sum = num1 + num2;

Some Java lines do not end in a semicolon. You will learn which ones don’t as you are introduced to other coding structures.

print and println Statements

This section discusses the use of print and println (pronounced print-line) statements.

print and println are two operations (or as we say in Java) … methods that can be called. These operations output text information to the console window in Eclipse.

We call methods when we want to accomplish some operation or task.

print and println StatementsHere are lines of code that call the two methods

print and println:

1. System.out.print(“Hello World”);

2. System.out.println(“Hello World”);

Think of methods as operations, but we always refer to them as methods in Java. Some other languages refer to them as functions or procedures.

When we say we are “calling a method”, what we mean is that we are executing an operation!

print and println StatementsYou may be wondering what the difference between the two

methods print and println are. print will display everything on one line but not start a new line of output for anything else that is printed. println will display everything on one line and then start a new line of output for any other print or println statement. Here is an example:

System.out.print(“Hello World! ”);

System.out.print(“How are you doing? ”);

System.out.print(“I am doing fine.”);

(Note the extra spaces at the end of the first two print statements

or the sentences would jam up together in output)

The output in the console window is all on one line:

Hello World! How are you doing? I am doing fine.

print and println StatementsAgain … print will display everything on one line but not start

a new line of output. println will display everything on one line and then start a new line of output.

Consider these three lines of code:

System.out. print(“Hello World! ”);

System.out. println(“How are you doing?”);

System.out. println(“I am doing fine.”);

The output is on two lines in the console window:

Hello World! How are you doing?

I am doing fine.

print Statements and Syntax ErrorsSo when we use the line of code:

System.out.print(“Hello World”);

We are calling the print method. Printing to a console window requires that we place System.out. prior to the word print.

Please notice the periods “.” (method selector operators) that separate the words System, out, and print.

Also, notice that the first S of System is capitalized (upper case). The reason is System is the name of a class.

Also, notice that after the word print there are parentheses that include in double quotes the information that we want to display to the screen and the entire line of code ends in a semicolon.

All of this is required and must be typed correctly or you will get a syntax error. Programs with syntax errors won’t run!

Compile-Time ErrorsSyntax errors are also called compile-time errors. These errors

keep a program from compiling and running until the errors are corrected. Examples are:

• misspelling key Java words like print, println, System, public, and class or leaving out curly braces where they are needed. (This kind of error is a syntax error.)

• forgetting to place a semicolon after Java lines that need it• not storing the class in a file with the same name. (These last

two kinds of errors are not syntax errors, but just compile-time errors.)

A compiler like Eclipse displays errors in the console window and …• indicates the type of error detected • indicates the file and line number where the error was detected

How Eclipse Points Out Errors• Eclipse will …

1. describe the error in the source code window

2. tell the line of code where the error is

3. suggest a solution to the error when you mouse over the bad code.

• Eclipse displays a red circle with an X in it to the left of any line that

has a syntax error.

• Eclipse will display a red box with an X in it for any line that has a

compile-time error.

• You can …– mouse over the red circle or red box and a message will pop-up

that describes the error. – click on the red circle or red box and then choose an option of

how to correct it. This saves time if you choose the correct fix, then the error will be corrected automatically for you.

The Readability of CodeIt is important for your code to be readable by others. In the real world,

programmers are on software teams as they develop and maintain software. So all code must be readable! Programmers have developed a standard for how code should be indented.

• The main factors that determine whether code is readable or not are– Spacing (referring to extra blank lines that space things out)– Indentation (referring to tabs or indentions on a specific line)

• Spacing and Indentation are just for programmer readability. The compiler ignores any kind of spacing and indentation. It just checks to make sure that everything is syntactically correct (spelled correctly)!

• Eclipse assists you with indenting segments of code as you type by automatically properly indenting your next line depending on the kind of Java code you are writing. But if you mess up the indenting then all you have to do is select all code by typing Control “a” on a PC or Apple “a” on a Mac, then type either Control “i” on a PC or Apple “i” on a Mac.

Chapter 2 Section 6

In this section, you will learn some very important information about:

1. Simple data types2. Numeric variables3. String variables4. The assignment operator =5. How to declare and construct a Scanner

object so you can read input from the keyboard

6. The new operator

Simple Java Data TypesIn Java, we have different kinds of data types. Here you will learn

three of them. We can store each of the three types of data values in different kinds of variables.

We can have …1. variables that can hold integers (type int)2. variables that can hold real numbers or what we refer to also as

floating-point numbers (type double)3. variables that refer to an object that can hold a string of

characters, in other words, a bunch of characters that make up a word (type String)

The data types int and double are simple numeric data types and we store those kind of values in simple variables that are not considered object variables.

However, Strings are objects and a String value must be stored in an object variable. Let’s look at how we do that with the assignment operator.

The Assignment OperatorThe assignment operator is the = character.

We use the assignment operator when we want to store a value in a variable.

Here is how to declare the different kinds of variables and store values in them:

int x = 129; // x is the int variable and 129 is being stored.

double y = 3.14159; // y is the double variable and π is stored.

String name = “Java”; /* name is the String variable and the word “Java” is being stored. (Notice the double quotes around Java since we are storing a String) Don’t place double quotes when you store numbers. */

Declaring and Using Scanner ObjectsWe also use the assignment operator when creating objects (constructing objects) so an object variable can refer to them.

If we want to receive input from the keyboard during a program, then we need to construct a Scanner object first. This allows the user to enter numbers or string values into a program.

To do this we need to import Java’s Scanner class with the line:

import java.util.Scanner;

Next, inside the main method, we need to construct the Scanner object with the line:

Scanner reader = new Scanner(System.in);

We need to use System.in as the parameter because this indicates the keyboard, which is the default input device for Java. Note: Scanner is a class so the S is capitalized.

Explanation of Constructing a Scanner ObjectIn the line of code :

Scanner reader = new Scanner(System.in);

the name of the class Scanner is used twice.

The first part of the line:

Scanner reader

declares reader to be an object variable of type Scanner. We need to do this or reader can’t refer to the Scanner object we construct.

The second part of the line:

new Scanner(System.in);

constructs the Scanner object and “attaches it” to the keyboard. The word new is the new operator that we use to construct things. The assignment operator makes reader refer to the Scanner object.

General Form for Constructing Objects

• In programming, the process of constructing an object is called instantiation.

• In general, constructing or instantiating and object takes the general form:

SomeClass someVariable = new SomeClass(some parameters);

You can see that the line of code below follows this form:

Scanner reader = new Scanner (System.in);

You can think of reader as something that is “scanning the keyboard waiting for input”.

Reading Input from the KeyboardAssume the following lines of code appear in the main method of a program.

Scanner reader = new Scanner (System.in);

System.out.print(“Enter your name and press return: ”);

String name = reader.nextLine();

System.out.print(“Enter your age and press return: ”);

int age = reader.nextInt();

System.out.print(“Enter your gpa and press return: ”);

double gpa = reader.nextDouble();

Notice we can receive different kinds of input using reader. We use

the object variable reader to “call the method nextLine() to receive a

String value from the keyboard. We use reader to “call the method

nextInt() to receive an integer and we use reader to “call the method

nextDouble() to receive a floating-point value from the keyboard.

Note: the print statements “prompt” the user to enter data, otherwise he or she wouldn’t know the computer is waiting for input!

Reading Input from the KeyboardWe can now “echo the input” (print the information back to the

screen) by using some println statements. In each line, we will print a literal string (something in double quotes) and the value contained a the variable. We use a plus sign to concatenate the literal string value and the value stored in the variable together to make a larger string that is then printed.

System.out.println(“Your name is: ” + name);

System.out.println(“Your age is: ” + age);

System.out.println(“Your gpa is: ” + gpa);

Notice that there are no double quotes around the variables name,

age, and gpa.

Now you know enough that you can understand the Convert.java program!

The Convert.java Program

Note the key Scanner lines identified by the red arrows.

More about Import Statements

In the Convert.java code you saw that the first line of code was

an import statement: import java.util.Scanner;

This tells the compiler where to find a class that will be used during the program. The class may be either in a Java library file or another file you have in a folder. The import statement contains the path name of where to find the class.

The Scanner class is found in a sub-package folder named util that is in the java package folder. so we import it using:

(Sometimes the first line of code is a package statement, depending on how you have Eclipse organized. More on that later.)

More about Numeric VariablesA numeric variable names a location in RAM memory where a

number can be stored.

When we use the line of code:double celsius;then enough RAM memory is allocated so that a floating-point value

can be stored. Java does this automatically for you!

Important points to remember about variables:

• A variable’s value may change during a program, but its name

remains constant.

• A variable’s type defines what kind of values can be stored in

RAM. The type of the variable cannot be changed while a program

is running. Remember when we say type we are referring to int,

double, String, or other data types.

Java’s Mathematical Operators

Java basically has 5 mathematical operators.

Addition is represented by the + sign.

Subtraction is represented by the - sign.

Multiplication is represented by the * sign.

Division is represented by the / sign.

Mod is represented by the % sign. Mod gives the remainder of

int division.

In the Convert.java program, the line of code:

celsius = ( fahrenheit - 32.0 ) * 5.0 / 9.0;

use three of the above mathematical operators.

Review of Simple and Object Variables

In the Convert.java program, simple variables like fahrenheit and celsius each hold a single floating-point number.

Object variables like reader and System.out hold references to objects.

Object variables are used to send messages to objects.

reader.nextDouble() sends the message “get me the next floating point number” from the keyboard.

Visualizing Simple and Object Variables

simple variables

object variables

object variables can send messages to objects

Chapter 2 Section 7: Graphics and GUIs

• Graphics and GUI programs in Java can run either

as a stand-alone application or as an applet.

• We will study how to use applets in upcoming

chapters.

• Standalone GUI applications run in windows.

• A window is a container for graphical components

to be displayed to the user.

JFrame Windows

Windows have numerous properties.• Width and height• Title bar• Ability to be dragged or resized by the user

The code for application windows is located in the class JFrame, which is imported from the package javax.swing. To use this class you need to import it with the statement:

import javax.swing.JFrame; or

import javax.swing.*;

The last import statement imports all the classes in the swing package not just JFrame.

GUIWindow1.java Code

This code is from Example 2-3 in the text and all it does is produce an empty window frame. I will

send it to you as GUIWindow1.java

GUIWindow1.java Output

Figure 2-12 from the text: GUI program with an empty window

An application window is really just an empty container that we can fill with other objects. One object we can place in the window is a panel.

Listing of JFrame Methods

This is Table 2-1 from the text that lists some commonly used JFrame methods. Note: The method setDefaultCloseIndicator(int i) above should be setDefaultCloseOperation().

Panels and ColorsA panel is a flat, rectangular area suitable for displaying other objects.

It may contain geometric shapes and images.

To use a panel you must import the JPanel class with:import javax.swing.JPanel;

You can use Java’s default colors or create your own RGB colors by

importing the Color class with:import java.awt.Color;

To change the current drawing and painting color to red, use the paint

brush g to call the setColor() method as follows:

g.setColor(Color.red);

Note the dot “.” between g and setColor and in parenthesis since red

is a color constant of the Color class we must use Color with a

capital C dot red.

Java’s Default Color Constants

If you were going to construct these colors by yourself, you would use the 3 integer values in the parenthesis.

Customized ColorsYou can construct a customized color if the color you need is not

one of the Color constants seen on the previous slide.

You can construct a new Color object by using three int values between 0 and 255 with:

Color aColor = new Color(redValue, greenValue, blueValue);

In this code, redValue, greenValue, blueValue must be integer values.

You would then use the code: g.setColor(aColor); (don’t use Color.aColor)

Here is an actual example:

Color brown = new Color(164, 84, 30);g.setColor(brown ); (don’t use

Color.brown)

GUIWindow2.java Code

This code is from Example 2-4 in the text and it produces an empty, pink panel. I will send it to you as GUIWindow2.java

Layout Managers and Multiple PanelsEvery container object (frame or panel) uses a layout manager

object to organize and lay out the graphical components contained in it.

The default layout manager for frames is an object of the class BorderLayout.

You can arrange up to five graphical objects in a container in the following positions:

NORTH, SOUTH, EAST, WEST, and CENTER

If we add fewer than 5 objects to a BorderLayout, the layout manager stretches some of them to fill the unoccupied areas.

An object of the GridLayout class divides a container into rows and columns.

BorderLayout with GUIWindow3.java

If you look at the code in GUIWindow3.java you can see how to set up a BorderLayout GUI Window. The code is too lengthy to show here.

GUIWindow4.java Code

This is Example 2-6

in the text that produces a

Frame with a grid containing 2 rows and 2 columns of

colored panels. See next slide.

GUIWindow4.java Output

Figure 2-14 from the text showing the 2-by-2 grid layout with four panels of different colors.