Lesson 2: First Java Programs

Lesson 2:

First Java Programs

Lesson 2: First Java Programs

Objectives:

– Discuss why Java is an important programming language.

– Explain the Java virtual machine and byte code.

– Choose a user interface style.– Describe the structure of a simple Java

program.


Objectives:

– Write a simple program.– Edit, compile, and run a program using a

Java development environment.– Format a program to give a pleasing,

consistent appearance.– Understand compile-time errors.– Write a graphics program.


- Applet- Assignment operator- Byte code - DOS development environment- Graphical user interface (GUI)- Hacking- Import statement- Integrated development

environment (IDE)- Interpreter

Vocabulary:

- Java virtual machine(JVM)

- Just-in-time compilation (JIT)

- Panel

- Panes

- Parameter

- Source code

- Statement

- Terminal I/O interface

- Variable

2.1 Why Java?• Java is the fastest growing programming

language in the world.

• Java is a modern object-oriented programming language.

• Java has benefited by learning from the less desirable features of early object-oriented programming languages.

2.1 Why Java?

Java is ideal for distributed, network-based applications.

✓Secure: Virus-free, tamper-free systems.

✓Robust: Supports development of programs that do not overwrite memory.

✓Portable: Yields programs that can be run on different computer types.

2.1 Why Java?• Java is ideally suited to develop

distributed, network-based applications because it:

– Enables the construction of virus-free, tamper-free systems (security)

– Supports the development of programs that do not overwrite memory (robust)

– Yields programs that can be run on different types of computers without change (portable)

2.1 Why Java?• Java supports advanced programming

concepts such as threads.– A thread is a process that can run

concurrently with other processes.

• Java resembles C++, the world’s most popular industrial strength programming language.

• Java however, runs more slowly than most modern programming languages because it is interpreted.

2.2 The Java Virtual Machine and Byte Code

• Java compilers translate Java into pseudomachine language called java byte code.

• To run java byte code on a particular computer, a Java virtual machine (JVM) must be installed.


• A Java virtual machine is a program that runs like a computer. It is called an interpreter.

• Disadvantage:– Runs more slowly than an actual computer

• To combat slower processing, some JVMs translate code when first encountered. This is known as just-in-time compilation (JIT).


• Advantages:

– Portability. Any computer can run an interpreter.This makes byte code portable.

– Applets. Applets are small Java programs already translated into byte code.• Applets run in a JVM incorporated in a web browser• Applets can be decorative (like animated characters on a web page.)• Applets can be practical (like continuous streams of stock market

quotes.)

– Security. It is possible to limit the capabilities of a Java program since it runs inside a virtual machine.


• Advantages:

- JVMs are getting faster.• Using JIT (just-in-time) compilations, which

translate byte code into machine language.

2.3 Choosing a User Interface Style

• There are two types of user interfaces available to use to create Java programs.

– Graphical User Interface (GUI)– Terminal I/O interface

• Figure 2-1 illustrates both interfaces used to create the same program.


Graphical user interface

(GUI)


Terminal I/O user interface


• There are 3 reasons for beginning with terminal I/O:

– It is easier to implement than a GUI– There are programming situations that require

terminal I/O– Terminal-oriented programs are similar in

structure to programs that process files of sequentially organized data. (What is learned here is easily transferred to that setting.)

2.4 HelloWorld• Figure 2-2 displays the results of a small

Java program, entitled “HelloWorld”

2.4 HelloWorld• A program is a sequence of instructions for a computer.

• The following is the bulk of instructions, or source code, for the HelloWorld” program.

2.4 HelloWorldThe Explanation:

➡ System.out is an object that displays characters in a terminal window.

➡ println is the message being sent to the object.➡ The quotations indicate what is to be displayed.➡ Semicolons mark the end of each statement.➡ The characters between the parentheses are the parameters.➡ The period (.) is the method selector operator.

Sending messages to objects always takes the following form:

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

2.4 HelloWorldThe Larger Framework:The program must be embedded in several lines of code, such as:

Program comments are in green, reserved words in blue, and code in black.

2.5 Edit, Compile, and Execute

• Figure 2-3 illustrates the edit, compile and execute steps.


✓ Edit– The programmer uses a word processor or editor to enter

the source code.– Save it as a text file with the extension .java.

✓ Compile– The programmer invokes the Java language compiler.– Translates the source code into Java byte code.

✓ Execute– The programmer instructs the JVM to load the byte code

into memory and execute.– The user and program can now interact.


• Development environments:– Unix

• standard text editor• command line activation of compiler and JVM

– DOS, using Microsoft Windows and NT OS• notepad text editor• command line activation of compiler and JVM from a DOS

window

– Integrated development environment, using Windows, NT, or MAC OS

• Examples: Symantec’s Visual Café, Microsoft’s Visual J++, NetBeans, or Borland’s J Builder


Unix or Linux Standard text editor Free

Microsoft WindowsNotepad and DOS

windowFree

Integrated development

environment (IDE)

BlueJ, Eclipse, or JGrasp

Not free, but combines editor,

compiler, debugger, and JVM


• Preparing your development environment:

– Create a directory, open a terminal window, use the cd command to move to your new directory

– Open notepad, create the file HelloWorld.java, type in the lines of code

– Save the file, go back to the terminal window, compile the program

– Run the program

2.5 Edit, Compile, and ExecuteThe program as typed into Notepad


• The following figures illustrate the steps necessary for preparing your development environment.


Compile-Time Errors:✓ Mistakes detected by the compiler are called syntax errors or

compile-time errors.✓ Typos made when editing.✓ Compiler prints a list of errors in the terminal window.


✓ Readability:✓ Programs may be maintained by other people.✓ Layout affects readability.

➡ Use indentation, blank lines, and spaces.


2.6 Temperature Conversion★ View the program’s source code:View the program’s source code:

import java.util.Scanner;import java.util.Scanner;

public class Convert public class Convert

{{

public static void main (String [ ] args) public static void main (String [ ] args)

{{

Scanner reader = new Scanner(System.in);Scanner reader = new Scanner(System.in); // This allows the user // This allows the user

// to enter data from // to enter data from

//// the keyboard the keyboard

double fahrenheit; double fahrenheit; // The temperature in // The temperature in // Fahrenheit// Fahrenheit

double celsius; double celsius; // The temperature in // The temperature in //// Celsius Celsius

System.out.print(“Enter degrees Fahrenheit: ”);System.out.print(“Enter degrees Fahrenheit: ”);

fahrenheit = reader.readDouble();fahrenheit = reader.readDouble();

celsius = (Fahrenheit – 32.0) * 5.0 / 9.0;celsius = (Fahrenheit – 32.0) * 5.0 / 9.0;

System.out.print(“The equivalent in Celsius is ”);System.out.print(“The equivalent in Celsius is ”);

System.out.println(celsius);System.out.println(celsius);

}}

}}

2.6 Temperature Conversion

➡ The following is an explanation of the program code:✓ Import statement✓ Instantiate or create an object✓ Declare the variables✓ Position the cursor after “Enter degrees Fahrenheit”✓ Assignment operators✓ Assignment statements are evaluated✓ Print text (and position the cursor)✓ Print the value of the variable✓ Statement to prevent the terminal window from

disappearing from the display (optional, only needed with certain development environments)

2.6 Temperature Conversion

➡ Temperature conversion program reads user input and performs computations.

➡ The first line of code is an import statement.

➡ Variables for Fahrenheit and Celsius.➡ Assignment statements use an operator

such as *, /, +, and -.

2.6 Temperature Conversion➡Variables and objects used in the conversion

program.

2.7 Graphics and GUIs: Windows and Panels

➡ A Simple Application Window:➡ Graphics and GUI programs in Java can be stand-alone

applications or applets.➡ Consistent features:

✓ Title bar with controls (maximize, zoom, etc.)✓ Width and height can be resized

➡ Code for application windows is in the class Jframe.✓ JFrame responds to messages to set the title bar and

window size.


Some commonly used JFrame methods


Panels and Colors:➡A Jframe has a container or pane to fill with objects.➡A panel is a rectangle used to display objects such a shapes and images.➡Panes are panels that contain related objects such as images and widgets.➡Colors in most computer system use RGB.

✓ Red, green, blue✓ Values 0-255


Layout Managers and Multiple Panels:➡Each container object uses a layout manager to control panel placement.➡BorderLayout class allows arrangement of up to five objects.

✓ North, south, east, west, center➡GridLayout uses rows and columns to arrange objects.

In this chapter, you learned:➡Java is the fastest growing programming language in the world. It is secure, robust, and portable. It is also similar to C++, the world’s most popular programming language.

Summary

➡The Java compiler translates Java into a pseudomachine language called Java byte code. Byte code can be run on any computer that has a Java virtual machine installed. The Java virtual machine (JVM) is a program that behaves like a computer—an interpreter.➡Java programs include variables, arithmetic

expressions, statements, objects, messages, and methods.

Summary

➡ Three basic steps in the coding process are editing, compiling, and running a program using a Java development environment. Programmers should pay attention to a program’s format to ensure readability.

Summary

➡ Java programs accomplish many tasks by sending messages to objects. Examples are sending text to the terminal window for output and receiving input data from the keyboard.

➡ There are several user interface styles, among them terminal based and graphical based.

Summary

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Lesson 2: First Java Programs