Getting Started with Java

1
Chapter 2

• Getting Started with Java

2
Topics

• Components of a Java Program
• classes
• methods
• comments
• import statements
• Declaring and creating objects
• Java identifiers
• Standard Java classes

3
The First Java Program

• This first program displays a window on the
  screen.
• The size of the window is 300 x 200 pixels
• The title is My First Java Program.
• The fundamental OOP concept illustrated by the
  program
• An object-oriented program uses objects.

4
The First Java Program

• / Chapter 2 Sample Program Displaying a Window
• File Ch2Sample1.java
• /
• import javax.swing.
• class Ch2Sample1
• public static void main(String args)
• JFrame myWindow
• myWindow new JFrame()
• myWindow.setSize(300, 200)
• myWindow.setTitle(My First Java Program)
• myWindow.setVisible(true)

5
UML diagram for Ch2Sample1

• This diagram shows the messages sent to myWindow

6
Class diagram for Ch2Sample1

• This diagram shows the dependency relationship
  between the classes in the program.
• The Ch2Sample1 class uses a JFrame object.
• To use an object in a program, first we declare
  and create and object. Then we send messages to
  it.

7
Declaring Objects

• When we declare an object, we must give it a name
  and say what class it belongs to.
• syntax
• the name must be a valid Java identifier
• the class must be defined and available
• if multiple objects are declared, the names are
  separated by commas

8
Java Identifiers

• A Java identifier is a sequence of letters,
  digits, underscores, and dollar signs used to
  name a class, object, method, etc.
• The first character in a Java identifier must be
  a letter.
• Uppercase and lowercase letters are
  distinguished.
• myWindow, mywindow, MYWINDOW
• No spaces are allowed in an identifier.

9
Java Naming Conventions

• Use of an uppercase letter for the first letter
  of class names
• class Account
• Use a lowercase letter for the first letter of
  object names.
• object account
• Use all upper case letters for constants
• final int WINDOW_HEIGHT

10
Instantiating Objects

• No object is created by the declaration.
• We create an object by invoking the new
  operation
• Example

11
object declaration and object creation

• declaration
• assigns a name to a memory address that will
  store the location of an object

• instantiation
• sets aside memory for the objects data
• stores the address of the objects data in the
  named memory location

12
Diagram to show memory state

• program diagram (or object diagram)
• state-of-memory diagram
• uses similar notation to UML but includes extra
  symbols

13
Another instantiation example

• The state after two new commands are executed.
• allocating a new object means the address of the
  original is forgotten
• an object with no references to it is garbage

14
Methods and Messages

• Syntax for sending messages
• The expressions sending a message and calling
  a method are synonymous.
• The object that receives a message must possess a
  corresponding method.
• Example

15
Program Components

• Comments
• Import Statement
• Class Declaration
• Method Declaration

16
Comments

• Comments are used to state the purpose of the
  program, explain the meaning of code, and provide
  other descriptions to help programmers use and
  understand the code.
• A single-line comment is preceeded by //
• A comment is any sequence of text that begins
  with the marker / and ends with the marker /.
• A javadoc comment begins with / and ends with /

17
Using Classes from Java Library

• In Java, classes are grouped into packages, and
  the Java system comes with numerous packages.
• To use a class from a package, we refer to the
  class in the program by using the following
  syntax
• ltpackage namegt.ltclass namegt
• This notation is called dot notation.

18
import statements

• Using the fully qualified name of a class can be
  cumbersome.
• Using the import statement solves this problem.
• import javax.swing.
• Now we can just use the name of the class

19
Class definition

• A Java program is composed of one or more
  classes.
• The syntax for declaring a class is
• A class member is either a data value or a
  method.
• Most classes will contain some of each

20
The main class

• One of the classes in the program must be
  designated as the main class.
• If we designate a class as a main class, we must
  define a method called main, because when a Java
  program is executed, the main method of a main
  class is executed first.
• The main class is sometimes called a driver class

21
Method Definition

• Syntax
• Example

22
Template for Java Program
23
Edit-Compile-Run Cycle
24
Editing

• Step One Edit the program.
• Type in the program, using a text editor, and
  save the program to a file.
• Use the name of the main class and the suffix
  .java for the filename.
• This file is called a source file.

25
Compiling

• Step 2 Compile the source file.
• The process of compiling the source file creates
  the bytecode file.
• the bytecode file is binary
• The name of the compiler-generated bytecode file
  will have the suffix .class while its prefix is
  the same as the source files.
• The class file will not be created if there are
  syntax errors in your program
• You'll get a list of error messages

26
Running the Program

• Step 3 Execute the bytecode file.
• A java interpreter will go through the bytecode
  file and execute the instructions in it.
• If your program is error-free, a window will
  appear on the screen.
• If an error occurs while running the program, the
  interpreter will catch it and stop its execution.