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CHAPTER 2 – Part A

CHAPTER 2 – Part A. AN INTRODUCTION TO OBJECTS AND CLASSES. Where to get the software:. Download Java 2 SKD 1.4.2 (J2SE) [offline installation] at http://java.sun.com/j2se/1.4.2/download.html To download BlueJ version 1.3.5 at http://www.bluej.org. Objects and Classes.

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CHAPTER 2 – Part A

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  1. CHAPTER 2 – Part A AN INTRODUCTION TO OBJECTS AND CLASSES

  2. Where to get the software: • Download Java 2 SKD 1.4.2 (J2SE) [offline installation] athttp://java.sun.com/j2se/1.4.2/download.html • To download BlueJ version 1.3.5 athttp://www.bluej.org

  3. Objects and Classes • Object: entity that you can manipulate in your programs (by invoking methods) • Each object belongs to a class • Class: Set of objects with the same behavior • Class determines legal methods"Hello".println() // Error"Hello".length() // OK

  4. Rectangle Class • Construct a rectangle:new Rectangle(5, 10, 20, 30)new Rectangle() • Use the constructed objectSystem.out.println(new Rectangle(5, 10, 20, 30));prints java.awt.Rectangle[x=5,y=10,width=20,height=30]

  5. Rectangle Shapes

  6. Syntax 2.1: Object Constructionnew ClassName(parameters) Examples: • new Rectangle(5, 10, 20, 30) Purpose: To construct a new object, initialize it with the construction parameters, and return a reference to the constructed object.

  7. A Rectangle Object

  8. Object Variables • Declare and optionally initialize:Rectangle cerealBox = new Rectangle(5, 10, 20, 30);Rectangle cerealBox; • Apply methods:cerealBox.translate(15, 25); • Share objects:r = cerealBox;

  9. Uninitialized and Initialized Variables Uninitialized Initialized

  10. Two Object Variables Referring to the Same Object

  11. Syntax 2.2: Variable DefinitionTypeName variableName;TypeName variableName = expression; • Examples: Rectangle cerealBox; String name ="Dave"; int n = 5; • Purpose: To define a new variable of a particular type and optionally supply an initial value

  12. Writing a Test Program for Imported Library Classes • Invent a new class, say MoveTest • Supply a main method • Place instructions inside the main method • Import library classes by specifying the package and class name:import java.awt.Rectangle; • You don't need to import classes in the java.lang package such as String and System

  13. Syntax 2.3 : Importing a Class from a PackageimportpackageName.ClassName ; • Example: • import java.awt.Rectangle; • Purpose: • To import a class from a package for use in a program

  14. File MoveRect.java 1 import java.awt.Rectangle; 2 3 public class MoveTest 4 { 5 public static void main(String[] args) 6 { 7 Rectangle cerealBox = new Rectangle(5, 10, 20, 30); 8 // print the original rectangle 9 System.out.println(cerealBox); 8 // move the rectangle 10 cerealBox.translate(15, 25); 11 // print the moved rectangle 12 System.out.println(cerealBox); 13 }

  15. A Simple Class public class Greeter { public String sayHello() { String message ="Hello,World!"; return message; } }

  16. Method Definition • access specifier (such as public) • return type (such as String or void) • method name (such as sayHello) • list of parameters (empty for sayHello) • method body in{ }

  17. Method Parameters public class Rectangle{ . . . public void translate(int x, int y) {method body} . . .}

  18. Syntax 2.4: Method Implementation • public class ClassName{ ... accessSpecifier returnType methodName(parameterType parameterName,...) { method body } ... } …Continue

  19. …Continue Example: • public class Greeter { public String sayHello() { String message ="Hello,World!"; return message; } } Purpose: • To define the behavior of a method A method definition specifies the method name, parameters, and the statements for carrying out the method's actions

  20. Syntax 2.5: The return Statementreturn expression; OR return; • Example: return message; • Purpose: To specify the value that a method returns, and exit the method immediately. The return value becomes the value of the method call expression.

  21. Testing a Class • Test class: a class with a main method that contains statements to test another class. • Typically carries out the following steps: • Construct one or more objects of the class that is being tested. • Invoke one or more methods. • Print out one or more results

  22. A Test Class for the Greeter Class public class GreeterTest { public static void main(String [] args)) { Greeter worldGreeter = new Greeter(); System.out.println(worldGreeter.sayHello()); } }

  23. Building a Test Program 1. Make a new subfolder for your program. 2. Make two files, one for each class. 3. Compile both files. 4. Run the test program.

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