Classes. Classes (continued) Syntax The general syntax for defining a class:

Transcription

1 Chapter 8: User-Defined Classes and ADTs Java Programming: Program Design Including Data Structures Chapter Objectives Learn about classes Learn about private, protected, public, and static members of a class Explore how classes are implemented Learn about the various operations on classes Java Programming: Program Design Including Data Structures 2 Chapter Objectives (continued) Examine constructors and finalizers Examine the method tostring Learn about the abstract data type (ADT) Java Programming: Program Design Including Data Structures 3 1

2 Classes class: collection of a fixed number of components Components: Members of a class Members are accessed by name Class categories/modifiers: private protected public Java Programming: Program Design Including Data Structures 4 Classes (continued) Private: members of class are not accessible outside class Public: members of class are accessible outside class Class members: can be methods or variables Variable members are declared like any other variables Java Programming: Program Design Including Data Structures 5 Syntax The general syntax for defining a class: If a member of a class is a named constant, you declare it just like any other named constant If a member of a class is a variable, you declare it just like any other variable Java Programming: Program Design Including Data Structures 6 2

3 Syntax (continued) If a member of a class is a method, you define it just like any other method If a member of a class is a method, it can (directly) access any member of the class data members and methods When you write the definition of a method, you can directly access any data member of the class (without passing it as a parameter) Java Programming: Program Design Including Data Structures 7 Syntax (continued) class Clock: Data Members (Instance Variables): private int hr; //store hours private int min; //store minutes private int sec; //store seconds Methods: public void settime(int hours, int minutes, int seconds) public int gethours() public int getminutes() public int getseconds() public void printtime() public void incrementseconds() public void incrementminutes() public void incrementhours() public boolean equals(clock otherclock) public void makecopy(clock otherclock) public Clock getcopy() Java Programming: Program Design Including Data Structures 8 Constructors Two types of constructors: With parameters Without parameters (default constructor) Java Programming: Program Design Including Data Structures 9 3

4 Constructors (continued) Constructors have the following properties: The name of a constructor is the same as the name of the class A constructor has no type A class can have more than one constructor All constructors of a class have the same name If a class has more than one constructor, any two constructors must have different signatures Java Programming: Program Design Including Data Structures 10 Constructors (continued) Constructors are automatically executed when a class object is instantiated With multiple constructors, which constructor executes depends on the type of values passed to the class object when the class object is instantiated Java Programming: Program Design Including Data Structures 11 Constructors (continued) class Clock: Constructors Default constructor is public Clock() Constructor with parameters: public Clock(int hours, int minutes,int seconds) Java Programming: Program Design Including Data Structures 12 4

5 Unified Modeling Language Class Diagrams Java Programming: Program Design Including Data Structures 13 Variable Declaration and Object Instantiation General syntax for using the operator new is: new classname() OR new classname(argument1, argument2,..., argumentn) Clock myclock; Clock yourclock; myclock = new Clock(); yourclock = new Clock(9, 35, 15); Java Programming: Program Design Including Data Structures 14 Variable Declaration and Object Instantiation (continued) Java Programming: Program Design Including Data Structures 15 5

6 Accessing Class Members To access a data member of a class object or method: referencevariablename.membername Example 8-1 myclock.settime(5, 2, 30); myclock.printtime(); yourclock.settime(x, y, z); if (myclock.equals(yourclock)) Java Programming: Program Design Including Data Structures 16 Assignment Operator: A Precaution Consider: myclock = yourclock; Copies the value of the reference variable yourclock into the reference variable myclock After execution, both variables refer to same object Java Programming: Program Design Including Data Structures 17 Assignment Operator: A Precaution (continued) Shallow copying: two or more reference variables of the same type point to the same object Deep copying: each reference variable refers to its own object Java Programming: Program Design Including Data Structures 18 6

7 The Copy Constructor Executes when an object is instantiated Initialized using an existing object Syntax: public ClassName(ClassName otherobject) Java Programming: Program Design Including Data Structures 19 Example: class Clock Java Programming: Program Design Including Data Structures 20 The Method tostring Public value-returning method Takes no parameters Returns address of a String object Output using print, println, printf methods Default definition creates String with name of object s class name followed by hash code of object Java Programming: Program Design Including Data Structures 21 7

8 The Modifier static In the method heading, specifies that the method can be invoked by using the name of the class If used to declare data member, data member invoked by using the class name Static data members of class exist even when no object of class type instantiated Static variables are initialized to their default values Java Programming: Program Design Including Data Structures 22 Example 8-3 The Modifier static (continued) public class Illustrate { private int x; private static int y; public static int count; public Illustrate() { x = 0; } public Illustrate(int a) { x = a; } Java Programming: Program Design Including Data Structures 23 The Modifier static (continued) void setx(int a) { x = a; } } public String tostring() { return("x = " + x + ", y = " + y + ", count = " + count); } public static void incrementy() { y++; } Illustrate illusobject = new Illustrate(); Illustrate.incrementY(); Illustrate.count++; Java Programming: Program Design Including Data Structures 24 8

9 The Modifier static (continued) Illustrate illusobject1 = new Illustrate(3); Illustrate illusobject2 = new Illustrate(5); Java Programming: Program Design Including Data Structures 25 Illustrate.incrementY(); Illustrate.count++; The Modifier static (continued) Java Programming: Program Design Including Data Structures 26 Finalizers Automatically execute when class object goes out of scope Have no parameters Only one finalizer per class Name of finalizer: finalize Java Programming: Program Design Including Data Structures 27 9

10 Accessor and Mutator Methods Accessor method: A method of a class that only accesses (that is, does not modify) the value(s) of the data member(s) Mutator method: modifies the value(s) of the data member(s) Java Programming: Program Design Including Data Structures 28 Creating Packages You can create packages using a reserved word package Define the class to be public. (If class is not public, it can only be used within package.) Choose name for package Organize package (create subdirectories) Java Programming: Program Design Including Data Structures 29 Creating Package for class Clock package jpfpatpd.ch08.clockpackage; public class Clock { //put instance variables and methods, //as before, here } import jpfpatpd.ch08.clockpackage.clock; Java Programming: Program Design Including Data Structures 30 10

11 The Reference this Refers to instance variables and methods of a class Used to implement cascaded method calls Java Programming: Program Design Including Data Structures 31 Inner Classes Defined within other classes Can be either a complete class definition or an anonymous inner class definition Used to handle events Java Programming: Program Design Including Data Structures 32 Abstract Data Type A data type that specifies the logical properties without the implementation details Java Programming: Program Design Including Data Structures 33 11

12 Programming Example: Candy Machine (Problem Statement) A new candy machine is bought for the gym, but it is not working properly. The machine sells candies, chips, gum, and cookies. In this programming example, we will write a program to create a Java application program for this candy machine so that it can be put into operation. We will divide this program in two parts. In the first part, we will design a non-gui application program. In the second part, we will design an application program that will create a GUI, as described in the second part. Java Programming: Program Design Including Data Structures 34 Programming Example: Candy Machine (Problem Statement continued) The non-gui application program should do the following: 1. Show the customer the different products sold 2. Let the customer make the selection 3. Show the customer the cost of the item selected 4. Accept money from the customer 5. Release the item Java Programming: Program Design Including Data Structures 35 Programming Example: Candy Machine (Input and Output) Input: The item selection and the cost of the item Output: The selected item Java Programming: Program Design Including Data Structures 36 12

13 Programming Example: Candy Machine Components: Cash register Dispenser Machine Java Programming: Program Design Including Data Structures 37 Programming Example: Candy Machine (continued) Java Programming: Program Design Including Data Structures 38 Programming Example: Candy Machine (continued) Java Programming: Program Design Including Data Structures 39 13

14 Programming Example: Candy Machine (continued) Java Programming: Program Design Including Data Structures 40 Programming Example: Candy Machine (continued) Java Programming: Program Design Including Data Structures 41 Programming Example: Candy Machine (continued) Java Programming: Program Design Including Data Structures 42 14

15 Programming Example: Candy Machine (continued) Java Programming: Program Design Including Data Structures 43 Chapter Summary Creating classes Members of a class: private protected public static Implementing classes Various operations on classes Java Programming: Program Design Including Data Structures 44 Chapter Summary (continued) Constructors Finalizers Method tostring Abstract data types Java Programming: Program Design Including Data Structures 45 15