Lecture 8 Objectives s Learn about classes s

Lecture 8 Objectives s Learn about classes. s Learn about private, protected, public, and static members of a class. s Explore how classes are implemented. s Learn about the various operations on classes. Java Programming: From Problem Analysis to Program Design, Second Edition

Lecture 8 Objectives s Examine constructors and finalizers. s Examine the method to. String. s Learn about the abstract data type (ADT). Java Programming: From Problem Analysis to Program Design, Second Edition

Classes s class: A reserved word; a collection of a fixed number of components. s Components: Members of a class. s Members are accessed by name. s Class categories/modifiers: – private – protected – public Java Programming: From Problem Analysis to Program Design, Second Edition

Classes s Private: Members of class are not accessible outside class. s Public: Members of class are accessible outside class. s Class members: Can be methods or variables. s Variable members are declared like any other variables. Java Programming: From Problem Analysis to Program Design, Second Edition

Syntax The general syntax for defining a class is: s If a member of a class is a named constant, you declare it just like any other named constant. s If a member of a class is a variable, you declare it just like any other variable. s If a member of a class is a method, you define it just like any other method. s If a member of a class is a method, it can (directly) access any member of the class—data members and methods. Therefore, when you write the definition of a method (of the class), you can directly access any data member of the class (without passing it as a parameter). Java Programming: From Problem Analysis to Program Design, Second Edition

Syntax class Clock: Data Members (Instance Variables): • private int hr; //store hours • private int min; //store minutes • private int sec; //store seconds Methods: • public void set. Time(int hours, int minutes, int seconds) • public int get. Hours() • public int get. Minutes() • public int get. Seconds() • public void print. Time() • public void increment. Seconds() • public void increment. Minutes() • public void increment. Hours() • public boolean equals(Clock other. Clock) • public void make. Copy(Clock other. Clock) • public Clock get. Copy() Java Programming: From Problem Analysis to Program Design, Second Edition

Constructors s Two types of constructors: s With parameters s Without parameters (default constructor) s Constructors have the following properties: s The name of a constructor is the same as the name of the class. s A constructor, even though it is a method, has no type. s A class can have more than one constructor. All constructors of a class have the same name. s If a class has more than one constructor, any two constructors must have different signatures. s Constructors are automatically executed when a class object is instantiated. s If there are multiple constructors, which constructor executes depends on the type of values passed to the class object when the class object is instantiated. Java Programming: From Problem Analysis to Program Design, Second Edition

Constructors class Clock: Constructors s Default constructor is public Clock(). Constructor with parameters: s public Clock(int hours, int minutes, int seconds) Java Programming: From Problem Analysis to Program Design, Second Edition

Unified Modeling Language Class Diagrams Java Programming: From Problem Analysis to Program Design, Second Edition 9

Variable Declaration and Object Instantiation The general syntax for using the operator new is: new class. Name() OR new class. Name(argument 1, argument 2, . . . , argument. N) Clock my. Clock; Clock your. Clock; my. Clock = new Clock(); your. Clock = new Clock(9, 35, 15); Java Programming: From Problem Analysis to Program Design, Second Edition

Variable Declaration and Object Instantiation Java Programming: From Problem Analysis to Program Design, Second Edition 11

Accessing Class Members The syntax to access a data member of a class object or method is: reference. Variable. Name. member. Name Example 8 -1 my. Clock. set. Time(5, 2, 30); my. Clock. print. Time(); your. Clock. set. Time(x, y, z); if (my. Clock. equals(your. Clock)). . . Java Programming: From Problem Analysis to Program Design, Second Edition 12

Assignment Operator: A Precaution my. Clock = your. Clock; s Copies the value of the reference variable your. Clock into the reference variable my. Clock. After this statement executes, both your. Clock and my. Clock refer to the same object. Java Programming: From Problem Analysis to Program Design, Second Edition 13

Assignment Operator: A Precaution s Shallow copying: Two or more reference variables of the same type point to the same object. s Deep copying: Each reference variable refers to its own object. Java Programming: From Problem Analysis to Program Design, Second Edition 14

The Copy Constructor s Executes when an object is instantiated. s Initialized using an existing object. s Syntax: public Class. Name(Class. Name other. Object) Java Programming: From Problem Analysis to Program Design, Second Edition

Example: class Clock Java Programming: From Problem Analysis to Program Design, Second Edition

The Method to. String s s Public value-returning method. Takes no parameters. Returns address of a String object. Output using print, println, printf methods. s Default definition creates String with name of object’s class name followed by hash code of object. Java Programming: From Problem Analysis to Program Design, Second Edition

The Modifier static s In the method heading, specifies that the method can be invoked by using the name of the class. s If used to declare data member, data member invoked by using the class name. s Static data members of class exist even when no object of class type instantiated. s Static variables are initialized to their default values. Java Programming: From Problem Analysis to Program Design, Second Edition

The Modifier static Example: 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: From Problem Analysis to Program Design, Second Edition

The Modifier static void set. X(int a) { x = a; } public String { return("x + ", } public static { y++; } to. String() = " + x + ", y = " + y count = " + count); void increment. Y() } Illustrate illus. Object = new Illustrate(); Illustrate. increment. Y(); Illustrate. count++; Java Programming: From Problem Analysis to Program Design, Second Edition

The Modifier static Illustrate illus. Object 1 = new Illustrate(3); Illustrate illus. Object 2 = new Illustrate(5); Java Programming: From Problem Analysis to Program Design, Second Edition 21

The Modifier static Illustrate. increment. Y(); Illustrate. count++; Java Programming: From Problem Analysis to Program Design, Second Edition 22

Finalizers s Automatically execute when class object goes out of scope. s Have no parameters. s Only one finalizer per class. s Name of finalizer: finalize. Java Programming: From Problem Analysis to Program Design, Second Edition

Accessor and Mutator Methods s Accessor method: A method of a class that only accesses (that is, does not modify) the value(s) of the data member(s). s Mutator method: A method of a class that modifies the value(s) of the data member(s). Java Programming: From Problem Analysis to Program Design, Second Edition

Creating Packages You can create packages using a reserved word package. s Define the class to be public. (If class is not public, it can only be used within package. ) s Choose name for package. s Organize package (create subdirectories). Java Programming: From Problem Analysis to Program Design, Second Edition

Creating Package for class Clock package itu. clock. Package; public class Clock { //put instance variables and methods, //as before, here } import itu. clock. Package. Clock; Java Programming: From Problem Analysis to Program Design, Second Edition

The Reference this s Refers to instance variables and methods of a class. s Used to implement cascaded method calls. Java Programming: From Problem Analysis to Program Design, Second Edition

Inner Classes s Defined within other classes. s Can be either a complete class definition or an anonymous inner class definition. s Used to handle events. Java Programming: From Problem Analysis to Program Design, Second Edition

Abstract Data Types A data type that specifies the logical properties without the implementation details. Java Programming: From Problem Analysis to Program Design, Second Edition

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. The non-GUI application program should do the following: 1. Show the customer the different products sold by the candy machine. 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: From Problem Analysis to Program Design, Second Edition

Programming Example: Candy Machine (Input and Output) s Input: The item selection and the cost of the item. s Output: The selected item. Java Programming: From Problem Analysis to Program Design, Second Edition

Programming Example: Candy Machine Components: s Cash register s Dispenser s Machine Java Programming: From Problem Analysis to Program Design, Second Edition

Programming Example: Candy Machine Java Programming: From Problem Analysis to Program Design, Second Edition

Lecture 8 Summary s Creating classes s Members of a class: s s private protected public static s Implementing classes s Various operations on classes Java Programming: From Problem Analysis to Program Design, Second Edition

Lecture 8 Summary s s Constructors Finalizers Method to. String Abstract data types Java Programming: From Problem Analysis to Program Design, Second Edition