Java Tutorial What is an Object? Variables Methods

Transcription

1 Java Tutorial What is an Object? 1 Variables represent the current state of the object Example: Bicycle: Current Speed is 10 mph Current Pedal cadence is 90 rpm (revolution per minutes) Current gear is 5th 2 Methods inspect or change the state of a particular object (bicycle). Brake method Change gears Change the pedal cadence 1

2 What is a Class? A blueprint that defines the instance variables and the methods common to all objects of a certain kind. After you've created the class, you can create any number of objects from the class. When you create an object of a class, the system allocates enough memory for the object. Constructors Creates a new object of the class. It initializes all the instance variables and does any work necessary to prepare the class to be used. Car c = new Car(); 2

3 class Car { String licenseplate; double speed; double maxspeed; void SetlicensePlate(String licenceplate1 ) { licenceplate = licenceplate1; double GetSpeed () { return speed; void SetSpeed (int speed1) { speed =speed1; class CarTest { public static void main(string args[]) { Car c = new Car(); c.setlicenseplate( Toronto66 ); c.setspeed (150); System.out.println(c.licensePlate); System.out.println(c.GetSpeed()); 3

4 Access Protection Public Private Protected Variable Method public violate encapsulation Provide services to clients private Enforce encapsulation Support other methods in the class 4

5 Inheritance Code reusability is claimed to be a key advantage of object-oriented languages. Class Vehicle { public int a; private int b; protected int c; public method1() { private method2() { Class Car extends Vehicle { // variables //Methods 5

6 Method Declarations A method declaration specifies the code that will be executed when the method is invoked (called) If the called method is in the same class, only the method name is needed compute mymethod mymethod(); 6

7 The called method is often part of another class or object main doit helpme obj.doit(); helpme(); 7

8 Method Overloading Method overloading is the process of giving a single method name multiple definitions float tryme(int x) { return x +.375; Invocation result = tryme(25, 4.32) float tryme(int x, float y) { return x*y; 8

9 Arrays of Objects The elements of an array can be object references The following declaration creates an array object called verbs and fills it with five String objects created using string literals String verbs[5] = {"play", "work", "eat", "sleep", walk ; verb Play Work eat sleep walk 9

10 Overriding Methods A subclass inherits methods from a super class. Sometimes, it is necessary for the sub class to modify the methods defined in the super class. public class Circle { protected double radius; protected double Area; public Circle(double radius) { this.radius = radius; Circle GetArea() Cylinder GetArea() public double getarea() { return (Area= Math.PI*radius*radius); //this method return the area of the circle // end of class circle 10

11 public class Cylinder extends Circle { protected double length; public Cylinder(double radius, double length) { super(radius); this.length = length; public double getarea() // method overriden here { return ( 2*super.getArea()+2*Math.PI*radius*length); //this method return the surface area of the cylinder // end of class Cylinder 11

12 Abstract Classes Create a common interface for all the classes derived from it. It can be expressed differently for each different subclass. If you inherit from an abstract class, you must provide method definitions for all the abstract methods. If you don t, then the derived class is also abstract. Abstract class express only the interface, not a particular implementation, so creating an object of abstract class makes no sense, 12

13 Interface more abstract than abstract class A collection of abstract methods and constants. Defines a set of methods but does not implement them. A class that implements the interface agrees to implement all the methods defined in the interface, thereby agreeing to certain behavior. An interface is a function specification about what a class do, not how it does. _ 13

14 this and super reference Access to overriden methods and variables can be obtained with the super keyword: public class A { int i=0; int j=3; void dosomething(){i=5; class B extends A { int j=0; void dosomething(){ j=10; // Call the overridden method super.dosomething(); // And can use the shadowed // property j=j+super.j; 14

15 We can also explicitly reference instance variables in the current object with the this keyword: public class A{ int x,y; void dosomething(int x, int y){ // x & y hold the values passed in // the argument variables. To access // the instance variables x,y we must // specify them with 'this'. this.x = x; this.y = y; If we have overloaded constructors, we can access other constructors from within one constructor using this(): public class A{ int x,y; int i,k; public A(int x, int y){ this.x = x; this.y = y; public A(int x, int y, int i, int k){ this(x,y);// Must be in first line this.i = i; this.k = k; 15

16 Similarly, we can access overridden constructors using super(): public class A { int i; public A(int i){this.i=i; class B extends A { int j; public B(int i, int j){ super(i);// Must be in // first line this.j=j; If there is no call to super class constructors in the first line of the sub-class, it will call the defualt constructor A() for the base class. 16

17 Parameters Passing When a method is called, the actual parameters in the invocation are copied into the formal parameters in the method header Passing parameters is similar to an assignment statement Pass by Value: Public static void main(string [] args){ double d = 2.0; changeme(d); System.out.println(d); public void changeme(double d) { //this has no effect on d outside of this method! d = 345.0; What s the output of d? 17

18 Pass by Reference: The actual parameter and the formal parameter become aliases of each other Suppose class Car has methods called setspeed() and getspeed(). Car ferrari = new Car(); ferrari.setspeed(100); changeparameters(ferrari); System.out.println( The speed = +ferrari.getspeed()); public void changeparameters(car c) { //changes the car s speed outside this method! c.setspeed(200.0); What gets printed out? 18

19 What is printed as a result of executing the following program? public class ArrayTest { public static void main(string[] args) { int[] test = new int[2]; test[0] = test[1] = 5; System.out.println(test[0] + "," + test[1]); fiddle(test, test[1]); System.out.println(test[0] + "," + test[1]); static void fiddle(int[] test, int element) { test[0] = 10; test[1] = 11; element = 12; System.out.println(test[0] + "," + test[1] + "," + element); test = new int[2]; test[0] = 20; test[1] = 21; System.out.println(test[0] + "," + test[1]); 19

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28 What is the output of a call to B.demo()? public class A { public void foo() { System.out.println("A foo"); bar(); public void bar() { System.out.println("A bar"); public class B extends A { public void foo() { System.out.println("B foo"); super.foo(); public void bar() { System.out.println("B bar"); public static void demo() { A a = new A(); B b = new B(); a.foo(); b.foo(); Answer: A foo A bar B foo A foo B bar A foo() bar() B foo() bar() demo() 28

29 (a) Comment out any lines that cause a compilation error. (b) Once the incorrect lines are eliminated, the class is compiled and run. What is the output? 6 29