Object-Oriented Programming - Lecture 5: Inheritance - Lê Hồng Phương

Lecture 5: Inheritance Lê Hồng Phương phuonglh@gmail.com Department of Mathematics, Mechanics and Informatics, Vietnam National University, Hanoi 2012-2013 Object-Oriented Programming: Inheritance 2 Content ● Inheritance ● Overriding and hiding methods ● Polymorphism ● Final methods and classes ● Abstract methods and classes 2012-2013 Object-Oriented Programming: Inheritance 3 Inheritance ● A class that is derived from another class is called a subclass (derived class, extended class, child class). ● The superclass is also called base class or parent class. ● In Java, excepting Object, every class has one and only one direct superclass. 2012-2013 Object-Oriented Programming: Inheritance 4 Inheritance ● A subclass inherits all the members (fields, methods, and nested classes) from its superclass. ● Constructors are not members, so they are not inherited by subclasses. ● The constructor of the superclass can be invoked from the subclass. 2012-2013 Object-Oriented Programming: Inheritance 5 Inheritance public class Bicycle { private int cadence = 0; private int speed = 0; private int gear = 1; void changeCadence(int newValue) { cadence = newValue; } void changeGear(int newValue) { gear = newValue; } void speedUp(int increment) { speed = speed + increment; } void applyBrakes(int decrement) { speed = speed - decrement; } void printStates() { System.out.println("(" + cadence + ", " + speed + ", " + gear + “)”); } } The fields represent the state of the object. The methods define its interaction with the outside world. 2012-2013 Object-Oriented Programming: Inheritance 6 Inheritance public class Bicycle { // ... public Bicycle(int c, int s, int g) { this.cadence = c; this.speed = s; this.gear = g; } // } A constructor defines how an object is initialized when constructed. Bicycle myBike = new Bicycle(30, 15, 4); 2012-2013 Object-Oriented Programming: Inheritance 7 Inheritance public class MountainBike extends Bicycle { private int seatHeight; public MountainBike(int height, int cadence, int speed, int gear) { super(cadence, speed, gear); seatHeight = height; } public int getSeatHeight() { return seatHeight; } } Call the constructor of the superclass to initialize its 3 fields. Bicycle MountainBike 2012-2013 Object-Oriented Programming: Inheritance 8 Inheritance ● A subclass inherits all the public and protected members of its parent, no matter what the package the subclass is in. ● If the subclass is in the same package as its parent, it also inherits the package-private members of the parent. ● A subclass does not inherit the private member of its parent class. – If it wants to access this member, it must call public or protected methods of the parent class. 2012-2013 Object-Oriented Programming: Inheritance 9 Overriding and hiding methods ● If you declare a field in the subclass with the same name as the one in the superclass, you hide it (not recommended). ● If you write a new instance method that has the same signature as the one in the superclass, you override it. ● If you write a new static method that has the same signature as the one in the superclass, you hide it. 2012-2013 Object-Oriented Programming: Inheritance 10 Overriding and hiding methods public class MountainBike extends Bicycle { //... @Override void applyBrakes(int decrement) { // ... } //... } The subclass has a method which overrides that of its superclass. ● This allows a class to inherit from a superclass whose behavior is similar and then to modify behavior as needed. ● Use @Override annotation to instruct the compiler (optional). 2012-2013 Object-Oriented Programming: Inheritance 11 Casting objects ● If class A is a subclass of class B then we can say A is B. – MountainBike is Bicycle – Car is Vehicle – Dog is Animal – A is Object ● Casting object: convert an object of one type to another type. 2012-2013 Object-Oriented Programming: Inheritance 12 Casting objects ● Implicit casting: ● Here, b is both a Bicycle and a MountainBike. ● Explicit casting: Bicycle b = new MountainBike(100, 30, 15, 4); if (b instanceof MountainBike) { MountainBike myBike = (MountainBike) b; } 2012-2013 Object-Oriented Programming: Inheritance 13 Polymorphism ● Polymorphism is the ability to create a behavior or an object that has more than one form. ● The exact behavior of an object is determined at run- time instead of at compile time. – This is called late binding or dynamic binding. ● Polymorphism is a very important concept of object- oriented programming. – You must completely understand this concept! 2012-2013 Object-Oriented Programming: Inheritance 14 Polymorphism ● Example: – Pet is class which defines an instance method named void say(). – Dog is a subclass of Pet which overrides the method say(). – Cat is another subclass of Pet which overrides the method say(). 2012-2013 Object-Oriented Programming: Inheritance 15 Polymorphism public class Pet { public void say() { System.out.println("I don't know who I am."); } } class Dog extends Pet { @Override public void say() { System.out.println("Wooof!"); } } class Cat extends Pet { @Override public void say() { System.out.println("Meow!"); } } Pet Dog Cat 2012-2013 Object-Oriented Programming: Inheritance 16 Polymorphism Pet Dog Cat public class PetTester { public static void main(String[] args) { Pet p; p = new Pet(); p.say(); p = new Dog(); p.say(); p = new Cat(); p.say(); } } 2012-2013 Object-Oriented Programming: Inheritance 17 Object as a superclass ● The class java.lang.Object is the root class of all the classes. ● Some important methods of Object that we may want to inherit and/or override are: protected Object clone() throws CloneNotSupportedException; public boolean equals(Object obj); protected void finalize() throws Throwable; public int hashCode(); public String toString(); Override this method if you want to convert an object to a nice string representation (e.g, for printing the object).