Java Inheritance: Understanding Extended Classes

1. Inheritance • INHERITANCE: extend existing classes by adding or redefining methods, and adding instance fields Suppose we have a class Vehicle: public class Vehicle{ String type; String model; }

2. We want a class which to represent a Car. This class will need all of the attributes of a standard Vehicle plus some more..

3. Inherited Fields are Private • Consider deposit method of CheckingAccountpublic void deposit(double amount){   transactionCount++;   // now add amount to balance   ...} • Can't just add amount to balance • balance is a private field of the superclass • Subclass must use public interface

4. and all data of Vehicle are automaticaly inherited by class Car • Ok to call deposit, getBalance on SavingsAccount object • Extended class = superclass, extending class = subclass • Inheritance is different from realizing an interface • Interface is not a class • Interface supplies no instance fields or methods to inherit

5. We want a class which to represent Cars. This class will need all of the behavior of a standard Vehicle plus some more.. • Constructor (calling superclass constructor

6. Object: The Cosmic Superclass • All classes extend Object • Most useful methods: • String toString() • boolean equals(Object otherObject) • Object clone()

7. The Object Class is the Superclass of Every Java Class

8. Overriding the toString Method • Returns a string representation of the object • Useful for debugging • Example: Rectangle.toString returns something likejava.awt.Rectangle[x=5,y=10,width=20,height=30] • toString used by concatenation operator • aString + anObjectmeans aString + anObject.toString() • Object.toString prints class name and object addressBankAccount@d2460bf • Override toString:public class BankAccount{   public String toString()   {      return "BankAccount[balance=" + balance + "]";   }   . . .}

9. Overriding the equals Method • equalstests for equal contents • == tests for equal location • Must cast the Object parameter to subclass • public class Coin{   public boolean equals(Object otherObject)   {      Coin other = (Coin)otherObject;      return name.equals(other.name) && value == other.value;   }}

10. Two References to Equal Objects

11. Two References to Same Object

12. Overriding the clone Method • Copying object reference gives two references to same objectBankAccount account2 = account1; • Sometimes, need to make a copy of the object • Use clone:BankAccount account2 = (BankAccount)account1.clone(); • Must cast return value because return type is Object • Define clone method to make new object:public Object clone(){   BankAccount cloned = new BankAccount();   cloned.balance = balance;   return cloned;} • Warning: This approach doesn't work with inheritance--see Advanced Topic 11.6