09.5.0 java07 inheritance

Inheritance

Introduction

• In real situations either when modeling real world objects such as vehicles, animals, etc. or when modeling abstract data structures such as queues, stacks, collections, windows, menu boxes the structure of different object families can be viewed as a kind of "family" tree.

• Java like most OO languages allows us to use these types of relationships to reuse code and functionality by making classes that use characteristics of "parent" classes

Defining the Differences

• Classes often share capabilities• We want to avoid re-coding these capabilities• Reuse of these would be best to

– Improve maintainability– Reduce cost– Improve “real world” modeling

Inheritance Defined

• When one class re-uses the capabilities defined in another class.

• The new subclass gains all the methods and attributes of the superclass.

Superclass

Subclass

Vehicle

Car Truck

Benefits of Inheritance

• Saves effort of “reinventing the wheel”• Allows us to build on existing code,

specializing without having to copy it, rewrite it, etc.

• To create the subclass, we need to program only the differences between the superclass and the subclass that inherits from it.

• Allows for flexibility in class definitions.

Example

Node

Data

setDatagetData

TreeNode

LeftRight

setLeftsetRightgetLeftgetRight

ListNode

Next

setNextgetNext

Inheritance: Example

class Node {

private int data;

public Node(int newData) {

setData(newData);

}

public void setData(int newData) {

data = newData;

}

public int getData() {

return data;

}

} // Node

Inheritance: Example

class ListNode extends Node {

private ListNode next; // Fields

public ListNode(int newData) { // Constructor

super(newData);

setNext(null);

}

public void setNext(ListNode newNext) {

next = newNext;

}

public ListNode getNext() {

return next;

}

} // ListNode

Inheritance: Example

class TreeNode extends Node {

private TreeNode left;

private TreeNode right;

public TreeNode(int newData) { // Constructor

super(newData);

setLeft(null);

setRight(null);

}

public void setLeft(TreeNode newLeft) {

left = newLeft;

}

public void setRight(TreeNode newRight) {

right = newRight;

}

Inheritance: Example

// class TreeNode (Continued)

public TreeNode getLeft() {

return left;

}

public TreeNode getRight() {

return right;

}

} // TreeNode

Two Types of Inheritance

• Extension– Adds attributes and methods

• Redefinition (Method Overriding)– Changes/modifies existing methods, specializing as

needed

• Consider a primitive bank account which allows only three kinds of transactions:– Deposits– Withdrawals– Ability to check current balance

Inheritance Example: Bank Accounts

The Base Class Bank_Account

Bank_Account

• balance

withdraw

deposit

setBalance

GetBalance

A Superclass Bank_Accountclass BankAccount {

private double balance;

public BankAccount() {setBalance(0);

} // constructor

public void setBalance(double balance) {this.balance = balance;System.out.println(

"New balance now: "+ getBalance());}

public double getBalance() { return balance;

}

A Superclass Bank_Account// class BankAccount continued

public void deposit(double amt) {setBalance(getBalance() + amt);

}

public void withdraw(double amt) {if(getBalance() < amt) {

amt = getBalance(); System.out.println ("Can only withdraw " + amt); }

setBalance(setBalance() - amt);}

} // BankAccount

Inheritance by Extension

• Imagine that we wish to create a new kind of Bank Account that is:– Identical to the base class in all respects, except one

– We want to add the ability for the account to earn interest

• Without inheritance, we’d have to write it from scratch, duplicating code, etc.

• With inheritance, we need code only the new capability and inherit the rest.

Illustration of InheritanceBankAccount

• balance

withdraw

deposit

GetBalance

setBalance

• RATE• MIN_BALANCE

calcInterest

SavingsAccount

Inheritance by Extension

class SavingsAccount extends BankAccount {

public final static double RATE = 0.023;

public final static double MIN_BALANCE = 500.00;

public double calcInterest() {

double interest;

if (getBalance() >= MIN_BALANCE)

interest = getBalance()*RATE;

else

interest = 0.00;

return interest;

}

}

Using Subclasses

class BankExample1 {

public static void main(String args[]) {

SavingsAccount mySavings;

double myInterest;

mySavings = new SavingsAccount();

mySavings.deposit(500.00);

myInterest = mySavings.calcInterest();

mySavings.deposit(myInterest);

mySavings.withdraw(

IOGadget.readDouble(

"Enter amount to withdraw"));

}

}

BankAccount

SavingsAccount

Inheritance by Redefinition

• Imagine that we wish to create a new kind of Savings Account that is identical to Savings Account in all respects, except:– We want to change the way in which withdrawals are handled– The base class already handles withdrawals, but now we want

a subclass that does them differently.

• Without inheritance, we’d have to rewrite it from scratch.

• With inheritance, we need code only the new way that we want withdrawals to work,

Illustration of Redefinition

• RATE• MIN_BALANCE

calcInterest

SavingsAccount

• overdraftOK• OVERDRAFTCHARGE

allowOverdraft

DeluxeSavings

withdraw

Inheritance with Redefinitionclass DeluxeSavings extends SavingsAccount {

private boolean overdraftOK;public final static double OVERDRAFTCHARGE = 20.0;public DeluxeSavings() {

super();setOverdraftOK(false);

}public void setOverdraftOK(boolean odok)

{ overdraftOK = odok; }public void withdraw(double amt) {

if (overdraftOK) { setBalance(getBalance() - amt); if (getBalance() < 0) setBalance(getBalance() - OVERDRAFTCHARGE); } else { super.withdraw(amt); } } // withdraw} // DeluxeSavings

super & this

• super means “look in the superclass”

• Constructor: super();

• Method: super.m();

• Field: super.x;

• this means “look in this class”

• Constructor: this();

• Method: this.m();

• Field: this.x;

Using Subclasses

class BankExample2 {public static void main(String args[]) {

double interest, amt1, amt2;DeluxeSavings mds = new DeluxeSavings();mds.deposit(250.00);amt1 = IOGadget.readDouble("Enter amount to

withdraw");mds.withdraw(amt1);mds.setOverdraftOK(true); interest = mds.calcInterest();mds.deposit(interest);amt2 = IOGadget.readDouble("Enter amount to

withdraw");mds.withdraw(amt2);

} // main} // Demo

Design & Use

• Declare common methods/attributes as high in the class hierarchy as possible

• All subclasses will inherit these capabilities

• Specialize (extend and redefine) in subclasses

• When a method is invoked, the request is serviced by When a method is invoked, the request is serviced by the lowest, most specific class and moves upthe lowest, most specific class and moves upas needed to find a matchas needed to find a match

Method Resolution

DeluxeSavings ds;

ds = new DeluxeSavings();

ds.deposit(1000.00);

ds.deposit(

ds.calcInterest());

ds.withdraw(500.00)

ds.setOverdraftOK(true);

ds.withdraw(2000.00);

DeluxeSavingsoverdraftOK

OVERDRAFT_CHARGEsetOverdraftOK

setOverdraftNOTwithdraw

BankAccountbalancedeposit

withdrawgetBalance

SavingsAccountRATE

MIN_BALANCEcalcInterest

Summary of Inheritance

• Extension take a base class and add new capabilities to it (methods, fields).

• Redefinition (method overriding) takes a base class and redefines an existing method, implementing it in a new way in order to change capability or performance.

• Both allow us to code only the differences.