Lecture 11

Inheritance

• Objects that inherit features of a parent class can be thought to contain an object of the parent type
• all methods and fields of the parent are "inherited"

Best Practices With Inheritance

• discard packaging
• make all instance fields private
• this means instance fields are not visible to child methods
• if you need to access instance fields of the parent from the child use the public interface

Why Do This?

• child inherits parent's methods and therefore inherits the public interface
• this simplifies access
• and prevents privacy problems via inheritance

public class Parent
{
protected mutable class x;
}

public class Child extends Parent
{public mutable class leakyMethod()
{return m;
}
}

Constructors

• form part of the public interface

public class Child extends Parent
{
public child ( ...parameters for child / parent )
//call parent constructor
super(...parameters for parent)
//assigns child parameter
}


Overriding Methods

• Child classes inherit all methods from the parent
• child has the option to re-implement these methods
  
• called overriding

Example

• All objects extend type Object by default

public class MyClass [extends Object]
[

• type object contains several default methods
• equals(), toString(), clone()

]

• declaration of an overridden method in the child must be exactly the same as the parent
• same return type
• same parameter type
• same number of parameters

Parent

public int myMethod(double a, int b)

Child

public int myMethod(double a, int b)

• if myMethod is overridden in a child class, use 'super.myMethod' to gain access to the parent method
• to prevent overriding use 'final'

Abstract Classes

• a student is either graduate or undergraduate
• there is no 'plain' student
• so it doesn't make sense to instantiate this class
• all students have letter grades
• child classes of student should be forced to implement getLetterGrade()

Solution

• declare Student an abstract class
• can't instantiate an object of type student
• declare getLetterGrade() an abstract method within abstract class student
• children must override getLetterGrade() or they will be abstract classes themselves

public abstract class Student
{
.
.
.
public abstract String getLetterGrade();

}