inheritance & polymorphism - 1

INHERITANCE

Focus

• To explain inheritance concept,• To apply inheritance concept in

programming.• To list the types of accessibility modifier

INHERITANCE

• Inheritance let us to create a new class from the existing classes.

• The new class is called subclass and the existing class is called superclass.

• The subclass inherits the properties of the superclass.

• The properties refer to the method or the attribute (data)

• Inheritance is ‘is-a’ relation

• Example : if a Circle inherits the Shape class, hence the Circle is a Shape. (see the next figure)

What is Inheritance?

INHERITANCE

Box

Example

The class Circle and Rectangle are derived from Shape and the class Box is derived from Rectangle. Every Circle and every Rectangle is Shape and every Box is a Rectangle

INHERITENCE

• If we need a new class with a same properties or method of the current class, we do not need to define a new class.

• A new class might be inherited from the current class.

When it is used?

INHERITANCE

• Single inheritance – Is a subclass that derived from a single/one superclass

(existing class)

• Multiple inheritance– Is a subclass that derived from more than one superclass– Not supported by Java

Types of Inheritances?

Geometry

Circle Triangle Square

Sphere Cone Cylinder Cubes

Single Inheritance

Multiple Inheritance

INHERITANCE

• Involve 2 classes :

– Super class.– Sub class

Rectangle

Box

Super class

Sub class

Properties/Characteristics?

Super class and Sub class

Rectangle

double length double width

Box

double height

INHERITANCE

• Super class – Rectangle• Sub class – Box• Attributes for Rectangle : length, width• Attribute for Box : height

Box class inherits the length and width of the Rectangle class (superclass)

keyword extends

• extends is the keyword to implement inheritance in Java.• Syntax

class SubClassName extends SuperClassName { // properties & methods }• E.g.

class Box extends Rectangle { // properties and coding }

Rectangle

Box

INHERITANCE

public class Rectangle{ private double length; private double width; public Rectangle() { length = 0; width = 0; }

public Rectangle(double L, double W) { setDimension(L,W); }

public void setDimension(double L, double W) { if (L >= 0) length = L; else length = 0;

if (W >= 0) width = W; else width = 0; }

public double getLength(){ return length;}

public double getWidth(){ return width;}

public double area(){ return length * width;} public void print(){ System.out.print(“length = “ + length); System.out.print(“width = “ + width);

}

} // end for Rectangle class

E.g. : Rectangle class

Rectangle

- length : double- width : double

+ Rectangle()+ Rectangle(double,double)+ setDimension(double,double) : void+ getLength() : double+ getWidth() : double+ area() : double+ print() : void

The class Rectangle has 9 members

UML class diagram of the Rectangle class

public class Box extends Rectangle{ private double height; public Box() { super(); height = 0; }

public Box(double L, double W, double H) { super(L,W); height = H; }

public void setDimension(double L, double W, double H) { setDimension(L,W); if (H >= 0) height = H; else height = 0; }

public double getHeight(){ return height;}

public double area(){ return 2 * (getLength() * getWidth()

+ getLength() * height + getWidth() * height);

} public double volume(){ return super.area() * height;}

public void print(){ super.print(); system.out.print(“height = “ + height);}

} // end for class Box extends

E.g. : Box class

Box

- height : double- length : double (can’t access directly)- width : double (cant’ access directly)

+ Box()+ Box(double,double)+ setDimension(double,double) : void+ setDimension(double,double,double) : void (overloads parent’s setDimension())

+ getLength() : double+ getWidth() : double+ getHeight() : double+ area() : double (overrides parent’s area())

+ volume() : double+ print() : void (overrides parent’s print())

The class Box has 13 members

UML class diagram of the class Box

• The class Box is derived from the class Rectangle• Therefore, all public members of Rectangle are public members of Box• The class Box overrides the method print and area• The class Box has 3 data members : length, width and height• The instance variable length and width are private members of the class Rectangle• So, it cannot be directly accessed in class Box• Use the super keyword to call a method of the superclass.

• To print the length and width, a reserve word super should be put into the print method of class Box to call the parent’s print() method.• The same case happen in the volume method where super.area() is being used to determine the base area of box.

• The method area of the class Box determines the surface area of the box.• To retrieve the length and width, the methods getLength and getWidth in class Rectangle is being used.• Here, the reserve word super is not used because the class Box does not override the methods getLength and getWidth

Defining Classes with Inheritance

• Case Study:– Suppose we want implement a class roster that contains

both undergraduate and graduate students.– Each student’s record will contain his or her name, three

test scores, and the final course grade.– The formula for determining the course grade is different

for graduate students than for undergraduate students.

Undergrads: pass if avg test score >= 70

Grads: pass if avg test score >= 80

Modeling Two Types of Students

• There are two ways to design the classes to model undergraduate and graduate students.– We can define two unrelated classes, one for

undergraduates and one for graduates.– We can model the two kinds of students by using

classes that are related in an inheritance hierarchy.

• Two classes are unrelated if they are not connected in an inheritance relationship.

Classes for the Class Roster

• For the Class Roster sample, we design three classes:– Student

– UndergraduateStudent

– GraduateStudent

• The Student class will incorporate behavior and data common to both UndergraduateStudent and GraduateStudent objects.

• The UndergraduateStudent class and the GraduateStudent class will each contain behaviors and data specific to their respective objects.

Inheritance Hierarchy

+ computeCourseGrade() : int + computeCourseGrade() : int

+ computeCourseGrade() : int

+ UndergraduateStudent() : void + GraduateStudent() : void

Definition of GraduateStudent & UndergraduateStudent classes

class GraduateStudent extends Student {

//constructor not shown

public void computeCourseGrade() { int total = 0; total = test1 + test2 + test3; if (total / 3 >= 80) { courseGrade = "Pass"; } else { courseGrade = "No Pass"; } }}

class UndergraduateStudent extends Student {

//Constructor not shown

public void computeCourseGrade() { int total = 0;

total = test1 + test2 + test3;

if (total / 3 >= 70) { courseGrade = "Pass"; } else { courseGrade = "No Pass"; } }

}

Declaring a Subclass

A subclass inherits data and methods from the superclass. In the subclass, you can also:

Add new data

Add new methods

Override the methods of the superclass Modify existing behaviour of parent

Overriding vs. Overloading

public class Test { public static void main(String[] args) { A a = new A(); a.p(10); } } class B { public void p(int i) { } } class A extends B { // This method overrides the method in B public void p(int i) { System.out.println(i); } }

public class Test { public static void main(String[] args) { A a = new A(); a.p(10); } } class B { public void p(int i) { } } class A extends B { // This method overloads the method in B public void p(double i) { System.out.println(i); } }

Inheritance Rules

1. The private members of the superclass are private to the superclass

2. The subclass can access the members of the superclass according to the accessibility rules

3. The subclass can include additional data and/or method members

Inheritance Rules (continued)

4. The subclass can override, that is, redefine the methods of the superclass

The overriding method in subclass must have similar Name

Parameter list

Return type

5. All members of the superclass are also members of the subclass– Similarly, the methods of the superclass (unless

overridden) are also the methods of the subclass– Remember Rule 1 & 2 when accessing a member

of the superclass in the subclass

• To call a superclass constructor

– super(); //must be the first statement in subclass’s constructor

• To call a superclass method

– super.methodname();

– this is only used if the subclass overrides the superclass method

6. (Using the Keyword super)

The keyword super refers to the direct superclass of a subclass . This keyword can be used in two ways:

Inheritance Rules (continued)

The Object Class is the Superclass of Every Java Class

(Accessibility Modifier)

• Sometimes , it is called visibility modifier• Not all properties can be accessed by sub class.• Super class can control a data accessing from subclass

by giving the type of accessing to the members and methods.

• A class can declare the data members or method as a public, private or protected.

• If it is not declared, the data or method will be set to default type.

INHERITANCE

Accessibility criteria

Modifier Same Class Same Package

Subclass Universe

private Yes No No No

default Yes Yes No No

protected Yes Yes Yes No

public Yes Yes Yes Yes

Member Accessibility

INHERITANCE

Sub class B

public int bprotected int c

Super class

int apublic int b

protected int cprivate int d

Sub class A

int apublic int b

protected int c

Package B

Package A

Data Accessibility

INHERITANCE

Refer to the previous slide

• Super class has 2 subclasses : Subclass A and Subclass B.

• Subclass A is defined in same package with superclass, subclass B is defined outside the package.

• There are 4 accessibility data types: public, protected, private and default.

• Subclass A can access all properties of superclass except private.

• But, subclass B can only access the properties outside the package which are public and protected.

Example: Visibility Modifiers

public class C1 { public int x; protected int y; int z; private int u; protected void m() { } }

public class C2 { C1 o = new C1(); can access o.x; can access o.y; can access o.z; cannot access o.u; can invoke o.m(); }

public class C3 extends C1 { can access x; can access y; can access z; cannot access u; can invoke m(); }

package p1;

public class C4 extends C1 { can access x; can access y; cannot access z; cannot access u; can invoke m(); }

package p2;

public class C5 { C1 o = new C1(); can access o.x; cannot access o.y; cannot access o.z; cannot access o.u; cannot invoke o.m(); }

What’s wrong with the code?How to fix it?

class ClassX{ private int m; public String toString() { return new String("(" + m + ")"); }}public class ClassY extends ClassX{ private int n; public String toString() { return new String("(" + m + " , " + n + ")"); }}

class TestAccesibility{ public static void main(String [] args) { ClassX x = new ClassX; ClassY y = new ClassY; System.out.println("x = " + x); System.out.println("y = " + y); }}

Inheritance and Constructors

• Unlike members of a superclass, constructors of a superclass are not inherited by its subclasses.

• You must define a constructor for a class or use the default constructor added by the compiler.

• The statement super();

calls the superclass’s constructor.• super(); must be the first statement in the

subclass contructor.

public Box(double l, double w, double h){ super(l,w); height = h;}

• A call to the constructor of the superclass must be in the first statement in the child constructor.

Rectangle myRectangle = new Rectangle(5, 3);Box myBox = new Box(6, 5, 4);

Superclass’s Constructor Is Always Invoked

A subclass constructor may invoke its superclass’s constructor. If none is invoked explicitly, the compiler puts super() as the first statement in the constructor. For example, the constructor of class A:

public A(double d) { // some statements

}

is equivalent to

public A(double d) { super(); // some statements

}

public A() {

}

is equivalent to

public A() { super();

}

Example on the Impact of a Superclass without no-arg Constructor

class Fruit { public Fruit(String name) { System.out.println("Fruit constructor is invoked"); }}

public class Apple extends Fruit { public Apple(String name) { System.out.println(“Apple constructor is invoked"); }} 

Find out the error in the program: