COMP200 INHERITANCE OOP using Java, from slides by Shayan Javed 2 - - PowerPoint PPT Presentation

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COMP200 INHERITANCE OOP using Java, from slides by Shayan Javed 2 - - PowerPoint PPT Presentation

Sep 09, 2023 467 likes •1.2k views

1 1 COMP200 INHERITANCE OOP using Java, from slides by Shayan Javed 2 Inheritance Derive new classes ( subclass ) from existing ones ( superclass ). Only the Object class (java.lang) has no superclass Every class =

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COMP200





INHERITANCE

OOP using Java, from slides by Shayan Javed

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Inheritance

Derive new classes (subclass) from existing

  • nes (superclass).

Only the Object class (java.lang) has no

superclass

Every class = subclass of Object.

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Inheritance

Code reuse – methods and properties. Use classes to model objects of the same type Define general class

Then define specialised classes.

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Inheritance

Can only inherit from one class. Can inherit from multiple classes in other

languages (C++)

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Example

Let’s say you create the following classes:

Circle Rectangle Triangle etc.

Share certain properties, but also specialised

properties

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Example

public class GeometricObject { private String Color; private String name; private float area; // constructors... public GeometricObject(String color, String name) { … } // get/set methods, etc. }

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Example

public class Circle extends GeometricObject { private double radius; // specific property public Circle(double radius, String color, String name) { this.radius = radius; this.color = color; this.name = name; } // get/set methods, etc. public double getArea() { return radius * radius * Math.PI; } }

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Example

An error in the constructor!

public Circle(double radius, String color, String name) { this.radius = radius; this.color = color; this.name = name; }

Why can’t we use “this.color” and

“this.name”?

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Example

They are private properties. NOT accessible by subclasses. Should use setColor() instead. public methods/properties inherited.

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Example

public class Rectangle extends GeometricObject { private double width, height; // specific properties public Rectangle(double w, double h, String color, String name) { this.height = h; this.width = w; setColor(color); setName(name); } // get/set methods, etc. public double getArea() { return width * height; } }

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The super keyword

Are constructors inherited?

  • Yes. super used for:

calling a super class constructor calling a super class method

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The super keyword

Constructors: super() calls the default constructor super(arguments) calls the constructors

according to the arguments

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The super keyword

Constructors:

public Circle(double radius, String color, String name) { super(color, name); this.radius = radius; }

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The super keyword

Constructors:

public Circle(double radius, String color, String name) { super(color, name); this.radius = radius; } public GeometricObject(String color, String name) { this.color = color; this.name = name; }

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The super keyword

What if we don’t add the super(..) constructor call?

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The super keyword

public Circle(double radius, String color, String name) { this.radius = radius; }

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The super keyword

public Circle(double radius, String color, String name) { this.radius = radius; } Is the same as: public Circle(double radius, String color, String name) { super(); this.radius = radius; }

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The super keyword

Called Constructor Chaining

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The super keyword

Calling the super class methods super.method(parameters)... Optional

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Protected fields

Using get/set methods to access properties of

the superclass is cumbersome…

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The protected keyword

Often want subclasses to directly access

properties/methods

Use protected instead of private/public Subclasses can now directly access

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Private

public class GeometricObject { private String Color; private String name; private float area; // constructors... // get/set methods, etc. }

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The protected keyword

public class GeometricObject { protected String Color; protected String name; protected float area; // constructors... // get/set methods, etc. }

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The protected keyword

public class Circle extends GeometricObject { private double radius; // specific property public Circle(double radius, String color, String name) { this.radius = radius; this.color = color; this.name = name; } }

Now works!

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Polymorphism

Important aspect of OOP

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Polymorphism

Important aspect of OOP “capability of an action or method to do

different things based on the object that it is acting upon. ”

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Polymorphism

Important aspect of OOP “capability of an action or method to do different

things based on the object that it is acting upon. ”

“characteristic of being able to assign a different

meaning or usage to something in different contexts - specifically, to allow an entity such as a variable, a function, or an object to have more than one form.”

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Polymorphism

1.

Overriding Methods

2.

Overloaded methods

3.

Dynamic (late) method binding

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Overriding Methods

Sometimes need to overwrite methods written

in the superclass

Re-define the method in the subclass.

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Overriding Methods

The Object class and toString() toString() = String output when you print the

  • bject
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Overriding Methods

The Object class and toString() toString() = String output when you print the

  • bject

Object class has default toString() method

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The toString() method

public String toString() { String str; ... return str; }

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The toString() method

Circle circle1 = new Circle(“Circle1”, “Red”, 3.5); System.out.println(circle1);

Output:

?

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The toString() method

Circle circle1 = new Circle(“Circle1”, “Red”, 3.5); System.out.println(circle1);

Output:

Circle@19821f

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The toString() method

So override the method

public String toString() { String str = “”; str += “Circle: ” + getName() + “,”; str += “Color: ” + getColor() + “,”; str += “Radius: ” + getRadius(); return str; }

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The toString() method

Circle circle1 = new Circle(“Circle1”, “Red”, 3.5); System.out.println(circle1);

Output:

Circle: Circle1,Color: Red,Radius: 3.5

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The equals() method

The Object class and equals() equals (Object obj) = returns whether the

  • bject passed in and this one are the same
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The equals() method

Default implementation:

public boolean equals(Object obj) { return (this == obj); } Compares references

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The equals() method

Circle circle1 = new Circle(3.5); Circle circle2 = new Circle(3.5);

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The equals() method

Circle circle1 = new Circle(3.5); Circle circle2 = new Circle(3.5);

circle1.equals(circle2); // ?

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The equals() method

Circle circle1 = new Circle(3.5); Circle circle2 = new Circle(3.5);

circle1.equals(circle2); // returns

false!

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The equals() method

Circle circle1 = new Circle(3.5); Circle circle2 = new Circle(3.5);

circle1.equals(circle2); // returns

false!

Circle circle3 = circle1; circle1.equals(circle3);

// ?

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The equals() method

Circle circle1 = new Circle(3.5); Circle circle2 = new Circle(3.5);

circle1.equals(circle2); // returns

false!

Circle circle3 = circle1; circle1.equals(circle3);

// returns true!

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The equals() method

Override it

public boolean equals(Object obj) { if (obj instanceof Circle) return this.radius == ((Circle)obj).getRadius(); else return false; }

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The instanceof operator

Test if object is of a specified type

if (object instanceof Class)

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The instanceof operator

Test if object is of a specified type

if (object instanceof Class)

Example:

Circle circle1 = new Circle(3.4); if (circle1 instanceof Circle) { … }

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The instanceof operator

Testing with the SuperClass:

if (subclassObj instanceof SuperClass) { … }

?

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The instanceof operator

Testing with the SuperClass:

if (Circle instanceof GeometricObject) { … }

?

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The instanceof operator

Testing with the SuperClass:

if (subclassObj instanceof SuperClass) { … }

returns true!

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The instanceof operator

Example:

Circle circle1 = new Circle(3.4);

if (circle1 instanceof GeometricObject) { … }

returns true!

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The instanceof operator

What if we write:

if (SuperClassObj instanceof subclass) { … }

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The instanceof operator

if (SuperClassObj instanceof subclass) { … }

returns true as long as the object is an instance

  • f the subclass
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The instanceof operator

Example:

Object is a superclass of Circle

public boolean equals(Object obj) { if (obj instanceof Circle) return this.radius == ((Circle)obj).getRadius(); else return false; }

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The instanceof operator

Be careful with null objects

String s = null; s instanceof String = false

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Object casting

So we can determine

if (SuperClassObj instanceof subclass)

Convert it to subclass by casting

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Object casting

So we can determine

if (SuperClassObj instanceof subclass)

Convert it to subclass by casting Can now access members/properties

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Object casting

Once again...

public boolean equals(Object obj) { if (obj instanceof Circle) { return this.radius == ((Circle)obj).getRadius(); } else return false; }

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Object casting

Once again...

public boolean equals(Object obj) { if (obj instanceof Circle) { Circle circle = (Circle)obj; // casting return this.radius == circle.getRadius(); } else return false; }

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Overloading Methods

Looked at it before. Can have multiple methods with the same

name.

But different parameters, return type

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Dynamic binding

Object type is determined at run-time ability of a program to resolve references to

subclass methods at runtime.

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Dynamic binding

public class GeometricObject { protected String Color; protected String name; protected float area; public float getArea() { return -1; } }

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Dynamic binding

// Circle public class Circle { public float getArea() { return Math.PI * radius * radius; } } // Rectangle public class Rectangle { public float getArea() { return width * height; } }

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Dynamic binding

GeometricObject[] objs = new GeometricObject[2];

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Dynamic binding

GeometricObject[] objs = new GeometricObject[2];

  • bjs[0] = new Circle(3);
  • bjs[1] = new Rectangle(1, 2);
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Dynamic binding

GeometricObject[] objs = new GeometricObject[2];

  • bjs[0] = new Circle(3);
  • bjs[1] = new Rectangle(1, 2);

for (GeometricObject i : objs) { System.out.println(i.getArea()); }

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Dynamic binding

GeometricObject[] objs = new GeometricObject[2];

  • bjs[0] = new Circle(3);
  • bjs[1] = new Rectangle(1, 2);

for (GeometricObject i : objs) { System.out.println(i.getArea()); }

What’s the output?

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Dynamic binding

Output: 9.0 2.0

The subclass getArea() methods are called. At run-time, Java dynamically determines the class.

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final Classes

Sometimes classes shouldn’t be allowed to be

extended.

The String class for example. Use the keyword

“final” final class String {

... }

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final Methods

Methods which can’t be overridden in the

subclasses.

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Summary...

Inheritance

super protected

Polymorphism

Overloading Overwriting Dynamic binding

instanceof Object casting final classes and methods

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Next lecture...

Abstract Classes Interfaces