20.5 Java polymorphism

Abstract Classes, Abstract Methods, Override Methods
Polymorphism
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1. Polymorphism
 What is Polymorphism?
 Types of Polymorphism
 Override Methods
 Overload Methods
2. Abstract Classes
 Abstract Methods
Table of Contents
2

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 From the Greek
 Such as a word having several different meanings
based on the context
 Often referred to as the third pillar of OOP, after
encapsulation and inheritance
What is Polymorphism?
5
Polys
(many)
Morphe
(shape/forms)
Polymorphos

 Ability of an object to take on many forms
Polymorphism in OOP
6
public interface Animal {}
public abstract class Mammal {}
public class Person extends Mammal implements Animal {}
Person IS-A Person
Person IS-A Mammal Person IS-AN Object
Person IS-AN Animal

 Variables are saved in reference type
 You can use only reference methods
 If you need object method you need to cast it or override it
Reference Type and Object Type
7
public class Person extends Mammal implements Animal {}
Animal person = new Person();
Mammal personOne = new Person();
Person personTwo = new Person();
Reference Type Object Type

 Check if object is an instance of a specific class
if (peter.getClass() == Person.class) {
((Person) peter).getSalary();
}
Keyword - instanceof
8
Mammal george = new Person();
Person peter = new Person();
Cast to object type and use its methods
if (george instanceof Person) {
((Person) george).getSalary();
}
Check object type of person

 Runtime polymorphism
 Compile time polymorphism
Types of Polymorphism
9
public class Shape {}
public class Circle extends Shape {}
public static void main(String[] args) {
Shape shape = new Circle();
}
int sum(int a, int b, int c){}
double sum(Double a, Double b){}
Method
overloading
Method
overriding

 Also known as Static Polymorphism
 Argument lists could differ in
 Number of parameters
 Data type of parameters
 Sequence of Data type of parameters
Compile Time Polymorphism
10
static int myMethod(int a, int b) {}
static Double myMethod(Double a, Double b) {}
Method overloading

 Create a class MathOperation, which should have method add()
 Must be invoked with two, three or four integers
Problem: MathOperation
11
MathOperation
+add(int a, int b): int
+add(int a, int b, int c): int
+add(int a, int b, int c, int d): int
Check your solution here :https://judge.softuni.bg/Contests/1592/Polymorphism-Lab

Solution: Math Operation
12
public class MathOperation {
public int add(int a, int b) {
return a + b;
}
public int add(int a, int b, int c) {
return a + b + c;
}
public int add(int a, int b, int c, int d) {
return a + b + c + d;
}
}

 Overloading can take place in the same class or in its sub-class
 Constructors in Java can be overloaded
 Overloaded methods must have a different argument list
 Overloaded method should always be the part of the same
class (can also take place in sub class), with same name but
different parameters
 They may have the same or different return types
Rules for Overloading Method
13

 Using of override method
Runtime Polymorphism
14
public static void main(String[] args) {
Rectangle rect = new Rectangle(3.0, 4.0);
Rectangle square = new Square(4.0);
System.out.println(rect.area());
System.out.println(square.area());
}
Method
overriding

 Also known as Dynamic Polymorphism
Runtime Polymorphism
15
public class Square extends Rectangle {
@Override
public Double area() {
return this.a * this.a;
}
}
Method overriding
public class Rectangle {
public Double area() {
return this.a * this.b;
}
}

 Overriding can take place in sub-class
 Argument list must be the same as that of the parent method
 The overriding method must have same return type
 Access modifier cannot be more restrictive
 Private, static and final methods can NOT be overriden
 The overriding method must not throw new or broader
checked exceptions
Rules for Overriding Method
16

Abstract Classes
17
Shape
Circle Rectangle

Abstract Classes
 Abstract class can NOT be instantiated
 An abstract class may or may not include abstract
methods
 If it has at least one abstract method, it must be
declared abstract
 To use an abstract class, you need to inherit it
18
public abstract class Shape {}
public class Circle extends Shape {}
Shape shape = new Shape(); // Compile time error
Shape circle = new Circle(); // polymorphism

Problem: Shapes
19
Shape
-Double perimeter
-Double area
+getPerimeter()
#setPerimeter(Double perimeter)
+calculatePerimeter
+calculateArea
Rectangle
-Double height
-Double width
+calculatePerimeter
+calculateArea
Circle
-Double radius
+calculatePerimeter
+calculateArea
Encapsulate
area

Solution: Shapes
20
public abstract class Shape {
private Double perimeter;
private Double area;
protected void setPerimeter(Double perimeter) {
this.perimeter = perimeter;
}
public Double getPerimeter() { return this.perimeter; }
protected void setArea(Double area) {this.area = area; }
public Double getArea() { return this.area; }
protected abstract void calculatePerimeter();
protected abstract void calculateArea();
}

Solution: Shapes
21
public class Rectangle extends Shape {
//TODO: Add fields
public Rectangle(Double height, Double width) {
this.setHeight(height); this.setWidth(width);
this.calculatePerimeter(); this.calculateArea(); }
//TODO: Add getters and setters
@Override
protected void calculatePerimeter() {
setPerimeter(this.height * 2 + this.width * 2); }
@Override
protected void calculateArea() {
setArea(this.height * this.width); } }

Solution: Shapes
22
public class Circle extends Shape {
private Double radius;
public Circle (Double radius) {
this.setRadius(radius);
this.calculatePerimeter();
this.calculateArea();
}
public final Double getRadius() {
return radius;
}
//TODO: Finish encapsulation
//TODO: Override calculate Area and Perimeter
}

 …
 …
 …
Summary
23
 Polymorphism - Definition and Types
 Override Methods
 Overload Methods
 Abstraction
 Classes
 Methods

 https://softuni.bg/modules/59/java-advanced

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20.5 Java polymorphism

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