Bài giảng Phát triển phần mềm phân tán - Bài 8: Polymorphism and generic programming - Vũ Thị Hương Giang
LESSON VIII.
Polymorphism and
generic programming
Vu Thi Huong Giang
SoICT (School of Information and
Communication Technology)
HUST (Hanoi University of Science
and Technology)
Objectives
• Master the polymorphism technique
• Understand the Java generic programming
Content
• Polymorphism
• Downcasting and upcasting
• Overloading
• Method call binding
• Generic programming
I. POLYMORPHISM
1. Example
Example
• An operation that can be performed on a 2DShape object can
also be performed on an object of one of three classes Triangle,
Circle, Quadrilateral.
– The super class 2DShape defines the common interface
– The subclasses Triangle, Circle, Quadrilateral have to follow this
interface (inheritance), but are also permitted to provide their own
implementations (overriding)
Once a method is requested
through the 2DShape class
reference, the objects 2DShape,
Triangle, Circle and Quadrilateral
respond differently
Object
Shape
….
2D Shape
3D Shape
Circle
Triangle
Quadrilateral
Sphere
Polyhedron
Example
public class 2DShape {
public void display() {
public class Point extends 2DShape {
private int x, y;
System.out.println("2D Shape");
...
}
public void display(){
}
System.out.print("(" + x + "," + y + ")");
}
}
public class Circle extends 2DShape{
public static final double PI =
public class Quadrilateral extends 2DShape {
private Point p1, p2, p3, p4;
3.14159;
private Point p;
private double r; //radious
.....
...
public void display(){
System.out.println("Quadrilateral: ");
p1.display(); p2.display();
p3.display(); p4.display();
System.out.println();
}
public void display(){
System.out.print("Circle: " +
r + ",");
p.display();
System.out.println();
}
}
}
Example
• There are many choice once a method is invoked
through a super class reference.
Object of super
Object of Circle class
class
Circle: 1, (0,0)
display()
Object of Point class
Application
program
2DShape
display()
(0,1)
display()
Object of Quadrilateral class
Quadrilateral:
(0,0), (0,1), (1,0), (1,1)
display()
2D Shape
7
Polymorphism
• Polymorphism means “many different forms” of
objects
– The ability of a reference variable to change behavior
according to what object instance it is holding.
• Objects of different subclasses are treated as
objects of a single super class
• Java choose the correct overridden method in the
appropriate sub class associated with the object
Exercise
public class 2DShapeUsage {
public static void main(String args[]) {
Point point = new Point();
Circle circle = new Circle();
Quadrilateral quadri = new Quadrilateral();
2DShape shape = new 2DShape();
// display() method of 2DShape class is called
shape.display(); // (1)
// shape refers to the Point object; display() method of Point class is called
shape = point; shape.display(); // (2)
// display() method of Circle class is called
shape = circle; shape.display(); // (3)
// display() method of Quadrilateral class is called
shape = quadri; shape.display(); // (4)
2DShape shape2 = new Point();
// display() method of Point class is called
shape2.display(); // (5)
}
}
II. DOWN CASTING AND
UP CASTING
Primitive type casting
• Java performs automatic primitive type casting when:
– Two types are compatible
– The destination type is larger then the source type
– Example:
• int i;
• double d = i;
• We have to perform manual primitive type casting
when:
– Two types are compatible
– The destination type is smaller then the source type
– Example:
• int i;
• byte b = i; byte b = (byte)i;
Reference type casting
• Java performs automatic reference type casting
when:
– Two types are compatible
– The destination type extends from the source type
Up casting
• We have to perform manual reference type
casting when:
– two types are compatible
– The source type extends from the destination type
Down casting
Up casting
• Substitute the reference of the sub class for the
reference of the super class in the inheritance
public class 2DShape {
public void display() {
// I
...
System.out.println("2D Shape");
Point point = new Point();
// 2 following statements are equivalent
2DShape shape = (2DShape) point;
2DShape shape = point;
}
}
public class Point extends 2DShape {
private int x, y;
...
public void displayPoint(){
// II
System.out.print(“Point”);
...
}
2DShape shape = new Point();
}
Up casting
• Java always remember what an object really is
during up casting
public class 2DShape {
public interface 2DShape {
public void display() {...}
public void display();
}
}
public class Point extends 2DShape {
public class Point implements 2DShape {
// interface’s methods
public void display() {...}
// class’ methods
public void displayPoint(){...}
}
public void displayPoint(){...}
}
...
...
Point point = new Point();
2DShape shape = point;
shape.display(); //OK
shape.displayPoint(); //impossible to call
Point point = new Point();
2DShape shape = point;
shape.display(); //OK
shape.displayPoint(); //impossible to call
14
Exercise: Implicit subclass object to
super class object conversion
• Refer to a super class object with a super class
reference
– Example ?
• Refer to a sub class object with a subclass reference
– Example ?
• Refer to a sub class object with a super class
reference is safe, but such code can only refer to
super class members
– Example ?
• Refer to a super class object with a subclass reference
is a syntax error
– Example ?
Downcasting
• Substitute the reference of the super class for the
reference of the sub class in the inheritance
hierarchy.
May not always suceed
public class 2DShape {
public void display() {
...
2DShape shape = new Point();
Point point = (Point) shape; // 1
point.displayPoint(); //possible to call
System.out.println("2D Shape");
}
}
public class Point extends 2DShape {
...
Point point = new Point();
2DShape shape = point;
if (shape instanceof 2DShape){
Point tempObj= (Point)shape; // 2
tempObj.displayPoint(); //possible to call
}
private int x, y;
...
public void displayPoint(){
System.out.print(“Point”);
}
}
16
Type Compatibility
public class 2DShape {
• Does it work ?
public void display() {
System.out.println("2D Shape");
...
}
Circle circle = new Circle();
2DShape shape = circle;
shape.changeWidth(20);
}
Syntax error
public class Circle extends 2DShape{
public static final double PI = 3.14159;
private Point p;
• How can you make it
work without changing
the classes definitions?
private double r; //radious
...
...
public void changeRadious(double rad){
Circle circle = new Circle();
2DShape shape = circle;
((Circle)shape).changeRadious(20); //ok
r = rad;
}
}
Problem with casting
public class 2DShape {
• Does it work ?
public void display() {...}
}
Point point = new Point();
Circle circle = new Circle();
TwoDimensionShape shape = circle;
((Point)shape).displayPoint();
public class Point extends 2DShape {
private int x, y;
...
public void displayPoint(){...}
}
Runtime exception
public class Circle extends 2DShape{
public static final double PI = 3.14159;
private Point p;
private double r; //radious
...
public void changeRadious(double rad){...}
}
III. Method overloading
• Overloading defines two or more methods with
the same name in a class.
• Overloaded methods are distinguished by their
signature
– same method name
– different parameters or different number of parameters
• Example:
public class Point extends 2DShape {
public void display(){
System.out.print("(" + x + "," + y + ")");
}
public void display(int x) {
System.out.print("x-coordinate: " + x);
}
}
19
Example
public class Point extends 2DShape {
// 1
public void display(){
System.out.print("(" + x + "," + y + ")");
}
// 2
public void display(int x) {
System.out.print("x-coordinate: " + x);
}
// 3- syntax error: Creating overloaded methods 2, 3 with identical parameters lists
public void display(int y) {
System.out.print(“y-coordinate: " + y);
}
// 4- syntax error: Methods 1, 4 can not be distinguished by return type
public boolean display() {
System.out.print("x-coordinate: " + x);
return true;
}
}
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