Interfaces

Here's an example to show how an interface (Shape) is defined, implemented (Square, Circle) and used (Main). Note: how Object.toString() is overriden and is used by printInfo()

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package org.megha.blog.example.part5;

/** An Interface for a shape that has an area and a perimeter */
public interface Shape {
	public double area();
	public double perimeter();
}

package org.megha.blog.example.part5;

/** A class that represents a circle */
public class Circle implements Shape {

	double radius;

	public Circle(double radius) {
		this.radius = radius;
	}

	public double area() {
		return Math.PI* radius*radius;
	}

	public double perimeter() {
		return 2*Math.PI*radius;
	}

	@Override
	public String toString() {
		return "circle of radius " + radius;
	}
}

package org.megha.blog.example.part5;

/** A square shape */
public class Square implements Shape {

	private double side;

	public Square(double side) {
		this.side = side;
	}

	public double area() {
		return side * side;
	}

	public double perimeter() {
		return 4 * side;
	}

	@Override
	public String toString() {
		return "square of side " + side;
	}
}

package org.megha.blog.example.part5;

/** Creates a square and a circle and prints their area and perimeter */
public class Main {

	private static void printInfo(Shape s) {
		System.out.println("for shape: " + s);
		System.out.println("area: " + s.area());
		System.out.println("perimeter: " + s.perimeter());
	}

	public static void main(String args[]) {
		Square square1 = new Square(5);
		printInfo(square1);

		Circle circle1 = new Circle(7);
		printInfo(circle1);
	}
}

Output of this program:

for shape: square of side 5.0
area: 25.0
perimeter: 20.0
for shape: circle of radius 7.0
area: 153.93804002589985
perimeter: 43.982297150257104