Abstract classes

Similar to Interfaces

Allows for abstraction.

• e.g: There are many 2D shapes. They all have an area and perimeter but they are all calculated differently.
• Abstraction gives a blueprint for a class to define some behaviors instead of copying them between several classes

Note:

• All abstract members must be implemented in order to compile (unless the class that implements the abstract class is also abstract)
• You cannot instance abstract classes (no “newing”) unless it’s a class that implements it
• The type of a variable can be an abstract class but it must be set to an instance of an implemented class.

// Allowed
Shape a = new Square();
// Not allowed; shape is abstract.
Shape b = new Shape();
// Allowed, too
Square c = new Square();

Abstract Methods

todo

public class Shape {
	public abstract double getArea();
}
public class Square extends Shape {
	int l = 5, w = 10; // example values
	
	@Override
	public double getArea() {
		return l * w;
	}
}