Decorator pattern

The decorator adds new functionality to an existing object without altering its structure.

decorator vs inheritance

Yes, same thing can be achieved through inheritance, but

• adding new functionality to a given class hierarchy may explode class hierarchy
• inheritance not always possible, for example final classes in Java
• decorator can operate on any implementation of a given interface
• testability, you don't need test every subclass
• decorator pattern encourages developers to write code that adheres to the SOLID design principles

example

interface Shape {
    void draw();
}

// draw any shape in frame
public class FrameShapeDecorator implements Shape {
    private Shape shape; 

    // shape can be rectangle, circle and etc
    public FrameShapeDecorator(Shape shape) {
        this.shape = shape;
    }

    @Override
    public void draw() {
        shape.draw();
        drawFrame();
    }

    // new functionality
    private void drawFrame() {/* ... */}
}