设计模式六原则 - 开闭原则

public class Line{
    public void draw(){
        System.out.println("draw a line.....");
    }
}

public class Circle{
    public void draw() {
        System.out.println("draw a circle.....");
    }
}

public class Canvas {
	/**
	 * 绘制
	 * 增加绘制线条，则修改CanvasDraw这个类源码，增加判断逻辑，违反开闭原则
	 */
	 public void draw(String type){
	    if(type.equals("circle")){
	       Circle circle = new Circle();
	       circle.draw();
	    }else if( type.equals("line")){
	       Line line = new Line();
	       line.draw();
	    }
	 }

}

public class Main {

    public static void main(String[] args){
    		Canvas canvas = new Canvas();
    		canvas.draw("circle");
    		canvas.draw("line");
    }
}

public class Line extends AbstractGraph{
    @Override
    public void draw(){
        System.out.println("draw a line.....");
    }
}

public class Circle extends AbstractGraph{
    @Override
    void draw() {
        System.out.println("draw a circle.....");
    }
}

public class Canvas {
    AbstractGraph abstractGraph;
    public void draw(){
        abstractGraph.draw();
    }
    public void setAbstractGraph(AbstractGraph graph){
        abstractGraph = graph;
    }

}

public class Main {
    public static void main(String[] args){
        //符合开闭原则的设计
        Canvas draw = new Canvas();
        draw.setAbstractGraph(new Line());
        draw.draw();
        draw.setAbstractGraph(new Circle());
        draw.draw();

    }
}