设计模式之开放封闭原则

Posted 2020-11-19 winter-is-coming

[Ivar Jacobson]: All systems change during their life cycles. This must be borne in mind when developing systems expected to last longer than the first version.

[Bertrand Meyer]:Software entities (classes, modules, functions, etc.) should be open for extension, but closed for modification.
软件实体（类、模块、函数等），应对扩展开放，但对修改封闭

开放封闭原则（Open Close Principle-OCP）(面向扩展开放，面向修改封闭）

面向扩展开放：也就是说模块的行为是能够被扩展的。当应用程序的需求变化时，可以使模块表现出全新的或与以往不同的行为，以满足新的需求。
面向修改封闭：模块的源代码是不能被侵犯的，任何人都不允许修改已有源代码。除非是改bug。

人话：

扩展新功能比较方便，不需要修改原有代码
优化旧功能，改动也应该在很小的范围内，不应该影响暴露出去的代码

OCP是面向对象的核心，关键在于抽象。

Talk is cheap, show me the code.

举一个计算多个图形周长之和的例子：

 // 对修改不封闭，对扩展也不开放
    public static double edgeLengthSum(List<Object> objects) {
        if (objects == null || objects.isEmpty()) {
            return -1;
        }

        double sum = 0;

        for (Object shape : objects) {
            if (shape == null) {
                continue;
            }
            if (shape instanceof Rectangle) {
                sum += (((Rectangle) shape).getHeight() + ((Rectangle) shape).getWidth()) * 2;
            } else if (shape instanceof Circle) {
                sum += 2 * Math.PI * ((Circle) shape).getRadius();
            }
//            else if (shape instanceof Triangle) {
//                sum += ((Triangle) shape).getSideLen() * 3;
//            }
        }

        return sum;
    }

修改后：

 
// 抽象是关键
abstract static class Shape {
     public abstract double edgeLength();
}


// 对修改封闭，对扩展也开放
    public static double edgeLengthSum(List<Shape> shapes) {

        if (shapes == null || shapes.isEmpty()) {
            return -1;
        }

        double sum = 0;

        for (Shape shape : shapes) {
            if (shape == null) {
                continue;
            }
            sum += shape.edgeLength();
        }

        return sum;
    }