闭门造车姚师傅

Polymorphism, the perspective of construct

Java
字数统计: 252阅读时长: 1 min
2017/05/27 Share

When polymorphism meet initialization of an object, what will happen.

Here is a pretty strange but reasonable example.

A piece of code

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/**
 * Created by boileryao on 5/27/2017.
 * Class: PolyConstructor
 */
public class PolyConstructor {
    public static void main(String[] args) {
        new ColorfulCircle("Blue");
    }
}

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

    Circle() {
        System.out.println("Circle, Before drawing");
        draw();
        System.out.println("Circle, After drawing");
    }
}

class ColorfulCircle extends Circle {
    String color;
    @Override
    void draw() {
        System.out.println("ColorfulCircle: " + color);
    }


    ColorfulCircle(String color) {
        this.color = color;
        System.out.println("ColorfulCircle, Before drawing");
        draw();
        System.out.println("ColorfulCircle, After drawing");

    }
}

Output of the code

Output:

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Circle, Before drawing
ColorfulCircle: null
Circle, After drawing
ColorfulCircle, Before drawing
ColorfulCircle: Blue
ColorfulCircle, After drawing

Explanation

Dynamic binding of method

​ Polymorphism, the parent’s method overrided by subclass before the construction of an object.

Construct of a object:

  • execute a sizeof() like operation to determine the memory needed, the set the allocated memory to ZERO.

    Here is the size of ColorfulCircle, containing field ‘color’.

  • Go upward to Object.class, then go down according the inherit level. Execute every class’s constructor to make the object more specific.

    Here, the inherit level is Object -> Circle -> ColorfulCircle, so make the allocated memory a Object first, then Circle, finally ColorfulCircle. Each step only do its own affair, other fileds owner by more specific classes will remain uninitialized(ZEROS, that is null for objects, 0 for numbers, false for boolean etc).

原文作者：Xiangyu Yao

原文链接：http://blog.boileryao.com/2017/05/poly-constructor/

发表日期：May 27th 2017, 8:42:07 pm

更新日期：November 20th 2018, 11:37:10 pm

版权声明：本文采用知识共享署名-非商业性使用 4.0 国际许可协议进行许可

CATALOG
  1. 1. A piece of code
  2. 2. Output of the code
  3. 3. Explanation
Total : 22
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