Structural Patterns Part 6/7: Flyweight Pattern

Structural Patterns Part 6/7: Flyweight Pattern

目录

• • Structural Patterns Part 67 Flyweight Pattern
  • 目录
  • Definition
  • UML Class Diagram
  • Implementation

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Definition

Flyweight Pattern 主要用来减少对象的创建次数从而降低内存占用，提升性能。Flyweight Pattern试图重用一些相似的对象并且存储起来(利用Map)，后续再有对象的创建请求时，先查一下Map中有没有，有的话直接返回该对象，没有的话再创建。

一句话：
Use sharing to support large numbers of fine-grained objects efficiently.

何时使用？当对象与对象之间非常相似，并且希望可以把已经创建的对象缓存起来重复使用时。

使用频率： Low

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UML Class Diagram

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Implementation

这里用到了围棋的例子。
棋盘上的棋子之间非常相似，除了位置不同之外就是黑白两色，如果一个棋盘上需要创建几百个对象实在是太浪费了，使用Flyweight Pattern。

// Shape.java
package designpatterns.structuralpatterns.flyweight;

public interface Shape {
    void draw();
}

// ShapeImpl.java
package designpatterns.structuralpatterns.flyweight;

class Chessman implements Shape {
    private int x;
    private int y;
    private String color;

    public Chessman(String color) {
        super();
        this.color = color;     
    }

    @Override
    public void draw() {
        if("BLACK".equalsIgnoreCase(color)){
            System.out.println("●" + toString());
        } else if("WHITE".equalsIgnoreCase(color)) {        
            System.out.println("○" + toString());
        } else {
            System.out.println("Black or White ?");
        }
    }

    public void setX(int x) {
        this.x = x;
    }

    public void setY(int y) {
        this.y = y;
    }

    @Override
    public String toString() {
        return " [X=" + x + ", Y=" + y + ", Color=" + color + "]";
    }
}

// FlyweightDemo.java
package designpatterns.structuralpatterns.flyweight;

import java.util.*;

public class FlyweightDemo {
    private static final String[] colors = {"BLACK", "WHITE"};

    public static void main(String[] args) {

        ChessBoard cb = new ChessBoard();

        for(int i = 0;i < 10; ++i) {
            Chessman chessman = cb.getChessman(getRandomColor());
            chessman.setX(getRandomXY());
            chessman.setY(getRandomXY());

            chessman.draw();
        }
        System.out.println("===========================");
        System.out.println("Chess Type Number: " + cb.getChessTypeNum());
    }

    public static String getRandomColor() {
        return colors[(int)(Math.random() * colors.length)];
    }

    public static int getRandomXY() {
        return (int)(Math.random() * 100);
    }
}

class ChessBoard {
    private HashMap<String, Shape> chessmenMap = new HashMap<>();

    public Chessman getChessman(String orogincolor) {
        String color = orogincolor.toUpperCase();
        Chessman chessman = (Chessman)chessmenMap.get(color);

        if(null == chessman) {
            chessman = new Chessman(color);
            chessmenMap.put(color, chessman);
            System.out.println("*** Creating a Chessman ***");
        }

        return chessman;        
    }

    public int getChessTypeNum() {
        return chessmenMap.size();
    }
}

// output
*** Creating a Chessman ***
○ [X=9, Y=28, Color=WHITE]
*** Creating a Chessman ***
● [X=55, Y=18, Color=BLACK]
● [X=92, Y=19, Color=BLACK]
○ [X=40, Y=40, Color=WHITE]
○ [X=43, Y=75, Color=WHITE]
○ [X=84, Y=24, Color=WHITE]
● [X=85, Y=12, Color=BLACK]
○ [X=9, Y=13, Color=WHITE]
● [X=27, Y=3, Color=BLACK]
○ [X=10, Y=76, Color=WHITE]
===========================
Chess Type Number: 2