享元模式

如想让某个类的一个实例能用来提供许多“虚拟实例”，就使用享元模式。

示例—景观设计

为客户的家庭景观做设计软件，需要新增树作为点缀。树有一个XY坐标位置，而且可以根据树的年龄动态地将自己绘制出来。用户可能要在他们的家庭景观设计中有非常非常多的树，所以每棵树的实例都要维护自己的状态，但引发了不易集中管理的问题。需要重新设计成一个实例就可以集中管理的模式。

UML图表示

享元模式-景观设计

代码演示

树

package Flyweight;

public class Tree {

    void display(int x, int y, int age){
        System.out.println("The "+ age  +" years old tree is being painted at " + x + "," + y + ". ");
    }
}

树管理类

package Flyweight;

public class TreeManager {
    private final int MAX_INDEX = 1000;
    int[][] treeArray = new int[MAX_INDEX][3];
    int currentIndex = 0;

    void newTree(int x, int y, int age){

        if (currentIndex == MAX_INDEX){
            System.out.println("Oops, you can't add any tree!!!");
            return;
        }

        treeArray[currentIndex][0] = x;
        treeArray[currentIndex][1] = y;
        treeArray[currentIndex][2] = age;
        currentIndex++;
    }

    void displayTrees(){
        Tree tree = new Tree();
        for (int i = 0; i <= currentIndex - 1; i++){
            tree.display(treeArray[i][0],treeArray[i][1],treeArray[i][2]);
        }
    }


}

测试代码

package Flyweight;

public class FlyweightDriver {
    public static void main(String[] args) {

        TreeManager treeManager = new TreeManager();

        treeManager.newTree(0,0,1);
        treeManager.newTree(0,1,1);
        treeManager.newTree(0,2,3);
        treeManager.newTree(0,3,2);
        treeManager.newTree(0,4,4);
        treeManager.newTree(0,5,2);
        treeManager.newTree(0,6,2);
        treeManager.newTree(0,7,3);
        treeManager.newTree(0,8,4);
        treeManager.newTree(0,9,2);

        treeManager.displayTrees();

    }
}

测试结果

The 1 years old tree is being painted at 0,0. 
The 1 years old tree is being painted at 0,1. 
The 3 years old tree is being painted at 0,2. 
The 2 years old tree is being painted at 0,3. 
The 4 years old tree is being painted at 0,4. 
The 2 years old tree is being painted at 0,5. 
The 2 years old tree is being painted at 0,6. 
The 3 years old tree is being painted at 0,7. 
The 4 years old tree is being painted at 0,8. 
The 2 years old tree is being painted at 0,9.