蝇量模式（Flyweight Pattern）

蝇量模式：让某个类的一个实例能用来提供许多“虚拟实例”。

在有大量对象时，有可能造成内存溢出，把其中共同的部分抽象出来，如果有相同的业务请求，直接返回在内存中已有的对象，避免重复创建。（JAVA中的String，如果没有则创建一个字符串保存在字符串常量池里，否则直接返回）

类图：

public interface Planet {
    public abstract void display(int x, int y);
}

public class Tree implements Planet {
    private int x;
    private int y;
    private String height;
    
    public int getX() {
        return x;
    }

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

    public int getY() {
        return y;
    }

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

    public Tree(String height) {
        this.height = height;
    }
    
    @Override
    public void display(int x, int y) {
        System.out.println(height + "Tree in x=" + x + " y=" + y);
    }

}

public class PlanetFactory {
    private static final HashMap<String, Planet> planetMap = new HashMap<String, Planet>();
    
    public static Planet getTree(String height) {
        Tree tree = (Tree) planetMap.get(height);
        if(tree == null) {
            tree = new Tree(height);
            planetMap.put(height, tree);
            System.out.println("Creating tree of height : " + height);//只有在map中找不到对象才创建一个，将这句话打印出来
        }
        return tree;
    }
}

public class Client {
    private static final String height[] = {"Tall", "Medium", "Short"};
    private static String getRandomHeight() {
        return height[(int) (Math.random() * height.length)];
    }
    private static int getRandomX() {
        return (int) (Math.random() * 100);
    }
    private static int getRandomY() {
        return (int) (Math.random() * 100);
    }
    public static void main(String[] args) {
        for(int i = 0; i < 20; i++) {
            Tree tree = (Tree) PlanetFactory.getTree(getRandomHeight());
            tree.display(getRandomX(), getRandomY());
        }

    }

}

某次测试结果：

Creating tree of height : Tall
TallTree in x=57 y=88
TallTree in x=10 y=24
Creating tree of height : Short
ShortTree in x=8 y=61
ShortTree in x=92 y=27
ShortTree in x=58 y=73
TallTree in x=22 y=25
ShortTree in x=95 y=3
Creating tree of height : Medium
MediumTree in x=0 y=1
MediumTree in x=95 y=38
MediumTree in x=35 y=56
TallTree in x=67 y=5
MediumTree in x=35 y=47
ShortTree in x=37 y=32
ShortTree in x=40 y=94
MediumTree in x=43 y=11
MediumTree in x=4 y=94
ShortTree in x=68 y=70
ShortTree in x=62 y=56
ShortTree in x=13 y=69
MediumTree in x=21 y=49