设计模式之：单例模式

单例模式的定义：Ensure a class has only one instance，and provides a global point of access it。也就是说，如果将一个类设计成单例模式，那么它将只允许一个实例。单例模式算是常见设计模式中最为简单直接的，下面可以看一下单例模式的通用代码：

package net.jerryblog.dp.sigleto;

public class Singleton {
    private static final Singleton instance = new Singleton();
    private Singleton(){
        
    }
    public static Singleton getInstance() {
        return instance;
    }
}

上面的情况下，一旦加载类，就会初始化instance，如果需要在用到instance的时候再实例化呢，可以采用懒汉式单例模式：

package net.jerryblog.dp.sigleto;

public class SingletonLazy {
    private static SingletonLazy instance = null;
    
    private SingletonLazy() {
        
    }
    
    public static synchronized SingletonLazy getInstance() {
        if(instance == null) {
            instance = new SingletonLazy();
        }
        return instance;
    }
    
}

单例模式还有一种扩展形式：有上限的多例模式。看代码：（这个例子是《设计模式之禅》里面的。）

package net.jerryblog.dp.sigleto;

import java.util.ArrayList;
import java.util.List;
import java.util.Random;

public class Emperor {
    
    private static List<String> nameList = new ArrayList<String>();
    private static List<Emperor> empList = new ArrayList<Emperor>(); 
    private static int curEmpNum = 0;
    private static int maxEmpNum = 3;
    
    private Emperor() {
        
    }
    
    private Emperor(String name) {
        nameList.add(name);
    }
    
    static {
        for(int i = 0; i < maxEmpNum; i++) {
            empList.add(new Emperor("皇帝：" + (i+1)));
        }
    }
    
    public static Emperor getInstance() {
        Random random = new Random();
        int index = random.nextInt(maxEmpNum);
        return empList.get(index);
    }
}