单例模式（Singleton Pattern）

情景：只有一个皇帝（Emperor），三个臣子（Minister）向一个皇帝汇报。

public class Emperor 
{
    private static final Emperor emperor = new Emperor();
    
    private Emperor()
    {
        
    }
    
    public static Emperor getInstance()
    {
        return emperor;
    }
    
    public static void say()
    {
        System.out.println("我是皇帝A");
    }
}

public class Minister 
{

    public static void main(String[] args) 
    {
        for(int n=0; n<3; n++)
        {
            Emperor emperor = Emperor.getInstance();
            emperor.say();
        }
    }
}

扩展情况：

有三个皇帝，大臣可以像其中一个皇帝汇报

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


public class Emperor 
{
    private static int maxNumOfEmperor = 2;
    private static ArrayList<String> nameList = new ArrayList<String>();
    private static ArrayList<Emperor> emperorList = new ArrayList<Emperor>();
    
    private static int countNumOfEmperor = 0;
    
    static
    {
        for(int n=0; n<maxNumOfEmperor; n++)
        {
            emperorList.add(new Emperor("皇" + (n + 1)));
        }
    }
    
    private Emperor()
    {
        
    }
    
    private Emperor(String name)
    {
        nameList.add(name);
    }
    
    public static Emperor getInstance()
    {
        Random random = new Random();
        
        countNumOfEmperor = random.nextInt(maxNumOfEmperor);
        
        return emperorList.get(countNumOfEmperor);
    }
    
    public static void say()
    {
        System.out.println(nameList.get(countNumOfEmperor));
    }
}

public class Minister 
{

    public static void main(String[] args) 
    {
        int ministerNum = 5;
        
        for(int n=0; n<ministerNum; n++)
        {
            Emperor emperor = Emperor.getInstance();
            
            emperor.say();
        }
    }

}