1.单例模式Singleton

单例模式：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。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);
	}
}