單例模式怕是很多程式設計師使用最多的設計模式了，面向物件的23種設計模式點這裡。廢話不多說，直接上程式碼：

public class Singleton {  //1.懶漢型
    private static Singleton instance;  
    private Singleton (){}  
    public static synchronized Singleton getInstance() {  
    if (instance == null) {  
        instance = new Singleton();  
    }  
    return
 
 instance;  
    }  
}  

public class Singleton {  //2.餓漢型
    private static Singleton instance = new Singleton();  
    private Singleton (){}  
    public static Singleton getInstance() {  
    return instance;  
    }  
} 

public class Singleton {  //3.內部型別，可看做餓漢變種
    private static class SingletonHolder {  
    private
 
 static final Singleton INSTANCE = new Singleton();  
    }  
    private Singleton (){}  
    public static final Singleton getInstance() {  
    return SingletonHolder.INSTANCE;  
    }  
}  

public class Singleton {  //4.雙重校驗鎖，1.5以後可用
    private static Singleton singleton;  
    private Singleton
 
 (){}  
    public static Singleton getSingleton() {  
    if (singleton == null) {  
        synchronized (Singleton.class) {  
        if (singleton == null) {  
            singleton = new Singleton();  
        }  
        }  
    }  
    return singleton;  
    }  
}  

public enum Singleton {  //5.列舉型，Effective Java作者提倡的。可防止反序列化
    INSTANCE;  
    public void whateverMethod() {  
    }  
}  

public class Singleton {  //6.靜態型，全部靜態方法和屬性
    private Singleton (){}  
    public static void whateverMethod() {  
    }  
}  

現在有六種，那麼推薦哪一種呢。其實我個人比較懶，所以推薦5和6這種字少的。6呢可以繼承，但重寫方法就違反了里氏替換原則，所以繼承的時候要記得只採取不同的子類擴充套件不同的方法。