举几个单例模式的例子——茴香豆的茴字有几种写法？

勤加载（饿汉模式）

 1 public class EagerSingleton {
 2   
 3   private EagerSingleton() {
 4   }
 5   
 6   private static EagerSingleton instance = new EagerSingleton();
 7   
 8   public static EagerSingleton getInstance() {
 9     return instance;
10   }
11 }

勤加载（static块）

 1 public class StaticBlockSingleton {
 2   
 3   private StaticBlockSingleton() {
 4   }
 5   
 6   private static StaticBlockSingleton instance;
 7   
 8   static {
 9     instance = new StaticBlockSingleton();
10   }
11   
12   public static StaticBlockSingleton getInstance() {
13     return instance;
14   }
15 }

懒加载（double-checked locking using volatile）

 1 public class DoubleCheckedSingleton {
 2   
 3   private DoubleCheckedSingleton() {
 4   }
 5   
 6   private volatile static DoubleCheckedSingleton instance;
 7   
 8   public static DoubleCheckedSingleton getInstance() {
 9     
10     if (instance == null) {
11       synchronized (DoubleCheckedSingleton.class) {
12         if (instance == null) {
13           instance = new DoubleCheckedSingleton();
14         }
15       }
16     }
17     return instance;
18   }
19 }

懒加载（内部静态类）

 1 public class InnerClassSingleton {
 2   
 3   private InnerClassSingleton() {
 4   }
 5   
 6   private static class Holder {
 7     private static InnerClassSingleton instance = new InnerClassSingleton();
 8   }
 9   
10   public static InnerClassSingleton getInstance() {
11     return Holder.instance;
12   }
13 }

懒加载（枚举）

 1 public enum EnumSingleton {
 2   
 3   INSTANCE;
 4   
 5   private Singleton instance;
 6   
 7   EnumSingleton() {
 8     instance = new Singleton();
 9   }
10   
11   public Singleton getInstance() {
12     return instance;
13   }
14   
15 }