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）

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

勤加载（static块）

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

懒加载（double-checked locking using volatile）

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

懒加载（内部静态类）

public class InnerClassSingleton {
  
  private InnerClassSingleton() {
  }
  
  private static class Holder {
    private static InnerClassSingleton instance = new InnerClassSingleton();
  }
  
  public static InnerClassSingleton getInstance() {
    return Holder.instance;
  }
}

懒加载（枚举）

public enum EnumSingleton {
  
  INSTANCE;
  
  private Singleton instance;
  
  EnumSingleton() {
    instance = new Singleton();
  }
  
  public Singleton getInstance() {
    return instance;
  }
  
}