Effective Java 第三条：用私有构造器或者枚举类型强化Singleton属性

Singleton指仅仅被实例化一次的类

public class Elvis {
  public static final Elvis INSTANCE = new Elvis();
  private Elvis() {
    ...
  }

  public void leaveTheBuilding() {
    ...
  }
}

public class Elvis {
  private static final Elvis INSTANCE = new Elvis();
  private Elvis() {
    ...
  }
  public static Elvis getInstance() {
    return INSTANCE;
  }
  public void leaveTheBuilding() {
    ...
  }
}

单元素枚举类型更加简洁，提供了序列化机制，绝对防止多次实例化

public enum Elvis {
  INSTANCE;
  public void leaveTheBuilding() {
    ...
  }
}

单元素的枚举类型已经成为实现Singleton的最佳方法

Relative Post—Java设计模式之singleton