java Singleton

//方法一
public class User {
    public static final User INSTANCE = new User();
    private User(){}
}

//调用
User user = User.INSTANCE;

//方法二
public class User {
   private static final User INSTANCE = new User();
   private User(){}
   public User getInstance(){
     return INSTANCE;
  }
}

//调用
User user = User.getInstance();