单例模式

单例模式的俩种方式：
饿汉式
public class Singleton {
  private static Singleton instance=new Singleton();
  private Singleton(){}
  static Singleton getInstance() {
      return instance;
  }
}
懒汉式
public class Singleton {
  private static Singleton instance=null;
  private Singleton(){}
  static Singleton getInstance() {
      if(instance==null)
      instance=new Singleton();
      return instance;
  }
}