/** 懒汉模式* 懒汉线程不安全* 懒汉加载慢，但是在内存作用域短，节省空间*/
public class SingletonA {//SinglettonA的唯一实例private static SingletonA instance = null;//将构造函数私有,防止外界构造SingletonA实例private SingletonA() {}//获取SingletonA的唯一实例，用synchronized关键字保证某一时刻只有一个线程调用此方法public static synchronized SingletonA getInstance(){//如果instance为空，便创建一个新的SingletonA实例，否则，返回已有的实例if(instance == null){instance = new SingletonA();}return instance;}public void print(){System.out.println("我是懒汉模式!");}public static void main(String[] args) {SingletonA s1 = SingletonA.getInstance();SingletonA s2 = SingletonA.getInstance();System.out.println(s1 == s2);}
}

/** 饿汉模式* 饿汉线程安全* 饿汉加载块，但是在内存中作用域长, 不节省空间*/
public class SingletonB {//SingletonB的唯一实例private static SingletonB instance = new SingletonB();//将构造函数私有，防止外界构造SingletonB实例private SingletonB() {}//获取SingletonB的实例public static SingletonB getInstance() {return instance;}public void print() {System.out.println("我是饿汉模式");}public static void main(String[] args) {SingletonB s1 = SingletonB.getInstance();SingletonB s2 = SingletonB.getInstance();System.out.println(s1 == s2);}}

public class UserFactory {public static User createUser(int i){//如果输入的是1,就创建它的子类，否则就创建父类if(i == 1){return new Alices();}return new User();}
}

public interface SellHouse {  void sell(double money);
}  

public class Hoster implements SellHouse {  @Override  public void sell(double money) {  System.out.println("祝你居住愉快");  }
}  

public class Medium implements SellHouse {  SellHouse hoster=new Hoster();  @Override  public void sell(double money) {  if(money>=1000){  hoster.sell(money);  }else{  System.out.println("你的价格太低了");  }  }
}  

public class Renter {  public static void main(String[] args) {  SellHouse renter=new Medium();  renter.sell(500);  }
}