设计模式的热点问题

1.单例模式

1.单例类只能有一个实例
2.单例类必须自己创建自己的唯一实例
3.单例类必须给所有其他对象提供这一实例
单例模式确保某一个类只有一个实例,选择单例模式就是为了避免不一致状态.
所以单例对象通常作为程序存放配置信息的载体.
1.饿汉式

public class Singlenton {
    private static Singlenton singlenton = new Singlenton();
    private Singlenton(){ }
    public Singlenton getInstance(){
        return singlenton;
    }
}

2.懒汉式

public class Singlenton {
    private static Singlenton singlenton = null;
    public static synchronized Singlenton getInstance(){
        if (singlenton == null){
            singlenton = new Singlenton();
        }
        return singlenton;
    }
}

3.枚举

enum Singlenton2{
        Singlenton2;
    }

4.静态内部类

public class Singlenton {
    private static class SingletonHolder{
        private static final Singlenton INSTANCE =new Singlenton();
    }
    private Singlenton(){}
    public static final Singlenton getInstance(){
        return SingletonHolder.INSTANCE;
    }
}

5.静态代码块

class Singlenton4{
        static {
            Singlenton4 singlenton4 = new Singlenton4();
        }
    }

2.工厂模式

1.简单工厂模式:不利于产生系列产品
2.工厂方法模式:又称多形性工厂
3.抽象工厂模式

工厂方法模式

//抽象产品角色
public interface Moveable {
    void run();
}
//具体产品角色
class Plane implements Moveable{
    @Override
    public void run() {
        System.out.println("Plane move");
    }
}
//抽象工厂
abstract class VehicleFactory{
    abstract Moveable create();
}
//具体工厂
class PlaneFactory extends VehicleFactory{
    @Override
    Moveable create() {
        return new Plane();
    }
}

抽象工厂模式

public abstract class AbstractFactory {
    public abstract Vehicle createVehicle();
}

class DefaultFactory extends AbstractFactory{
    @Override
    public Vehicle createVehicle() {
        return new Car();
    }
}

class Test{
    public static void main(String[] args) {
        AbstractFactory abstractFactory = new DefaultFactory();
        Vehicle v = abstractFactory.createVehicle();
        v.run();
    }
}