单例模式

1. 懒汉单例模式

class A {
    private static A a = null ;
    private A() {}
    public static A getAInstance() {
        a = new A();
        return a ;
    }       
}

2.恶汉单例模式

class B {
    private static B b = new B() ;
    private B() {}
    public static B getBInstance() {
        return b ;
    }       
}

单例模式中，只有一个实例。

3.工厂模式

import java.io.*;
class CarFactory {    
　　
　　/**
　　*　单例模式　
　　*/

    private static CarFactory factory = null;
    private CarFactory (){}
    public static CarFactory getFactoryInstance(){
        factory = new CarFactory();
        return factory;
    }
    Car buildCar(String CarName) {
        switch(CarName) {
            case "BMW" : return new BMW();
            case "QQ" : return new QQ();
            default : break;
        }
        return null;
    }
}
interface Car {
    void build();
}
class BMW implements Car {
    public void build(){
        System.out.println("我是宝马我自豪，别摸我");
    }
}
class QQ implements Car {
    public void build(){
        System.out.println("我是QQ我小巧，0油耗");
    }
}
public class Test04 {
    public static void main(String[] args) throws Exception {
        
        BufferedReader reader = new BufferedReader(
            new InputStreamReader(System.in)
        ); 
        System.out.println("要什么车？(BMW ? QQ ?)");
        String input = reader.readLine();
        
        CarFactory factory = CarFactory.getFactoryInstance();
        Car car = factory.buildCar(input);
        car.build();
        
    }
}

3.工厂方法模式

import java.io.*;
interface Factory {
    Goods build();
}
//汽车工厂
class CarFactory implements Factory {
    public Goods build() {
        return new Car();
    }
}
//水
class WaterFactory implements Factory {
    public Goods build() {
        return new Water();
    }
}
interface Goods {
    void order() ;
}
class Car implements Goods {
    public void order(){
        System.out.println("汽车！开走吧！");
    }
}
class Water implements Goods {
    public void order(){
        System.out.println("水！喝吧！");
    }
}

public class Test05 {
    public static void main(String[] args) throws Exception{
        BufferedReader reader = new BufferedReader(
            new InputStreamReader(System.in)
        ); 
        System.out.println("要什么东西？(CarFactory ? WaterFactory ?)");
        String input = reader.readLine();
        Class factoryClass = Class.forName(input);
        Factory factory = (Factory)factoryClass.newInstance();
        //Factory factory = new WaterFactory();
        Goods goods = factory.build();
        goods.order();
    }
}