[设计模式]创建型设计模式

简介

创建型设计模式：创建对象的同时，隐藏对象创建的细节

代码

1. 简单工厂模式

```java
package me.maxiaolong.designpattern;

/**
 * @author maxiaolong
 * created at 2020/7/2
 * 简单工厂模式，工厂类只需要知道参数便可以获得相应对象
 * 优点：
 * 缺点：1) 如果增加产品，必须要修改工厂类（违背开闭原则）
 * 例子：
 *  冰箱厂 生产大冰箱，中冰箱，小冰箱
 */
public class SimpleFactoryClient {
    public static void main(String[] args) {
        // 创建工厂的过程可以使用简单工厂进一步隐藏创建细节
        ShapeFactory shapeFactory = new ShapeFactory();
        Shape circle = shapeFactory.getShape("Circle");
        circle.draw();
    }
}

class ShapeFactory{
    ShapeFactory(){
    }

    public Shape getShape(String shapeDescription){
        switch (shapeDescription){
            case "Triangle":
                return new Triangle();
            case "Rectangle":
                return new Rectangle();
            case "Circle":
                return new Circle();
            default:
                return null;
        }
    }
}


interface Shape{

    /**
     * 绘制图形
     */
    void draw();
}

class Triangle implements Shape{

    @Override
    public void draw() {
        System.out.println("draw a triangle shape");
    }
}

class Rectangle implements Shape{

    @Override
    public void draw() {
        System.out.println("draw a rectangle shape");
    }
}

class Circle implements Shape{

    @Override
    public void draw() {
        System.out.println("draw a circle shape");
    }
}
```

2. 工厂方法模式

```java
package me.maxiaolong.designpattern;

/**
 * @author maxiaolong
 * created at 2020/7/3
 * 相比于简单工厂，增加了抽象工厂类(抽象工厂类仅用于创建高层接口) 产品依然是单一种类产品
 * 例子：增加具体产品不需要修改原有工厂代码，只需要在增加具体产品的基础上增加具体工厂（开闭原则） 即增加产品等级更加满足开闭原则
 */
public class FactoryMethodClient {
    public static void main(String[] args) {
        LogFactory logFactory = new FileLogFactory();
        Log log = logFactory.createLog();
        log.writeLog();
    }
}

interface LogFactory{

    /**
     * 创建Log
     * @return
     */
    Log createLog();
}

class FileLogFactory implements LogFactory{

    @Override
    public Log createLog() {
        return new FileLog();
    }
}

class DatabaseLogFactory implements LogFactory{

    @Override
    public Log createLog() {
        return new DatabaseLog();
    }
}

interface Log{
    /**
     * 写日志
     */
    void writeLog();
}

class FileLog implements Log{

    @Override
    public void writeLog() {
        System.out.println("write file log");
    }
}

class DatabaseLog implements Log{

    @Override
    public void writeLog() {
        System.out.println("write database log");
    }
}
```

3. 抽象工厂模式

```java
package me.maxiaolong.designpattern;

/**
 * @author maxiaolong
 * created at 2020/7/3
 * 增加产品族
 *
 */
public class AbstractFactoryClient {
    public static void main(String[] args) {
        // 工厂new的操作可以使用简单工厂代替
        AbstractFactory factory = new BigFactory();
        AbstractIceBox iceBox = factory.createIceBox();
        iceBox.describeIceBox();
    }
}

interface AbstractFactory{
    /**
     * 生产冰箱
     * @return
     */
    AbstractIceBox createIceBox();

    /**
     * 生产空调
     * @return
     */
    AbstractAirCondition createAirCondition();
}

class BigFactory implements  AbstractFactory{

    @Override
    public AbstractIceBox createIceBox() {
        return new BigIceBox();
    }

    @Override
    public AbstractAirCondition createAirCondition() {
        return new BigAirCondition();
    }
}

class MediumFactory implements AbstractFactory{

    @Override
    public AbstractIceBox createIceBox() {
        return new MediumIceBox();
    }

    @Override
    public AbstractAirCondition createAirCondition() {
        return new MediumAirCondition();
    }
}

class SmallFactory implements AbstractFactory{

    @Override
    public AbstractIceBox createIceBox() {
        return new SmallIceBox();
    }

    @Override
    public AbstractAirCondition createAirCondition() {
        return new SmallAirCondition();
    }
}

interface AbstractIceBox {
    /**
     * 生产冰箱接口
     */
    void describeIceBox();
}

class BigIceBox implements AbstractIceBox {

    @Override
    public void describeIceBox() {
        System.out.println("describe big icebox");
    }
}

class MediumIceBox implements AbstractIceBox {

    @Override
    public void describeIceBox() {
        System.out.println("describe medium icebox");
    }
}

class SmallIceBox implements AbstractIceBox {

    @Override
    public void describeIceBox() {
        System.out.println("describe small icebox");
    }
}

interface AbstractAirCondition {
    /**
     * 生产空调接口
     */
    void describeAirCondition();
}

class BigAirCondition implements AbstractAirCondition {

    @Override
    public void describeAirCondition() {
        System.out.println("describe big air condition");
    }
}

class MediumAirCondition implements AbstractAirCondition {

    @Override
    public void describeAirCondition() {
        System.out.println("describe medium air condition");
    }
}

class SmallAirCondition implements AbstractAirCondition {

    @Override
    public void describeAirCondition() {
        System.out.println("describe small air condition");
    }
}
```

4. 建造者模式

```java
package me.maxiaolong.designpattern;

/**
 * @author maxiaolong
 * created at 2020/7/3
 * 定义：创建一个包含多个组成部分的复杂对象，可以返回一个完整的产品对象给用户
 */
public class BuilderClient {
    public static void main(String[] args) {
        Builder builder = new ConcreteBuilder();
        Director director = new Director(builder);
        Product product = director.construct();
    }
}

/**
 * 负责安排复杂对象的建造次序
 */
class Director{

    private Builder builder;

    Director(Builder builder){
        this.builder = builder;
    }

    public Builder getBuilder() {
        return builder;
    }

    public void setBuilder(Builder builder) {
        this.builder = builder;
    }

    /**
     * 控制对象构建顺序
     */
    public Product construct(){
        this.builder.buildPartA();
        this.builder.buildPartB();
        this.builder.buildPartC();
        return this.builder.getResult();
    }
}

/**
 * 产品以及部分
 */
class Product{
    private String partA;
    private String partB;
    private String partC;

    public String getPartA() {
        return partA;
    }

    public void setPartA(String partA) {
        this.partA = partA;
    }

    public String getPartB() {
        return partB;
    }

    public void setPartB(String partB) {
        this.partB = partB;
    }

    public String getPartC() {
        return partC;
    }

    public void setPartC(String partC) {
        this.partC = partC;
    }
}

abstract class Builder{

    /**
     * 申明为受保护的，便于子类使用
     */
    protected Product product = new Product();

    abstract void buildPartA();

    abstract void buildPartB();

    abstract void buildPartC();

    public Product getResult(){
        return this.product;
    }
}

class ConcreteBuilder extends Builder{

    @Override
    public void buildPartA(){
        this.product.setPartA("partA");
    }

    @Override
    public void buildPartB(){
        this.product.setPartB("partB");
    }

    @Override
    public void buildPartC(){
        this.product.setPartC("partC");
    }
}
```

5. 单例模式

```java
package me.maxiaolong.designpattern;

import com.sun.xml.internal.bind.v2.runtime.unmarshaller.XsiNilLoader;

/**
 * @author maxiaolong
 * created at 2020/7/3
 */
public class SingletonClient {

}

/**
 * 饿汉模式
 */
class SingletonV1{
    private static SingletonV1 singletonV1 = new SingletonV1();

    private SingletonV1(){}

    public static SingletonV1 getInstance(){
        return singletonV1;
    }
}

/**
 * 懒汉模式
 */
class SingletonV2{
    private static SingletonV2 singletonV2;

    private SingletonV2(){}

    /**
     * 懒汉模式线程不安全
     * @return
     */
    public static SingletonV2 getInstance(){
        if(singletonV2==null){
            //没有设置为临界区，可自由访问
            singletonV2 = new SingletonV2();
        }
        return singletonV2;
    }
}

class SingletonV3{
    /**
     * volatile关键字可以使指令重排序失效
     */
    private static volatile SingletonV3 singletonV3;

    private SingletonV3(){}

    public static SingletonV3 getInstance(){
        if(singletonV3==null){
            // 如果对整个方法加synchronized，同步块太大，不利于并发
            synchronized (Object.class){
                if(singletonV3==null){
                    // 过程 1. 内存空间开辟 2. 指针指向 3. 对象初始化 其中2，3是进行指令重排序优化的
                    singletonV3 = new SingletonV3();
                }
            }
        }
        return singletonV3;
    }
}

/**
 * 延迟初始化占位类模式
 * 实现延迟初始化的同时，保证线程安全
 * 原理：1. 外部类加载时不要求内部类加载，即SingletonV4加载时，不要求内部类InstanceHolder被加载；只有当外部类调用getInstance静态方法时内部类才加载并完成初始化操作；
 *      2. 多线程进行类的初始化时，JVM保证只有一条线程进行类的初始化，其它线程则会被阻塞。
 */
class SingletonV4{
    public static class InstanceHolder{
        public static SingletonV4 singletonV4 = new SingletonV4();
    }

    private SingletonV4(){}

    public static SingletonV4 getInstance(){
        return InstanceHolder.singletonV4;
    }
}
```