创建性-构造者设计模式

前言

我们在使用Retrofit等这些第三方框架的时候，发现他们的使用都很方便，比如Retrofit retrofit = new Retrofit.Builder().build()，和我们通常直接new一个对象不同，他是交给Builder类，通过build()函数来构造一个Retrofit对象。

刚开始使用，可能不太明白，明明可以直接new出来，为什么要交给Builder来构造呢？那么通过学习了构造者设计模式，才明白，原来当使用者构造对象时，构造函数的参数一般大于等于五个，就可以考虑使用构造者设计模式，这样编写代码的好处，在于结构清晰，也易于扩展。

在实际工作中，我们可能接触了很多这种链式创建对象的场景，比如AlertDialog，发现这种代码在使用的时候，真的很香，不仅实现了参数传递的灵活性，也避免了因参数过多造成的混乱。总之，除了使用官方封装的这些类以外，自己在实现代码的同时，也想要动手实现一个建造者设计模式。

实现

下面我们以电脑为例，创建一个抽象类Computer，我们想想，电脑都有什么属性，品牌，型号，操作系统，用途，类型，名字等等。这些属性如果都用构造方法实现的话，会很长，而且也不易于扩展，不够灵活。

 static class Computer{String name;String brand;String os;String display;String type;String use;public void setName(String name) {this.name = name;}public void setBrand(String brand) {this.brand = brand;}public void setOs(String os) {this.os = os;}public void setDisplay(String display) {this.display = display;}public void setType(String type) {this.type = type;}public void setUse(String use) {this.use = use;}}

比如，我们需要创造一个macbook笔记本，用传统思维，直接创造一个类，然后继承Computer,通过构造函数的方法重载，完成各种需求的笔记本。这样一套下来，代码臃肿不说，也不宜与后期的维护，如果有新的属性，又要重载一个构造函数，比较繁琐。

    static class MacBook extends Computer{public MacBook() {}// ……//各种构造函数// ……        @Overridepublic String toString() {return "MacBook{" +"name='" + name + '\'' +", brand='" + brand + '\'' +", os='" + os + '\'' +", display='" + display + '\'' +", type='" + type + '\'' +", use='" + use + '\'' +'}';}}

如果有一个专门的类，来创建不同需求的笔记本，这个问题就能很好的解决。

那么这个专门的类就是Builder类，通过构造者设计模式进行优化，我们这里具体到了MacBook笔记本，在实际开发工作中，这个可以使用泛型T代替，更加灵活。

 static abstract class Builder {abstract MacBookBuilder builderName(String name);abstract MacBookBuilder builderBrand(String brand);abstract MacBookBuilder builderOs(String os);abstract MacBookBuilder builderType(String type);abstract MacBookBuilder builderUse(String use);abstract MacBookBuilder builderDisPlay(String display);abstract Computer build();}

接下来，我们将利用MacBookBuilder 来构造笔记本，代码如下：

 static class MacBookBuilder extends Builder{MacBook macBook = new MacBook();@OverrideMacBookBuilder builderName(String name) {macBook.name = name;return this;}@OverrideMacBookBuilder builderBrand(String brand) {macBook.brand = brand;return this;}@OverrideMacBookBuilder builderOs(String os) {macBook.os = os;return this;}@OverrideMacBookBuilder builderType(String type) {macBook.type = type;return this;}@OverrideMacBookBuilder builderUse(String use) {macBook.use = use;return this;}@OverrideMacBookBuilder builderDisPlay(String display) {macBook.display = display;return this;}@OverrideComputer build() {return macBook;}}

好了，我们通过测试类，测试一下，附上完整代码：

package com.example.lib;public class BuilderTest {public static void main(String[] args) {Computer macbook = new MacBookBuilder().builderName("macbook 2023").builderOs("mac/os").builderBrand("apple").builderType("Laptops").builderDisPlay("Retina显示器").builderUse("studyProgram").build();System.out.println(macbook.toString());}static class Computer{String name;String brand;String os;String display;String type;String use;public void setName(String name) {this.name = name;}public void setBrand(String brand) {this.brand = brand;}public void setOs(String os) {this.os = os;}public void setDisplay(String display) {this.display = display;}public void setType(String type) {this.type = type;}public void setUse(String use) {this.use = use;}}static class MacBook extends Computer{public MacBook() {}@Overridepublic String toString() {return "MacBook{" +"name='" + name + '\'' +", brand='" + brand + '\'' +", os='" + os + '\'' +", display='" + display + '\'' +", type='" + type + '\'' +", use='" + use + '\'' +'}';}}static abstract class Builder {abstract MacBookBuilder builderName(String name);abstract MacBookBuilder builderBrand(String brand);abstract MacBookBuilder builderOs(String os);abstract MacBookBuilder builderType(String type);abstract MacBookBuilder builderUse(String use);abstract MacBookBuilder builderDisPlay(String display);abstract Computer build();}static class MacBookBuilder extends Builder{MacBook macBook = new MacBook();@OverrideMacBookBuilder builderName(String name) {macBook.name = name;return this;}@OverrideMacBookBuilder builderBrand(String brand) {macBook.brand = brand;return this;}@OverrideMacBookBuilder builderOs(String os) {macBook.os = os;return this;}@OverrideMacBookBuilder builderType(String type) {macBook.type = type;return this;}@OverrideMacBookBuilder builderUse(String use) {macBook.use = use;return this;}@OverrideMacBookBuilder builderDisPlay(String display) {macBook.display = display;return this;}@OverrideComputer build() {return macBook;}}
}