23种设计模式之builder模式

最近在看图解设计模式，其中使用的是java来实现，但是发现其中有的看完后并不知道这个模式是用来干嘛的。一知半解下。通过网上资料辅助理解。就此记录。

后来发现，并不是看不懂，只是书上的例子有些比较老，对比网上的多个例子用的简化方法，书中写的例子都是经典案例，这里记录两种案例。

为什么要用builder模式

对于一个实体类，有两种方法实现构造
方法一、参数不同的构造函数

public class Computer {...public Computer(String cpu, String ram) {this(cpu, ram, 0);}public Computer(String cpu, String ram, int usbCount) {this(cpu, ram, usbCount, "键盘");}public Computer(String cpu, String ram, int usbCount, String keyboard) {this(cpu, ram, usbCount, keyboard, "显示器");}
}

缺点：构造函数多，当参数多时，含义不明，不易分辨。

new Computer( 1, 2, 3, 4, … )//参数多了就很繁琐

方法二、javabean模式

public class bean{public String username;public String password;XXXset();//实现set和get方法XXXget();
}

缺点：在使用中途，如果其中的属性值发生变化，要修改的不仅是字段，还有set和get方法，要改的多了，就很烦，还容易漏删、漏改。

builder模式就是为了解决这两个缺点。

什么时候用

当一个类的构造函数的参数多于四个，且有的参数是可选的，考虑使用builder模式

builder模式是什么

明白了为什么用builder和什么时候用后，就开始学习怎么用了。

常用方法（实现后可链式调用）

Computer为主类，其中定义一个静态内部类Builder，定义构造赋值必须的参数，定义set方法赋值可选的参数。

public class Computer {private final String cpu;//必须private final String ram;//必须private final int usbCount;//可选private final String keyboard;//可选private final String display;//可选private Computer(Builder builder){this.cpu=builder.cpu;this.ram=builder.ram;this.usbCount=builder.usbCount;this.keyboard=builder.keyboard;this.display=builder.display;}public static class Builder{//静态内部类private String cpu;//必须private String ram;//必须private int usbCount;//可选private String keyboard;//可选private String display;//可选public Builder(String cup,String ram){this.cpu=cup;//必选的参数，通过构造赋值this.ram=ram;}public Builder setUsbCount(int usbCount) {this.usbCount = usbCount;//可选的参数，通过set方法赋值return this;//链式编程的实现，返回当前的对象实例}public Builder setKeyboard(String keyboard) {this.keyboard = keyboard;return this;//链式编程的实现，返回当前的对象实例}public Builder setDisplay(String display) {this.display = display;return this;//链式编程的实现，返回当前的对象实例}        public Computer build(){return new Computer(this);//链式编程调用结束，返回当前的对象实例}}//省略getter方法
}

测试：

Computer computer=new Computer.Builder("因特尔","三星").setDisplay("三星24寸")//通过链式调用，观感明了，参数设置见名知意.setKeyboard("罗技").setUsbCount(2).build();//调用结束不要忘了

传统方法

public class Computer {private String cpu;//必须private String ram;//必须private int usbCount;//可选private String keyboard;//可选private String display;//可选public Computer(String cpu, String ram) {this.cpu = cpu;this.ram = ram;}public void setUsbCount(int usbCount) {this.usbCount = usbCount;}public void setKeyboard(String keyboard) {this.keyboard = keyboard;}public void setDisplay(String display) {this.display = display;}@Overridepublic String toString() {return "Computer{" +"cpu='" + cpu + '\'' +", ram='" + ram + '\'' +", usbCount=" + usbCount +", keyboard='" + keyboard + '\'' +", display='" + display + '\'' +'}';}
}

抽象Builder类

public abstract class ComputerBuilder {public abstract void setUsbCount();public abstract void setKeyboard();public abstract void setDisplay();public abstract Computer getComputer();
}

具体Builder类，

//第一个具体bulider
public class MacComputerBuilder extends ComputerBuilder {private Computer computer;public MacComputerBuilder(String cpu, String ram) {computer = new Computer(cpu, ram);}@Overridepublic void setUsbCount() {//具体的Builder类中包含其特有的set方法，且提前定义值computer.setUsbCount(2);}@Overridepublic void setKeyboard() {computer.setKeyboard("苹果键盘");}@Overridepublic void setDisplay() {computer.setDisplay("苹果显示器");}@Overridepublic Computer getComputer() {return computer;}
}//第二个具体bulider
public class LenovoComputerBuilder extends ComputerBuilder {private Computer computer;public LenovoComputerBuilder(String cpu, String ram) {computer=new Computer(cpu,ram);}@Overridepublic void setUsbCount() {computer.setUsbCount(4);}@Overridepublic void setKeyboard() {computer.setKeyboard("联想键盘");}@Overridepublic void setDisplay() {computer.setDisplay("联想显示器");}@Overridepublic Computer getComputer() {return computer;}
}

Director类

public class ComputerDirector {public void makeComputer(ComputerBuilder builder){builder.setUsbCount();builder.setDisplay();builder.setKeyboard();}
}

测试（未完）

public static void main(String[] args) {ComputerDirector director=new ComputerDirector();//1ComputerBuilder builder=new MacComputerBuilder("I5处理器","三星125");//抽象builder用具体的builder来实例化director.makeComputer(builder);//通过具体的Builder类来指导DirectorComputer macComputer=builder.getComputer();//4System.out.println("mac computer:"+macComputer.toString());ComputerBuilder lenovoBuilder=new LenovoComputerBuilder("I7处理器","海力士222");director.makeComputer(lenovoBuilder);Computer lenovoComputer=lenovoBuilder.getComputer();System.out.println("lenovo computer:"+lenovoComputer.toString());
}

题外话：StringBuilder是可以用链式编程的


StringBuilder builder = new StringBuilder().builder.append("blake").append("bob").append("alice").append("linese").append("eve");

就是利用以下方式实现链式编程的

@Overridepublic StringBuilder append(String str) {super.append(str);return this;//返回当前的对象实例}

参考：秒懂设计模式之建造者模式（Builder pattern）