设计模式的理解：构造器模式（Builder Pattern）

构造器模式，将一个复杂对象的构建与其表示分离，使得同样的构建过程可以创建不同的表示。

通俗的话来讲，就是把成员变量的初始化与构造函数进行分离。

考虑到当一个类的构造函数参数个数超过4个，而且这些参数有些是可选的参数，有些参数需要she'zhi考虑使用构造者模式。

解决的问题

当一个类的构造函数参数超过4个，而且这些参数有些是可选的时，我们通常有两种办法来构建它的对象。例如我们现在有如下一个类计算机类Computer，其中cpu与ram是必填参数，而其他3个是可选参数，那么我们如何构造这个类的实例呢,通常有两种常用的方式：

第一：折叠构造函数模式（telescoping constructor pattern ）

class Computer {string cpu;//必须string ram;//必须int usbCount;//可选string keyboard;//可选string display;//可选public:Computer(string cpu, string ram) {this(cpu, ram, 0);}Computer(string cpu, string ram, int usbCount) {this(cpu, ram, usbCount, "罗技键盘");}Computer(string cpu, string ram, int usbCount, string keyboard) {this(cpu, ram, usbCount, keyboard, "三星显示器");}Computer(string cpu, string ram, int usbCount, string keyboard, string display) {this->cpu = cpu;this->ram = ram;this->usbCount = usbCount;this->keyboard = keyboard;this->display = display;}
}

虽然，在C++语言中，可以在形参中赋默认值，但是在其他语言中，上面的表达方式弊端就十分的明显，阅读起来不方便，操作起来也不方便，当构造函数参数超过10个以上时，你在穿参初始化时很容易把值传错给其他的参数。所以builder模式就产生了

一、优化的builder模式：

两个主要的实现思路，将必须初始化的数据设置成构造函数，且构造函数不能太长，将可选的成员变量另外去赋值。

#include "iostream"
using namespace  std;
class Computer {
private:Computer() {}string cpu;//必须string ram;//必须int usbCount;//可选string keyboard;//可选string display;//可选
public:
class Builder{string cpu;//必须string ram;//必须int usbCount;//可选string keyboard;//可选string display;//可选public:Builder(string cup,string ram){this->cpu=cup;this->ram=ram;}const string &getCpu() const {return cpu;}const string &getRam() const {return ram;}int getUsbCount() const {return usbCount;}const string &getKeyboard() const {return keyboard;}const string &getDisplay() const {return display;}Builder& setUsbCount(int usbCount) {this->usbCount = usbCount;return *this;}Builder& setKeyboard(string keyboard) {this->keyboard = keyboard;return *this;}Builder& setDisplay(string display) {this->display = display;return *this;}Computer * build(){return new Computer(*this);}};//省略getter方法Computer(Builder builder){this->cpu=builder.getCpu();this->ram=builder.getRam();this->usbCount=builder.getUsbCount();this->keyboard=builder.getKeyboard();this->display=builder.getDisplay();}};
int main(){Computer * computerPtr =  Computer::Builder("因特尔","三星").setDisplay("三星24寸").setKeyboard("罗技").setUsbCount(2).build();return  0;
}

优化的builder省去了Director角色，而把Builder嵌套在实体类中，充当一个内部类。

二、传统Builder 模式

最原先的builder 模式有一个director类，

1.在builder抽象类中，将实体类Product的可选成员参数的赋值设置为抽象函数，必选成员变量设置为Product实体类和 Builder派生类的构造函数。

2.Procuct 为 Builder派生类的子对象

3.director 负责初始化 Builder。

1）定义实体类

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 + '\'' +'}';}
}

2）抽象Builder类和派生类

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

//苹果电脑构建者
public class MacComputerBuilder extends ComputerBuilder {private Computer computer;public MacComputerBuilder(String cpu, String ram) {computer = new Computer(cpu, ram);}@Overridepublic void setUsbCount() {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;}
}

//联想电脑构建者类
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;}
}

3) director 类

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

使用案例：

public static void main(String[] args) {ComputerDirector director=new ComputerDirector();//1ComputerBuilder builder=new MacComputerBuilder("I5处理器","三星125");//2director.makeComputer(builder);//3Computer 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());
}