Creational Patterns Part 4/5: Builder Pattern

Creational Patterns Part 4/5: Builder Pattern

目录

Creational Patterns Part 45 Builder Pattern

目录

Definition

UML Class Diagram

Implementation

Definition

Builder Pattern使用一些简单的对象来build一个复杂的对象，逐步完成build的过程。在Factory Pattern中一个method call就可以创建出一个对象，而如果一个对象的创建需要准备很多事情，就可以使用Builder Pattern。

何时使用？创建对象需要事先做很多事时/当构造方法或静态工厂的参数过多时。

使用频率：

Medium Low

UML Class Diagram

Implementation

现在要组装一台电脑，那么通常需要主板、内存、显卡、屏幕和键盘等设备。也就是说在组装电脑之前，要提前准备好以上这些对象，可以用Builder Pattern来实现。

// Device.java
package designpatterns.creationalpatterns.builder;

public interface Device {
String name();
Integer price();
}

// DeviceImpl.java
package designpatterns.creationalpatterns.builder;

class MainBoard implements Device {

@Override
public String name() {
return "MainBoard";
}

@Override
public Integer price() {
return 1000;
}
}

class Memory implements Device {

@Override
public String name() {
return "Memory";
}

@Override
public Integer price() {
return 500;
}
}

class VideoCard implements Device {

@Override
public String name() {
return "VideoCard";
}

@Override
public Integer price() {
return 2000;
}
}

class Screen implements Device {

@Override
public String name() {
return "Screen";
}

@Override
public Integer price() {
return 1000;
}
}

class KeyBoard implements Device {

@Override
public String name() {
return "KeyBoard";
}

@Override
public Integer price() {
return 500;
}
}

// BuilderDemo.java
package designpatterns.creationalpatterns.builder;

import java.util.*;

public class BuilderDemo {
public static void main(String[] args) {
ComputerBuilder builder = new ComputerBuilder();
// The client does't know or care about how the object is created
Computer computer = builder.assembleComputer();
computer.showParts();
}
}

class ComputerBuilder {

public Computer assembleComputer() {
Computer computer = new Computer();

computer.add(new MainBoard());
computer.add(new Memory());
computer.add(new VideoCard());
computer.add(new Screen());
computer.add(new KeyBoard());

return computer;
}
}

class Computer {
private List<Device> deviceList = new ArrayList<Device>();

public void add(Device device) {
deviceList.add(device);
}

public void showParts() {
System.out.println(String.format("%-10s %5s", "Device", "Price"));
System.out.println("——————————————————");

Integer totalPrice = 0;
for(Device device: deviceList) {
System.out.println(String.format("%-10s %5d", device.name(), device.price()));
totalPrice += device.price();
}
System.out.println("——————————————————");
System.out.println(String.format("%-10s %5d", "Computer", totalPrice));
}
}

// output
Device     Price
——————————————————
MainBoard   1000
Memory       500
VideoCard   2000
Screen      1000
KeyBoard     500
——————————————————
Computer    5000