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,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:
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
}
第一:折叠构造函数模式(telescoping constructor pattern ),这个我们经常用,如下代码所示
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, "罗技键盘");
}
public Computer(String cpu, String ram, int usbCount, String keyboard) {
this(cpu, ram, usbCount, keyboard, "三星显示器");
}
public 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;
}
}
第二种:Javabean 模式,如下所示
public class Computer {
...
public String getCpu() {
return cpu;
}
public void setCpu(String cpu) {
this.cpu = cpu;
}
public String getRam() {
return ram;
}
public void setRam(String ram) {
this.ram = ram;
}
public int getUsbCount() {
return usbCount;
}
...
}
第一种主要是使用及阅读不方便。你可以想象一下,当你要调用一个类的构造函数时,你首先要决定使用哪一个,然后里面又是一堆参数,如果这些参数的类型很多又都一样,你还要搞清楚这些参数的含义,很容易就传混了。。。那酸爽谁用谁知道。
第二种方式在构建过程中对象的状态容易发生变化,造成错误。因为那个类中的属性是分步设置的,所以就容易出错。
为了解决这两个痛点,builder模式就横空出世了。
package com.cjian.GOF.builder;
/**
* @description:
* @author: CJ
* @time: 2021/11/30 9:13
*/
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(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 {
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;
this.ram = ram;
}
public Builder setUsbCount(int usbCount) {
this.usbCount = usbCount;
return this;
}
public Builder setKeyboard(String keyboard) {
this.keyboard = keyboard;
return this;
}
public Builder setDisplay(String display) {
this.display = display;
return this;
}
public Computer build() {
return new Computer(this);
}
}
@Override
public String toString() {
return "Computer{" + "cpu='" + cpu + '\'' + ", ram='" + ram + '\'' + ", usbCount=" + usbCount + ", keyboard='"
+ keyboard + '\'' + ", display='" + display + '\'' + '}';
}
}
package com.cjian.GOF.builder;
/**
* @description:
* @author: CJ
* @time: 2021/11/30 9:15
*/
public class Demo {
public static void main(String[] args) {
Computer computer = new Computer.Builder("因特尔", "三星").setDisplay("三星24寸")
.setKeyboard("罗技")
.setUsbCount(2)
.build();
System.out.println(computer);
}
}
Computer{cpu='因特尔', ram='三星', usbCount=2, keyboard='罗技', display='三星24寸'}
刚好最近codecheck,可以使用到