java Builder原理和实现
I_belong_to_jesus 人气:0首先给一个简单的Builder设计模式的例子：
主实现类代码如下：
/**
* 实体类 包含一个静态内部类 Builder
*/
public class CompanyClient {
public String companyName;
// 用final修饰的成员变量表示常量，只能被赋值一次，赋值后值无法改变！ final修饰的变量有三种：静态变量、实例变量和局部变量。
public String companyAddress;
public double companyRegfunds;
public String mPerson;
public String mType;
//构造方法
public CompanyClient() {
this(new Builder());
}
//构造方法
public CompanyClient(Builder builder){
this.companyName = builder.companyName;
this.companyAddress = builder.companyAddress;
this.companyRegfunds = builder.companyRegfunds;
this.mPerson = builder.person;
this.mType = builder.type;
}
public String getCompanyName() {
return companyName;
}
public String getCompanyAddress() {
return companyAddress;
}
public double getCompanyRegfunds() {
return companyRegfunds;
}
public String getmPerson() {
return mPerson;
}
public String getmType() {
return mType;
}
public Builder newBuilder() {
return new Builder(this);
}
@Override //重写toString方法后，当print这个对象的时候，会默认的调用toString()这个方法。
public String toString() {
return "CompanyClient{" +
"companyName='" + companyName + '\'' +
", companyAddress='" + companyAddress + '\'' +
", companyRegfunds=" + companyRegfunds +"千万"+
", mPerson=" + mPerson +
", mType='" + mType + '\'' +
'}';
}
/**
*静态内部类 Builder
*/
public static class Builder{
public String companyName;
public String companyAddress;
public double companyRegfunds;
public String person;
public String type;
//构造方法
public Builder() {
companyName = companyName;
companyAddress = companyAddress;
companyRegfunds = companyRegfunds;
person = person;
type = type;
}
//构造方法
Builder(CompanyClient companyClient){
this.companyName = companyClient.companyName;
this.companyAddress = companyClient.companyAddress;
this.companyRegfunds = companyClient.companyRegfunds;
this.person = companyClient.mPerson;
this.type = companyClient.mType;
}
public Builder setCompanyName(String name) {
companyName = name;
return this;
}
public Builder setCompanyAddress(String address) {
companyAddress = address;
return this;
}
public Builder setCompanyRegfunds(double regfunds) {
companyRegfunds = regfunds;
return this;
}
public Builder setmPerson(String per) {
person = per;
return this;
}
public Builder setmType(String typeStr) {
type = typeStr;
return this;
}
//构建一个实体
public CompanyClient build() {
return new CompanyClient(this);
}
}
}
测试类实现代码如下：
public class TestBuilder {
public static void main(String[] args) {
CompanyClient client = new CompanyClient.Builder()
.setCompanyName("alibaba")
.setCompanyAddress("wangjing")
.setCompanyRegfunds(5)
.setmPerson("10000")
.build();
System.out.println(client);
System.out.println("---------------------");
CompanyClient.Builder builder = new CompanyClient.Builder();
builder.setCompanyName("huawei");
builder.setCompanyAddress("haidian");
builder.setCompanyRegfunds(20);
builder.setmType("communication");
CompanyClient client1 = builder.build();
System.out.println(client1);
System.out.println("---------------------");
CompanyClient.Builder build1 = client1.newBuilder();
build1.setCompanyName("baidu");
CompanyClient client2 = build1.build();
System.out.println(client2);
}
}
输出结果如下：
CompanyClient{companyName='alibaba', companyAddress='wangjing', companyRegfunds=5.0千万, mPerson=10000, mType='null'}
---------------------
CompanyClient{companyName='huawei', companyAddress='haidian', companyRegfunds=20.0千万, mPerson=null, mType='communication'}
---------------------
CompanyClient{companyName='baidu', companyAddress='haidian', companyRegfunds=20.0千万, mPerson=null, mType='communication'}
首先看main函数中的第一行代码：
CompanyClient client = new CompanyClient.Builder()
.setCompanyName("alibaba")
.setCompanyAddress("wangjing")
.setCompanyRegfunds(5)
.setmPerson("10000")
.build();
之所以可以这么执行，是因为内部静态类Builder里定义了一份与CompanyClient类一模一样的变量，通过一系列的成员函数进行设置属性值，但是返回值都是this，也就是都是Builder对象，最后提供了一个build函数用于创建CompanyClient对象，返回的是CompanyClient对象，对应的构造函数在CompanyClient 类中进行定义，也就是构造函数的入参是Builder对象，然后依次对自己的成员变量进行赋值，对应的值都是Builder对象中的值。此外Builder类中的成员函数返回Builder对象自身的另一个作用就是让它支持链式调用，使代码可读性大大增强。
总结下，Java环境下builder设计模式：
定义一个静态内部类Builder，内部的成员变量和外部类一样
Builder类通过一系列的方法用于成员变量的赋值，并返回当前对象本身（this）
Builder类提供一个build方法或者create方法用于创建对应的外部类，该方法内部调用了外部类的一个私有构造函数，该构造函数的参数就是内部类Builder
外部类提供一个私有构造函数供内部类调用，在该构造函数中完成成员变量的赋值，取值为Builder对象中对应的值
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