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java的Builder原理和實現(xiàn)詳解

更新時間：2021年12月29日 10:32:08 作者：I_belong_to_jesus

大家好，本篇文章主要講的是java的Builder原理和實現(xiàn)詳解，感興趣的同學(xué)趕快來看一看吧，對你有幫助的話記得收藏一下，方便下次瀏覽

首先給一個簡單的Builder設(shè)計模式的例子：

主實現(xiàn)類代碼如下：

/**
 * 實體類 包含一個靜態(tài)內(nèi)部類 Builder
 */
public class CompanyClient {
    public String companyName;
    // 用final修飾的成員變量表示常量，只能被賦值一次，賦值后值無法改變！ final修飾的變量有三種：靜態(tài)變量、實例變量和局部變量。
    public String companyAddress;
  
    public double companyRegfunds;
    public String mPerson;
    public String mType;     
     //構(gòu)造方法 
     public CompanyClient() {
         this(new Builder());
     }
     //構(gòu)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 //重寫toString方法后，當(dāng)print這個對象的時候，會默認(rèn)的調(diào)用toString()這個方法。
     public String toString() {
         return "CompanyClient{" +
                 "companyName='" + companyName + '\'' +
                 ", companyAddress='" + companyAddress + '\'' +
                 ", companyRegfunds=" + companyRegfunds +"千萬"+
                 ", mPerson=" + mPerson +
                 ", mType='" + mType + '\'' +
                 '}';
     }
     /**
     *靜態(tài)內(nèi)部類 Builder
     */
     public static class Builder{
        public String companyName;
        public String companyAddress;
        public  double companyRegfunds;
        public  String person;
        public String type;
        //構(gòu)造方法
        public Builder() {
            companyName = companyName;
            companyAddress = companyAddress;
            companyRegfunds = companyRegfunds;
            person = person;
            type = type;
  
        }
        //構(gòu)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;
        }
         //構(gòu)建一個實體
         public CompanyClient build() {
             return new CompanyClient(this);
         }
    }
 }

測試類實現(xiàn)代碼如下：

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);
    }
}

輸出結(jié)果如下：

CompanyClient{companyName='alibaba', companyAddress='wangjing', companyRegfunds=5.0千萬, mPerson=10000, mType='null'}
---------------------
CompanyClient{companyName='huawei', companyAddress='haidian', companyRegfunds=20.0千萬, mPerson=null, mType='communication'}
---------------------
CompanyClient{companyName='baidu', companyAddress='haidian', companyRegfunds=20.0千萬, mPerson=null, mType='communication'}

首先看main函數(shù)中的第一行代碼：

CompanyClient client = new CompanyClient.Builder()
                .setCompanyName("alibaba")
                .setCompanyAddress("wangjing")
                .setCompanyRegfunds(5)
                .setmPerson("10000")
                .build();

之所以可以這么執(zhí)行，是因為內(nèi)部靜態(tài)類Builder里定義了一份與CompanyClient類一模一樣的變量，通過一系列的成員函數(shù)進(jìn)行設(shè)置屬性值，但是返回值都是this，也就是都是Builder對象，最后提供了一個build函數(shù)用于創(chuàng)建CompanyClient對象，返回的是CompanyClient對象，對應(yīng)的構(gòu)造函數(shù)在CompanyClient?類中進(jìn)行定義，也就是構(gòu)造函數(shù)的入?yún)⑹?strong>Builder對象，然后依次對自己的成員變量進(jìn)行賦值，對應(yīng)的值都是Builder對象中的值。此外Builder類中的成員函數(shù)返回Builder對象自身的另一個作用就是讓它支持鏈?zhǔn)秸{(diào)用，使代碼可讀性大大增強(qiáng)。

總結(jié)下，Java環(huán)境下builder設(shè)計模式：

定義一個靜態(tài)內(nèi)部類Builder，內(nèi)部的成員變量和外部類一樣

Builder類通過一系列的方法用于成員變量的賦值，并返回當(dāng)前對象本身（this）

Builder類提供一個build方法或者create方法用于創(chuàng)建對應(yīng)的外部類，該方法內(nèi)部調(diào)用了外部類的一個私有構(gòu)造函數(shù)，該構(gòu)造函數(shù)的參數(shù)就是內(nèi)部類Builder

外部類提供一個私有構(gòu)造函數(shù)供內(nèi)部類調(diào)用，在該構(gòu)造函數(shù)中完成成員變量的賦值，取值為Builder對象中對應(yīng)的值

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