java - 返回 null 的构建器模式

Question

我正在使用构建器模式，但在设置 feilds 后，其返回 nullpointerException:

package com.test.sample1;

 public class MessageID {

public int BOI1;
public int DOI2;
public int CO3;

public MessageID(int BOI1,int DOI2,int CO3){
    this.BOI1=BOI1;
    this.DOI2=DOI2;
    this.CO3=CO3;
}


public int getBOI1(){
    return BOI1;
}
public int getDOI2(){
    return DOI2;
}
public int CO3(){
    return CO3;
    }
 }

这里是 BaseServiceRequest.java::

        package com.test.sample;

        import com.test.sample1.MessageID;

        public class BaseServiceRequest   {


             MessageID messageID;

            public BaseServiceRequest(BaseServiceRequestBuilder builder) {
                this.messageID = builder.messageID;

            }

            public MessageID getMessageID() {

                return messageID;
            }

            public static class BaseServiceRequestBuilder {

                private MessageID messageID;

                public BaseServiceRequestBuilder messageID(MessageID messageID) {   
                    this.messageID =messageID;      
                return this;
                }

            }
        }

这是主类::

       package com.test.sample;

      import com.test.sample.BaseServiceRequest.BaseServiceRequestBuilder;
      import com.test.sample1.MessageID;

      public class Wrapper {

        static BaseServiceRequestBuilder builder = new BaseServiceRequestBuilder();
        static BaseServiceRequest request;

  public static void main(String[] args) {
    test(builder);
    int syso = request.getMessageID().getBOI1();
    System.out.println(syso);

  }

   public static void test(BaseServiceRequestBuilder builder) {
   builder.messageID(new MessageID(23, 34, 12));
  }

  }

我应该在 request.getMessageID().getBOI1() 处得到 23 的输出，
但出现空指针异常

Answer 1

那是因为您正在调用构建器的构建方法并丢弃结果。保存结果:

request = builder.messageID(new MessageID(23, 34, 12));

请注意，这并不是真正的构建器模式的示例；这真的是一个工厂。大多数有用的 Builder 类都使用流畅的 API 来使过程清晰且不易出错，并且 Builder 对象包含构造新实例所需的中间状态，如下所示:

request = new BaseServiceRequestBuilder().boi1(23).doi2(34).co3(12).build();

如果您已经拥有 MessageID 对象(可能来自某些传入请求)，则构建器也可能接受它作为参数:

request = new BaseServiceRequestBuilder().messageID(id).build();