Question

我正在使用构建器模式，但在设置了它返回的nullpointerException：

之后

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()得到23的输出，
但得到一个空指针异常

Answer 1

那是因为你正在调用builder的构建方法并抛弃结果。保存结果：

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

请注意，这不是Builder模式的一个例子;它真的是一个工厂。最有用的Builder类使用流畅的API 来使流程清晰且不易出错，并且Builder对象包含构造新实例所需的中间状态，例如：

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

如果您已经拥有MessageID对象，可能来自某个传入请求，则Builder也可能接受它作为参数：

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

生成器模式返回null

1 个答案: