java 使用“旧”对象引用创建具有 Builder 模式的新对象

Create new object with Builder pattern with "old" object reference

提问by sk2212

I am playing around with the Builder pattern and get stuck how to add a new "property" to a new-created object:

我正在尝试使用 Builder 模式，但遇到了如何向新创建的对象添加新“属性”的问题：

public class MsProjectTaskData {
  private boolean isAlreadyTransfered;
  private String req;    

   public static class Builder {    
    private boolean isAlreadyTransfered = false;

    public Builder withTransfered(boolean val) {
        isAlreadyTransfered = val; 
        return this;
    }    
    public MsProjectTaskData build() {
        return new MsProjectTaskData(this);
    }
   }

   private MsProjectTaskData(Builder builder) {
     isAlreadyTransfered = builder.isAlreadyTransfered;
   }

  public MsProjectTaskData(String req) {
    this.req = req;
  }
}

I can create a new object with Builder like this:

我可以像这样用 Builder 创建一个新对象：

MsProjectTaskData data = new MsProjectTaskData.Builder().withTransfered(true).build();

But with this approach the reqstring from a new-created object is lost (of course).

但是使用这种方法，req来自新创建对象的字符串丢失了（当然）。

Is there a possibility to create a new object with the new set isAlreadyTransferedvariable and with the "old" reqstring from a "old" object?

是否有可能使用新的设置isAlreadyTransfered变量和来自“旧”对象的“旧”字符串创建新对象？req

Maybe I have to pass the old object reference to the Builder but I do not know how to do this. Maybe the use of Builder pattern is not really usefull for this approach?

也许我必须将旧对象引用传递给 Builder，但我不知道该怎么做。也许 Builder 模式的使用对于这种方法并不是真的有用？

EDIT: (After comment from Eugene)

编辑：（在尤金评论后）

Think, I got it:

想想，我明白了：

public static class Builder {   
 private boolean isAlreadyTransfered = false;
 private MsProjectTaskData data;

 public Builder(MsProjectTaskData data) {
     this.data = data;
 }

 public Builder withTransfered(boolean val) {
     isAlreadyTransfered = val; 
     data.setAlreadyTransfered(isAlreadyTransfered);
     return this;
 }   
 public MsProjectTaskData build() {
     return data;
 }
}

Seems to work or is something wrong with the code above? Can I use this approach without consideration?

似乎有效还是上面的代码有问题？我可以不加考虑地使用这种方法吗？

回答by Eugene

Make the Builder constructor take as an argument the "old" object and set whatever you want from it to the new one.

使 Builder 构造函数将“旧”对象作为参数，并将您想要的任何内容设置为新对象。

Edit

编辑

You need to read a bit more about the builder pattern to get a better grasp at what it is and if you reallyneed it.

您需要阅读更多有关构建器模式的信息，以便更好地了解它是什么以及您是否真的需要它。

The general idea is that Builder pattern is used when you have optionalelements. Effective Java Item 2 is your best friend here.

一般的想法是当您有可选元素时使用 Builder 模式。Effective Java Item 2 是您最好的朋友。

For your class, if you want to build one object from another and use a Builder pattern at the same time, you

对于您的类，如果您想从另一个对象构建一个对象并同时使用构建器模式，您

1. Either pass the "old" object in the Builder constructor
2. Create a method fromor fromOld, etc.

1. 在 Builder 构造函数中传递“旧”对象
2. 创建一个方法from或fromOld等。

So how does that looks like? I am going to provide only the first one you can figure out the second on your own.

那么它看起来如何呢？我将只提供第一个，您可以自己找出第二个。

class MsProjectTaskData {
    private final String firstname;
    private final String lastname;
    private final int age;

    private MsProjectTaskData(Builder builder){
        this.firstname = builder.firstname;
        this.lastname  = builder.lastname;
        this.age       = builder.age;
    }

    public static final class Builder{
        //fields that are REQUIRED must be private final
        private final String firstname;
        private final String lastname;

        //fields that are optional are not final
        private int age;

        public Builder(String firstname, String lastname){
            this.firstname = firstname;
            this.lastname  = lastname;
        }

        public Builder(MsProjectTaskData data){
            this.firstname = data.firstname; 
            this.lastname  = data.lastname;
        }

        public Builder age(int val){
            this.age = val; return this;
        }

        public MsProjectTaskData build(){
            return new MsProjectTaskData(this);
        }
    }

    public String getFirstname() {
         return firstname;
    }

    public String getLastname() {
         return lastname;
    }

    public int getAge() {
         return age;
    }
}

And how you will create one object from another:

以及如何从另一个对象创建一个对象：

   MsProjectTaskData.Builder builder = new MsProjectTaskData.Builder("Bob", "Smith");
   MsProjectTaskData oldObj = builder.age(23).build();
   MsProjectTaskData.Builder newBuilder = new MsProjectTaskData.Builder(oldObj);
   MsProjectTaskData newObj = newBuilder.age(57).build();
   System.out.println(newObj.getFirstname() + " " + newObj.getLastname() + " " + newObj.getAge()); // Bob Smith 57