Create new object with Builder pattern with “old” object reference

问题

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

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:

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

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

Is there a possibility to create a new object with the new set isAlreadyTransfered variable and with the "old" req string from a "old" object?

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?

EDIT: (After comment from Eugene)

Think, I got it:

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?

回答1:

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

Edit

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

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

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

Either pass the "old" object in the Builder constructor
Create a method from or fromOld, etc.

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

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:

   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

回答2:

I would change it to

   public class MsProjectTaskData {
    private boolean transfered;
    private String request;    

    public static class Builder {    
        private boolean transfered = false;
        private String request;

        public Builder() {
            // empty
        }

        public Builder(MsProjectTaskData old) {
            this.request(old.request);
            this.transfered(old.transfered);
        }

        public Builder request(String val) {
            request = val; 
            return this;
        }

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

    private MsProjectTaskData(Builder builder) {
        transfered = builder.transfered;
        request = builder.request;
    }

}

And used like this

MsProjectTaskData data = new  MsProjectTaskData.Builder().transfered(true).request("request").build();
MsProjectTaskData changedData = new MsProjectTaskData.Builder(data).transfered(false).request("changeRequest").build();
MsProjectTaskData sameData = new MsProjectTaskData.Builder(data).build();

I took the liberty to change the boolean to "transfered", I feel it is easier to understand

来源：https://stackoverflow.com/questions/16216642/create-new-object-with-builder-pattern-with-old-object-reference

标签

java

design-patterns

builder