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

i playing around builder pattern , stuck how add new "property" 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 new object builder this:

msprojecttaskdata data = new msprojecttaskdata.builder().withtransfered(true).build(); 

but approach req string new-created object lost (of course).

is there possibility create new object new set isalreadytransfered variable , with "old" req string "old" object?

maybe have pass old object reference builder not know how this. maybe use of builder pattern not usefull approach?

edit: (after comment eugene)

think, 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 work or wrong code above? can use approach without consideration?

make builder constructor take argument "old" object , set whatever want new one.

edit

you need read bit more builder pattern better grasp @ , if really need it.

the general idea builder pattern used when have optional elements. effective java item 2 best friend here.

for class, if want build 1 object , use builder pattern @ same time, you

1. either pass "old" object in builder constructor
2. create method from or fromold, etc.

so how looks like? going provide first 1 can figure out second on 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 required must private final         private final string firstname;         private final string lastname;          //fields optional 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 create 1 object 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