Copying fields from different instance of base into derived in one go - how to do this?

[Edward Winchester]

Hello,

Lets say I have 2 classes like this:

public class Base {

  private String          field1;
  private int               field2;
  private List<String> field3;

  // standard getters and setters present but not shown here...
}

public class Derived extends Base {

  private String d1;

  // standard getters and setters present but not shown here...
}

I have an instance of type Base. I want to create a new Derived whose Base portion is initialised from the existing Base instance. Preferably with new copies of the members.

Base b = new Base();
// initialise fields of b...

Derived d = new Derived(/* how to copy all parts of the Base across? */);

I was going to create a constructor on Derived that accepts a Base and copy all the Base members individually but wondered if there was a better, more maintainable way where I don't need to list all members of Base? If Base gets extended then it'd be nice for the copy to continue to work.

Don't really want to use serialisation.

Thanks,

PUK

[Campbell Ritchie]

You mean you want to add a copy constructor to the Base class? Then have a Derived constructor like this?

public Derived(Base b, String d1)
{
    super(b);
    this.d1 = d1;
}

Try that.

By the way: we say subclass and superclass in Java.

[Edward Winchester]

Campbell Ritchie wrote:You mean you want to add a copy constructor to the Base class? Then have a Derived constructor like this?

public Derived(Base b, String d1)
{
    super(b);
    this.d1 = d1;
}

Try that.

That still means a list of fields needs to be maintained. Good thing is it moves that list to the class in question.

By the way: we say subclass and superclass in Java.

My C++ background kicking in, apologies.

Thanks for your help.

PUK

[shukla raghav]

i would advice that get rid of this habit of relating Java and C++, they are two different languages with similar syntaxes thats all. or else you may fall into trouble switching mindsets. using clone() method is a better approach for creating copies of the objects

[Edward Winchester]

shukla raghav wrote:i would advice that get rid of this habit of relating Java and C++, they are two different languages with similar syntaxes thats all. or else you may fall into trouble switching mindsets.

OK, makes sense.

using clone() method is a better approach for creating copies of the objects

clone() would create me a distinct superclass object. That's not much use if I have inheritance.

I could have avoided inheritance and aggregated the "superclass" portion in the main class. Then I would not inherit the setters and getters.

clone() probably is of use to me here though:

public class subclass extends superclass {

  public subclass(superclass s) {
    super(s.clone());
  }
}

Hope I'm understanding things correctly.

PUK

[Maneesh Godbole]

"P Uk",
Please check your private messages for an important administrative matter.

[Campbell Ritchie]

I disagree with shukla raghav. The clone() method is not helpful there. If you have a superclass copy constructor, you can simply copy all the fields from the old instance to the new one. If the fields are primitives or immutable reference types, there is no need for any further action. If however the fields are mutable (eg List<String>) they might require duplication. There is a method in the Collections class which can copy Lists easily.

You are not changing the state and structure of the classes, but adding a bit to their interface. The fields are the same, and you ahve simply added an overloaded constructor.

And I thought you were a C# chap

[Edward Winchester]

Maneesh Godbole wrote:"P Uk",
Please check your private messages for an important administrative matter.

Done. Thank you.

Ed

[Maneesh Godbole]

Edward Winchester wrote:
Done.
Ed