Java: Setting "this" instance to an object of the same type

Question

I have a POJO that has access to a method that returns an object of the same type (usually with some fields populated). Is it possible to implement a method in the POJO that calls this getter method and sets the instance of the POJO to reference the object that is returned? I would like to avoid setting a copy() function for every POJO that I create.

For example:

public class DAO {
    public static BaseEntity get(int id) {
        // returns some POJO
    }
}

public abstract class BaseEntity {
    public void get(int id) {
        BaseEntity entity = DAO.get(id);
        // sets "this" to the entity
    }
}

public class POJO extends BaseEntity {
    int foo;
    String bar; 
}

What has that got to do with a 'set this' ? Since you can't instantiate BaseEntity, there will be no 'this' anyway. — Stultuske
– Stultuske, Commented Jan 20, 2015 at 7:48
POJO extends BaseEntity, so I'd like to be able to do something like pojo.get(123) to populate its fields. — user2066880
– user2066880, Commented Jan 20, 2015 at 7:49
I understand what you are trying to do. I just don't get the logic of your 'how'. I would recommend an interface (not an abstract class) and a (concrete) implementation of that interface. — Stultuske
– Stultuske, Commented Jan 20, 2015 at 7:51

Eran · Accepted Answer · 2015-01-20 07:49:49Z

2

You can't assign to this.

You can have your get method return the new instance :

public BaseEntity get(int id) {
    return DAO.get(id);
}

And then assign that returned instance to the reference of the original instance:

BaseEntity pojo = new POJO ();
...
pojo = pojo.get(5);

answered Jan 20, 2015 at 7:49

Eran

395k57 gold badges726 silver badges793 bronze badges

Sign up to request clarification or add additional context in comments.

4 Comments

user2066880 Over a year ago

I see, this makes sense. Something that seems weird to me is that pojo = pojo.get(5) seems a bit redundant. If I could just call pojo.get(5) and the object is updated, then that seems a lot more natural. Is my thought process correct?

Peter Lawrey Over a year ago

The first new POJO(); is likely to be redundant.

Eran Over a year ago

@PeterLawrey The idea was to call an instance method that changes that instance (since that's what the OP seems to want), so an initial instance must be created in order to call pojo.get(5). Of course, you can directly call the static method.

Eran Over a year ago

@user2066880 For that purpose, pojo.get(5) would have to change the state of the existing instance. It can't change the pojo variable to refer to a different instance.

Peter Lawrey · Accepted Answer · 2015-01-20 07:58:57Z

1

If I could just call pojo.get(5) and the object is updated,

What you really want is just

 BaseEntity pojo = DAO.get(id);

The only way to change a reference to an object is to return it. Trying to wrapping it appears to just make your code more complicated in this case.

answered Jan 20, 2015 at 7:58

Peter Lawrey

535k83 gold badges770 silver badges1.2k bronze badges

Collectives™ on Stack Overflow

Java: Setting "this" instance to an object of the same type

2 Answers 2

4 Comments

Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

2 Answers 2

4 Comments

Comments

Your Answer

Sign up or log in

Post as a guest

Related