Java: How to deep-clone a HashSet neatly?

Question

I have a HashSet of MyObject that I need to clone. If MyObject implements a copy-constructor, what is the easiest, neatest way to clone Set myObjects

Obviously I could do something like:

 Set<MyObject> myNewObjects = new Set<MyObject>(); 
 for(MyObject obj: myObjects) myNewObjects.add(new MyObject(obj));

But I'm doing this as part of a loooong copy-construcotr, and I'd really like to just be able to do it in one line like:

public myClass(MyClass toClone){
    //... 
    this.myObjects = new Set<MyObjects>(toClone.getmyObjects()); 
    //... 
}

Any suggestions?

Hi David, every article I've come across has strongly warned me off using clone(), as it's implementation is inherently broken... do you disagree or was this just an FYI? — Paul, Jun 26 '14 at 18:08
You can always override clone, but yes, it's kinda broken by default. — David Ehrmann, Jun 26 '14 at 18:12
There is no fully generic way to deep clone anything in Java. You will have to use `MyObject`'s constructor directly. — Louis Wasserman, Jun 26 '14 at 18:14

score 15 · Answer 1 · answered Jun 26 '14 at 18:06

15

If you are using Java 8, you can do something like this:

Set<MyObject> set1 = new HashSet<>();
Set<MyObject> set2 = set1.stream().map(MyObject::new).collect(Collectors.toSet());

Keep in mind, however, that using Collectors.toSet():

There are no guarantees on the type, mutability, serializability, or thread-safety of the Set returned

Although current implementation in Java 8 Update 5 uses regular HashSet.

answered Jun 26 '14 at 18:06

izstas

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Java 6 legacy system :( – Paul Jun 26 '14 at 18:07

Giovanni Botta · Answer 2 · 2014-06-26T18:07:25.963

4

You can use Google Guava's Iterables to do something like this:

Set<MyObject> newSet = new HashSet<MyObject>(
  Iterables.transform(myObjects,new Function<MyObject, MyObject>() {
    @Override
    public MyObject apply(MyObject input) {
      return new MyObject(input);
    }
  }));

Or if you want it immutable use Guava's immutable set:

Set<MyObject> newSet = ImmutableSet.copyOf(
  Iterables.transform(myObjects,new Function<MyObject, MyObject>() {
    @Override
    public MyObject apply(MyObject input) {
      return new MyObject(input);
    }
  }));

Unfortunately it does not get any more compact than what you already have unless you can use Java 8.

edited Jun 26 '14 at 18:07

answered Jun 26 '14 at 17:57

Giovanni Botta

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@Paul asked for a deep copy. – David Ehrmann Jun 26 '14 at 17:57
Hi Giovanni, I was under the impression that the HashSet Copy constructor would give me a shallow copy? I mean, it's not going to call the copy constructors of each element in the set? – Paul Jun 26 '14 at 18:01
@Paul I didn't realize you were actually creating new instances, my apologies. I'll edit. – Giovanni Botta Jun 26 '14 at 18:04
1

@Paul Please take a look at the edit, and once again apologies for misreading! – Giovanni Botta Jun 26 '14 at 18:08

score 0 · Answer 3 · answered Nov 22 '17 at 12:36

Set<MyObject> set = new HashSet<>();
        for (MyObject value : myHash) {
            set.add(value.copy());
        }

value.copy() is the function that clones your object (you need to create a method like that and create a new object and assign the fields accordingly to make deep cloning work.).

Java: How to deep-clone a HashSet neatly?

3 Answers3