CODEWITHSUNDEEP
javaarraylist
Vik H
Vik H

Reputation: 11

Difference of 2 Ways of Adding Elements to Java ArrayList Using Iterator

What is the difference between:

for (ArrayList<Integer> a : result)
    temp.add(new ArrayList<Integer>(a));

and

for (ArrayList<Integer> a : result)
    temp.add(a);

?

Upvotes: 1

Views: 36

Answers (4)

kirti
kirti

Reputation: 4609

This is adding a new Arraylist with item of a in temp while iterating result arraylist 

 for (ArrayList a : result)
 temp.add(new ArrayList(a));

This is adding a in temp while iterating result arraylist

    for (ArrayList a : result)
 temp.add(a);

Upvotes: 0

Boris the Spider
Boris the Spider

Reputation: 61148

Lets start with a very simple example:

final List<String> a = new ArrayList<>(Arrays.asList("a", "b", "c"));
System.out.println(a);

Output:

[a, b, c]

Now, lets copy using =

final List<String> b = a;
b.add("d");
System.out.println(b);
System.out.println(a);
System.out.println(a == b);

Output:

[a, b, c, d]
[a, b, c, d]
true

Okay, now lets copy using new:

final List<String> c = new ArrayList<>(b);
c.add("e");
System.out.println(c);
System.out.println(b);
System.out.println(a);
System.out.println(c == b);

Output:

[a, b, c, d, e]
[a, b, c, d]
[a, b, c, d]
false

Okay, so when we copy using = we copy the reference to the same List. When we copy using the copy constructor we actually create an entirely separate List that contains the same elements.

So a and b are references to some same list, they will always return the same result, and any action on a will affect b - think of them as aliases for some other entity.

c is a physical copy of a (and b for that matter) where we have constructed a new entity and copied the contents of a into it.

So, to answer your question; your first method copies the references and your second method copies the contents.

Upvotes: 1

Tom
Tom

Reputation: 17577

for (ArrayList a : result) temp.add(new ArrayList(a));

Will add a new ArrayList to temp that contains the elements of a. See: http://docs.oracle.com/javase/7/docs/api/java/util/ArrayList.html#ArrayList%28java.util.Collection%29

Adding or deleting stuff in a, won't change anything for the list that was added to temp.

for (ArrayList a : result) temp.add(a);

Will add a to temp. Due to the same reference, changes of a affect the list that was added to temp.

Upvotes: 1

Hagai
Hagai

Reputation: 703

new ArrayList(a) actually copies the Arraylist that you already have, see the javadoc of the constructor

So when you have

ArrayList A = new Arraylist();
//fill the arraylist
ArrayList B = new ArrayList(a);

A and B will actually contain the same elements, but A == B will return false

Upvotes: 1

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