ArrayList, java. Whats the difference between these two constructors

Question

I'm kind of a newbie still in java, can you tell me whats the difference between these two constructors?

First:

public class Plan
{

   ArrayList<Point2D> points;

   public Plan(ArrayList<Ponto2D> points)
   {
       this.points = new Arraylist<Point2D>(points);
   }

}

and this : second:

public class Plan
{

    public Plan(ArrayList<Point2D> lpoints)
    {
        points = new ArrayList<Point2D>();
        for(Point2D p : lpoints) point.add(p.clone());
    }

}

Please be careful with indent, your code is really hard to read — realUser404, Aug 26 '15 at 14:36
The first constructor takes a collection and assigns it to the `points` ArrayList, the second constructor takes a ArrayList, iterates through it, and adds to `points` after initializing `points`. — Honinbo Shusaku, Aug 26 '15 at 14:38
@Abdul, the first one doesn't assign the passed list, it copies the list. — Mick Mnemonic, Aug 26 '15 at 14:39
@Abdul Also note the use of `Point2D.clone()` in the second example. — Edd, Aug 26 '15 at 14:40
@MickMnemonic Ah ok, I just looked it up, and learned what a `shallow copy` is — Honinbo Shusaku, Aug 26 '15 at 14:42

score 5 · Answer 1 · edited May 23 '17 at 11:45

5

The first constructor is a shallow copy, the second one a deep copy.

Answer given by S.Lott for this question.

Shallow copies duplicate as little as possible. A shallow copy of a collection is a copy of the collection structure, not the elements. With a shallow copy, two collections now share the individual elements.

Deep copies duplicate everything. A deep copy of a collection is two collections with all of the elements in the original collection duplicated.

edited May 23 '17 at 11:45

Community

1
1

answered Aug 26 '15 at 14:39

NiziL

5,068
23
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I believe the first constructor is actually cloning the list, so not sharing the elements – realUser404 Aug 26 '15 at 14:46
@realUser404 It's a trap ! (:P) The first constructor clone the list, but share the same elements, the second one duplicate the elements thanks to `clone()`. You can take a look at the [OpenJDK implementation](http://grepcode.com/file/repository.grepcode.com/java/root/jdk/openjdk/6-b14/java/util/ArrayList.java) of `ArrayList`, which use `System.arraycopy(...)` behind the hood. – NiziL Aug 26 '15 at 14:50

score 3 · Answer 2 · answered Aug 26 '15 at 14:42

3

this.points = new Arraylist<Point2D>(points);

This takes a whole collection and uses the collection to initlaize your points ArrayList.

for(Point2D p : lpoints) point.add(p.clone());

That has the same result but adds each element of the lpoints-collection one by one to your points list.

So for your usage, use the first possibility.

answered Aug 26 '15 at 14:42

Christian

22,585
9
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in second case, it adds a **copy** of the point2D, which is not quite the same – Steve Marion Aug 26 '15 at 16:03

score 1 · Answer 3 · answered Aug 26 '15 at 14:39

1

in first case, the parameter ArrayList share the same points (p1.equals(p2) would be true, p1 == p2 would be true) in the second case, they have different copy of the points (p1.equals(p2) would be true, but p1 == p2 would be false)

answered Aug 26 '15 at 14:39

Steve Marion

289
1
9

score 1 · Answer 4 · 2015-08-26T15:00:55.070

In your first constructor you use the default Java Constructor for creating an ArrayList that takes a collection as it's argument.

(From Java Docs)

public ArrayList(Collection < ? extends E > c)

Constructs a list containing the elements of the specified collection, in the order they are returned by the collection's iterator.

This is basically same as writing your own iterator (From your example).

public Plan(ArrayList<Point2D> lpoints) {
  points = new ArrayList<Point2D>();
  for(Point2D p : lpoints) 
      point.add(p.clone());
}

In this example however we use a method called .clone() because we don't want each object's shallow copy. We want to duplicate them.

[Edit]: Both the examples don't do the same thing. First one is a shallow copy and second one a deep copy.

ArrayList, java. Whats the difference between these two constructors

4 Answers4