Filling array of Java objects in initialization

Question

I have an implemented data structure called List (it's a singly linked list). I have the following code in my Java program:

List[] Adjacent = new List[N]; // Trying to create an array of N List objects

Suppose N = 6. I want Adjacent to be an array of six elements where each one of them is a List object itself. But when I try to access it, the array is actually an array of N null references. What is going on?

Of course I did the following to solve my problem:

for (int p = 0; p < N; p++) 
    Adjacent[p] = new List();

But that's not what I really want. Is there a more efficient/nice way to do it?

I would avoid the class name List. It is the name of a widely used interface... — ppeterka
– ppeterka, Commented Oct 1, 2013 at 11:50
@rodrigoalves correct to "references", Java doesn't have pointers. — Peter Lawrey
– Peter Lawrey, Commented Oct 1, 2013 at 11:55

Peter Mortensen · Accepted Answer · 2015-08-14 05:07:24Z

3

When you have

List list;

this is a reference, not an object. To have an object you have to create one with new List(...). There is no way to work around this. You have to initialise your references to actual objects.

Also, it is far worse to be creating a class which does the same thing with a similar name to something already built in. I suggest you use a List of List like this:

List<List<String>> listofLists = new ArrayList<>();
for(int i = 0; i < N; i++) listOfLists.add(new ArrayList<>());

It is more efficient for you to reuse existing classes.

edited Aug 14, 2015 at 5:07

Peter Mortensen

31.4k22 gold badges110 silver badges134 bronze badges

answered Oct 1, 2013 at 11:50

Peter Lawrey

535k83 gold badges770 silver badges1.2k bronze badges

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

Comments

Koziołek · Accepted Answer · 2013-10-01 11:52:25Z

3

Nope. After create new array you should fill it manualy. Of course we have Arrays.fill(array, object) method but it will set the same object to all positions in array.

answered Oct 1, 2013 at 11:52

Koziołek

2,9042 gold badges30 silver badges50 bronze badges

Comments

Holger · Accepted Answer · 2013-10-01 12:11:12Z

There will be a “nice” way to to it with Java 8:

MyObject[] array=Stream.generate(MyObject::new).limit(N).toArray(MyObject[]::new);

Stream.generate(MyObject::new) will create an unlimited stream of objects provided by a Supplier which is the default constructor of MyObjectin our case, limit(N) obviously limits the number of objects and toArray(MyObject[]::new) will store the objects into an array constructed by an IntFunction<A[]> that constructs an array using a specified int which is set to the equivalent of new MyObject[i] in our case. The stream will provide the limit N for that i parameter.

Though the last part is not the real spirit of streams. Normally, you would direct the stream to the consumer dealing with the objects and omit the array completely.

Grim · Accepted Answer · 2013-10-01 12:03:18Z

0

Suppose N is always 6 ...

List[] l = new List[]{new List(),new List(),
       new List(),new List(),new List(),new List()};

answered Oct 1, 2013 at 12:03

Grim

2,13312 gold badges63 silver badges138 bronze badges

Collectives™ on Stack Overflow

Filling array of Java objects in initialization

4 Answers 4

Comments

Comments

Comments

Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

4 Answers 4

Comments

Comments

Comments

Comments

Your Answer

Sign up or log in

Post as a guest

Related