CODEWITHSUNDEEP
javaarraysoopinitialization
rodrigoalvesvieira
rodrigoalvesvieira

Reputation: 8062

Filling array of Java objects in initialization

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?

Upvotes: 1

Views: 2857

Answers (4)

Peter Lawrey
Peter Lawrey

Reputation: 533502

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.

Upvotes: 3

Holger
Holger

Reputation: 298153

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.

Upvotes: 3

Grim
Grim

Reputation: 1976

Suppose N is always 6 ...

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

Upvotes: 0

Koziołek
Koziołek

Reputation: 2874

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.

Upvotes: 3

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