CODEWITHSUNDEEP
javaarraysgenericsarraylist
mtmurdock
mtmurdock

Reputation: 13062

How do I make a generic list in Java?

I am looking to reinvent the wheel a little and create my own generic array-backed list class in Java similar to ArrayList. Yes I know this is silly, but it is an academic pursuit. The problem is that you cannot instantiate an array of a generic type

public class MySuperCoolList<E> {
   E[] array;

   public MySuperCoolList<E> () {
      array = new E[10]; // ERROR: cannot do this!
   }
}

Surely there must be a solution to this problem because Java's ArrayList is doing the same thing. The question is, how? How can I instantiate an array of a generic type E? And how is it done in ArrayList (if anyone knows)?

Upvotes: 0

Views: 1676

Answers (3)

obataku
obataku

Reputation: 29646

public MySuperCoolList<E>(final Class<? extends E> type) {
  array = (E[]) Arrays.newInstance(type, 10);
}

See Arrays.newInstance. This is how Arrays.copyOf works.

I've placed a PoC here.

int[] vals = (int[]) Array.newInstance(Integer.TYPE, 10);
vals[0] = 500;
System.out.println(vals);
System.out.println(vals.length);
System.out.println(Arrays.toString(vals));

As you can see, the output is as expected:

[I@fb53f6
10
[500, 0, 0, 0, 0, 0, 0, 0, 0, 0]

Upvotes: 0

Akash Yadav
Akash Yadav

Reputation: 2421

In this case , you might want to use Array of Object Type , cause object type can accomodate everything and the code goes like,

public class MySuperCoolList<E> {
    Object[] array;

    public MySuperCoolList () {
       array = new Object[10];
    }

    public E get(int index){
       return (E) array[index];
    }

    public void put(int index,E val) {
      array[index] = val;
    }

}

Upvotes: 0

Mark Byers
Mark Byers

Reputation: 838106

And how is it done in ArrayList (if anyone knows)?

It's open source. Take a look at the source code for ArrayList:

/**
 * The array buffer into which the elements of the ArrayList are stored.
 * The capacity of the ArrayList is the length of this array buffer.
 */
private transient Object[] elementData;

Upvotes: 4

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