CODEWITHSUNDEEP
javaarraylist
Stryker1050
Stryker1050

Reputation: 1

Length of ArrayList in Java

How do I find the size of an ArrayList in Java? I do not mean the number of elements, but the number of indexes. 

public static void main(String[] args) {
    ArrayList hash = new ArrayList(5);
    System.out.println(hash.size());
}

Prints out "0." Using:

    System.out.println(hash.toArray().length);

Also prints out a "0." I have looked in http://docs.oracle.com/javase/7/docs/api/java/util/ArrayList.html but I do not see a method that will help me. Is my ArrayList reverting to a size of 0 if I do not add anything to it?

EDIT The assignment is to create a hash table using ArrayList. I am supposed to create a hash function using the formula

    double hashkey = Math.floor(hash.size()*(Math.E*key-Math.floor(Math.E*key)));

Where key is an integer. hashkey then becomes the index where the value will be stored. I am using hash.size() as a placeholder at the moment, but that value should be the capacity of my ArrayList.

Upvotes: 0

Views: 9128

Answers (3)

Stephen C
Stephen C

Reputation: 718678

There is no such method in the ArrayList API. The capacity of an ArrayList is hidden by design.

However, I think that your question is based on a misunderstanding.

How do I find the size of an ArrayList in Java? I do not mean the number of elements, but the number of indexes.

In fact, the size of a List, the number of elements in a List, and the number of indexes (i.e. indexable positions) for a List ... are all the same thing.

The capacity of an ArrayList is something different. It is the number of elements that the object could contain, without reallocating the list's backing array. However, the fact that the list has a capacity N does NOT mean that you can index up to N - 1. In fact, you can only index up to size() - 1, irrespective of the capacity.

Now to deal with your examples:

ArrayList list = new ArrayList(5);
System.out.println(list.size());

This prints out zero because the list has zero elements. The ArrayList() and ArrayList(int) constructors both create and return lists that are empty. The list currently has space for 5 elements (because you gave it an initial capacity of 5) but you can't index those slots. 

System.out.println(list.toArray().length);

This prints zero because when you copy the list's contents to an array (using toArray()), the array is the same size as the list. By definition.

This does not mean that the list's backing array has changed. On the contrary, it is still big enough to hold 5 elements without reallocation ... just like before.

But ... I hear you say ... the array's length is zero!

Yes, but that is not the backing array! The toArray() method allocates a new array and copies the List contents into that array. It does NOT return the actual backing array.

Upvotes: 2

padawan
padawan

Reputation: 1315

Maybe you should encapsulate your ArrayList in a class and add another attribute private int capacity in that class as well.

public class AdvancedArrayList<T>
{
  private int capacity;
  private ArrayList<T> list;
  public AdvancedArrayList<T>(int capacity)
  {
    this.capacity = capacity;
    list = new ArrayList<>();
  }
  public ArrayList<T> getList()
  {
    return list;
  }
  public int getCapacity()
  {
    return capacity;
  }
  public void addElement(T element)
  {
     if(list.size() < capacity)
        list.add(element);
     else
        System.out.println("Capacity is full");
  }
}

Notice that size is different than capacity.

Upvotes: 1

Shreyos Adikari
Shreyos Adikari

Reputation: 12744

ArrayList.size() will give the current size.That's why hash.size() giving you the current size of your ArrayList hash. It will not give you the capacity.
You just initialized the list. Have not add any elements to your arraylist, that's why its giving 0.

Upvotes: 4

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