java: ArrayList - how can i check if an index exists ?

Question

Hi. I'm using ArrayList and i add data in specific indexes, how can i check if a specific index exists ?

should i just get() and check the value or wait for an exception or is there another way ?

update

thank you for your answers but i add stuff in specific indexes

once i will add at index 8, other times at index 2. the length of the list will not show me the available indexes.

Answer 1

The method arrayList.size() returns the number of items in the list - so if the index is greater than or equal to the size(), it doesn't exist.

Answer 2

If your index is less than the size of your list then it does exist, possibly with null value. If index is bigger then you may call ensureCapacity() to be able to use that index.

If you want to check if a value at your index is null or not, call get()

Answer 3

You can check the size of an ArrayList using the size() method. This will return the maximum index +1

Answer 4

You could check for the size of the array.

package sojava;
import java.util.ArrayList;

public class Main {
    public static Object get(ArrayList list, int index) {
        if (list.size() > index) { return list.get(index); }
        return null;
    }

    public static void main(String[] args) {
        ArrayList list = new ArrayList();
        list.add(""); list.add(""); list.add("");        
        System.out.println(get(list, 4));
        // prints 'null'
    }
}

Answer 5

This is what you need ...

public boolean indexExists(final List list, final int index) {
    return index >= 0 && index <= list.size();
}

Why not use an plain old array? Indexed access to a List is a code smell I think.

Answer 6

Regarding your update (which probably should be another question). You should use an array of these objects instead an ArrayList, so you can simply check the value for null:

Object[] array = new Object[MAX_ENTRIES];
..
if ( array[ 8 ] == null ) {
   // not available
}
else {
   // do something
}

Best-Practice

If you don't have hundred of entries in your array you should consider organizing it as a class to get rid of the magic numbers 3,8 etc.

Control flow using exception is bad practice.