java collection Out of memory error

Question

Im trying to time the Arraylist and Linkedlist by populating more than million rows and get the below error after the Arraylist population,

Exception in thread "main" java.lang.OutOfMemoryError: Java heap space at java.lang.Integer.valueOf(Integer.java:642) at scratch.Collectionss.main(Collectionss.java:25)

How do I avoid this error, I tried setting the l1 = null but that gives me a error,

public class Collectionss {
    public static void main(String[] args){
        // 
        long starttime = System.currentTimeMillis();
        List<Integer> l1 = new ArrayList<Integer>();
        for (int i = 1; i <= 10000000; i++){
            l1.add(i);
        }
        System.out.println(l1.size());

        long endtime = System.currentTimeMillis();

        System.out.println(endtime - starttime);

        //
        long starttime1 = System.currentTimeMillis();
        List<Integer> l2 = new LinkedList<Integer>();
        for (int i = 1; i <= 10000000; i++){
            l2.add(i);
        }
        System.out.println(l2.size());

        long endtime1 = System.currentTimeMillis();

        System.out.println(endtime1 - starttime1);

    }

}

Answer 1

You should be running these tests in separate methods as the optimisation of the first loop can interfere with the optimisation of the second i.e. the second one can be slower, just because it is second.

I suggest you run both tests at least 10 (or for 2 seconds) and use System.nanoTime() which has higher resolution.

If you do this and you still run out of memory I suggest you increase the maximum memory size. If you are running 32-bit windows, the default is very low. You can increase it with -Xmx1g on the command line

---

If you run the following, you can see that GCs have the biggest impact which is not surprising as the problem most produces garbage

import java.util.ArrayList;
import java.util.LinkedList;
import java.util.List;

public class Collectionss {

    public static final int TO_ADD = 10000000;

    public static void main(String[] args) {
        for (int i = 0; i < 5; i++) {
            long timeAL = timeAddToArrayList();
            long timeLL = timeAddsToLinkedList();
            System.out.printf("Time to add %,d Integer to ArrayList %.3f sec, LinkedList %.3f%n",
                    TO_ADD, timeAL / 1e9, timeLL / 1e9);
        }
    }

    private static long timeAddToArrayList() {
        long starttime = System.nanoTime();
        List<Integer> l1 = new ArrayList<Integer>();
        for (int i = 1; i <= TO_ADD; i++) {
            l1.add(i);
        }
        assert TO_ADD == l1.size();

        return System.nanoTime() - starttime;
    }

    private static long timeAddsToLinkedList() {
        long starttime = System.nanoTime();
        List<Integer> l2 = new LinkedList<Integer>();
        for (int i = 1; i <= TO_ADD; i++) {
            l2.add(i);
        }
        assert TO_ADD == l2.size();

        return System.nanoTime() - starttime;
    }
}

prints

Time to add 10,000,000 Integer to ArrayList 0.238 sec, LinkedList 1.326
Time to add 10,000,000 Integer to ArrayList 1.193 sec, LinkedList 0.971
Time to add 10,000,000 Integer to ArrayList 0.841 sec, LinkedList 0.048
Time to add 10,000,000 Integer to ArrayList 0.349 sec, LinkedList 1.128
Time to add 10,000,000 Integer to ArrayList 0.064 sec, LinkedList 0.048

However add System.gc() before each test and you get

Time to add 10,000,000 Integer to ArrayList 0.241 sec, LinkedList 2.130
Time to add 10,000,000 Integer to ArrayList 0.070 sec, LinkedList 0.072
Time to add 10,000,000 Integer to ArrayList 0.067 sec, LinkedList 0.053
Time to add 10,000,000 Integer to ArrayList 0.069 sec, LinkedList 0.048
Time to add 10,000,000 Integer to ArrayList 0.065 sec, LinkedList 0.051

Answer 2

try increasing your heap size and run your program

Answer 3

If you are dealing with large data sets, then its always better to initialise your ArrayList with the correct size, in your case.

List<Integer> l1 = new ArrayList<Integer>(10000000);, otherwise your ArrayList will be having a default size of 10, and every time once the size is exceeded, the add method will create a new array of increased size copying all the contents of the old array inside your arraylist.

See the source below from ArrayList ensureCapacity method.

if (minCapacity > oldCapacity) {
    Object oldData[] = elementData;
    int newCapacity = (oldCapacity * 3)/2 + 1;
        if (newCapacity < minCapacity)
    newCapacity = minCapacity;
        // minCapacity is usually close to size, so this is a win:
        **elementData = Arrays.copyOf(elementData, newCapacity);**
}

And if its still happens, even after initialising then increase your heap size as others suggested in the post.

Answer 4

You need to increase the heap availbable to the JVM via the-Xmx4g flag. This will increase the heap size to 4GB. You don't need a command line for that. IDE also accept JVM parameters. E. g. eclipse: Run Configurations... -> Arguments -> VM Arguments

As @crush mentioned you could save memory (resp. the GC some work), if you initialize the ArrayList with the needed capacity.

Answer 5

I think you can specify the heap size whenever you run your program. If you are executing on the command line, whenever you execute using "java " include a parameter: "-Xms1024m -Xmx1024m" or whatever you want your heap size to be. Look jvm launch parameters to get the exact usage.

Answer 6

You are adding 10000000 items in your List collection. Each time will surely add to the memory consumption and finally crossing the heap size.

And you are creating such a collection twice.

You need to figure out why do you need such a big collection. If you need such big collections then you need to have desired increase heap space.