Question

The Java code is as follows:

Random r = new Random(1234697890);
HashMap<Integer, List<Integer>> map = new HashMap<Integer, List<Integer>>();
List<Integer> list = new ArrayList<Integer>();

for(int i=0;i<100000;i++){
    for(int j=0;j<1000;j++){
        list.add(r.nextInt(100000));
    }
    map.put(i, list);
    map.remove(i);
}

when i reaches 37553 , java.lang.OutOfMemoryError: Java heap space happens.
It seems that garbage collection does not happen in the loop.
Now I wonder how to fix the problem.

Answer 1

Try rewriting the code as follows and you should not get OOME’s …

Random r = new Random(1234697890);
HashMap<Integer, List<Integer>> map = new HashMap<Integer, List<Integer>>();

for(int i=0;i<100000;i++){
    List<Integer> list = new ArrayList<Integer>();
    for(int j=0;j<1000;j++){
        list.add(r.nextInt(100000));
    }
    map.put(i, list);
    map.remove(i);
}

The problem with your original code is that:

• you only create one list,
• you keep adding more and more elements to it, and
• that list only becomes garbage when the code completes … because it is “in scope” the whole time.

Moving the list declaration inside the loop means that a new ArrayList is created and filled in each loop iteration, and becomes garbage when you start the next iteration.

---

Someone suggested calling System.gc(). It won’t help at all in your case because there is minimal¹ garbage to be collected. And in general it is a bad idea because:

• the GC is guaranteed to have run immediately before an OOME is thrown,
• the JVM can figure out better than you can when is the best (i.e. most efficient) time to run the GC,
• your call to System.gc() may be totally ignored anyway. The JVM can be configured so that calls to System.gc() are ignored.

_{1 – The pedant in me would like to point out that map.put(i, list); map.remove(i); is most likely generating an Integer object that most likely becomes garbage. However, this is “chicken feed” compared to your indefinitely growing ArrayList object.}