CODEWITHSUNDEEP
javaloopsdata-structures
Ravi K M
Ravi K M

Reputation: 87

Java OutOfMemoryError: Strange behavior

Below program (prog1) throws OutOfMemoryError error. Rightly so. But if I add sysout below line 5(prog2), it wont throw error. Any reason for this strange behaviour?

prog1:

public static void main(String[] args) {
    List<String> myList = new ArrayList<>();     
    try {
        while (true) {
            myList.add("My String"); //5
            }
    } catch (Exception e) {
        e.printStackTrace();
    }
}

prog2:

public static void main(String[] args) {
    List<String> myList = new ArrayList<>();        
    try {
        while (true) {
            myList.add("My String");
            System.out.println(myList);
        }
    } catch (Exception e) {
        e.printStackTrace();
    }
}

Upvotes: 1

Views: 50

Answers (1)

Stephen C
Stephen C

Reputation: 719239

The second version will also run out of memory eventually. (If you don't get bored and kill it first.)

The problem is that the print statement is repeatedly printing a list that is getting longer and longer.

By the time you have added N items to the list in the second version, you will have printed lists of sizes 1 + 2 + 3 + ... + N. That is N * (N + 1) / 2 copies of the string "My String" ... and some more. That is O(N^2).

If you are writing that to a /dev/null it will take a long time. If you write it to a file, longer. If you write it to a console displaying on your screen ... come back in a few hours.

As an experiment, replace 

    System.out.println(myList);

with 

    System.out.print("X");

Now you will only print O(N) characters instead of O(N^2) ... and the OOME will happen a lot sooner.

Upvotes: 2

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