ArrayList vs ArrayList - both storing values from 0 to 9 .. which takes more memory?

Question

I was asked this question... ArrayList of Integer vs ArrayList of String - both storing values from 0 to 9 ... which takes more memory?

Answer 1

Assuming lists are created as follows:

    List<Integer> integers = new ArrayList<>();
    integers.add(0);
    integers.add(1);
    // ...
    List<String> strings = new ArrayList<>();
    strings.add("0");
    strings.add("1");
    // ...

There two levels of answering this question.

First is about knowing memory consumption of Integer and String.

• How much memory does a string use in Java 8?
• Integer vs int: with regard to memory

So, a String with one character takes 40 bytes (32 on Java 8). An Integer takes 16 bytes.

On this level, list of strings takes more memory than list of integers.

On the second level you have to know that string literals are interned and boxing an int with Integer.valueOf uses cache for values between -128 and at least 127. So basically both add("0") and add(0) will use cached objects and one could say that no additional memory is used.

So depending on whether you consider string pool/integer cache, the answer is either "list of strings takes more memory" or "both lists take the same amount of memory".

If this is an interview question, you should probably give both answers.

ps. I personally wouldn't have known the size of String or Integer out of the head, but would have guessed String to take more memory.

Answer 2

Here is a test result using this tool, it shows that ArrayList<Integer> occupies less memory:

public static void main(String[] args) {
    ArrayList<Integer> integerArrayList = new ArrayList<>();
    ArrayList<String> stringArrayList = new ArrayList<>();
    for (int i = 0; i < 10; i++) {
        integerArrayList.add(i);
        stringArrayList.add(String.valueOf(i));
    }
    System.out.println(RamUsageEstimator.sizeOf(integerArrayList));  // 240
    System.out.println(RamUsageEstimator.sizeOf(stringArrayList));  // 560
}