Does ArrayList.clear() method frees memory?

Question

If I'm not mistaken, ArrayList contains the value of memory locations in which the variables you've added to the List are stored. So, my assumption is that when you call ArrayList.clear() method it only frees the aforementioned values (of memory locations) but doesn't free those memory locations themselves. I'll try to illustrate this with an example:

Let's say you've got current status of memory:

[Memory location] (type of variable) *value*

[1000] (int) 32

[1002] (int) 12

[1003] (float) 2.5

And you add them to the list myList, so it contains pointers to the 1000, 1002, 1003 memory locations.

When you call myList.clear() the pointers will be nullified, but the memory locations 1000, 1002, 1003 would still contain previously given values. Am I wrong?

http://stackoverflow.com/questions/6757868/map-clear-vs-new-map-which-one-will-be-better can help you — Pankaj Kumar, Aug 30 '13 at 08:16
Note that `ArrayList` never contains primitives like `int`, but objects like `Integer`. This ends up being fairly important to the answer. — Sean Owen, Aug 30 '13 at 08:57

Karthik T · Accepted Answer · 2013-08-30T08:22:58.113

15

You are correct because the memory is cleared asynchronously by the Garbage collector, and not directly by such library functions. clear would only null them all out, and update the ArrayList state accordingly.

If any or all of those elements are not not referenced by any other portion of your code, they become eligible for garbage collection, and might be destroyed + de-allocated any time from shortly after that call to the end of time.

edited Aug 30 '13 at 08:22

answered Aug 30 '13 at 08:16

Karthik T

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Thanks I'll make sure I manually take care of the discarded variables :) Cheers and nice avatar – nstosic Aug 30 '13 at 08:21
@NitroNbg, There should be no need to manually do anything, as long as they are not referenced, they will be de allocated. And thanks :) – Karthik T Aug 30 '13 at 08:24

score 6 · Answer 2 · answered Aug 30 '13 at 08:19

6

clear only nulls backing array elements

public void clear() {
    modCount++;
    for (int i = 0; i < size; i++)
        elementData[i] = null;
    size = 0;
}

but if we call ArrayList.trimToSize after clear backing array will shrink

public void trimToSize() {
    modCount++;
    int oldCapacity = elementData.length;
    if (size < oldCapacity) {
        elementData = Arrays.copyOf(elementData, size);
    }
}

answered Aug 30 '13 at 08:19

Evgeniy Dorofeev

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This ends up, in the context of the question, wasting space doesnt it? Since it is now not only the cleared elements, but also the Array itself that is in GC limbo – Karthik T Aug 30 '13 at 08:34

score 6 · Answer 3 · edited Dec 27 '18 at 07:48

6

No,It wont clear the memory.After you clear().

See the source code of clear()

 public void More ...clear() {
    modCount++;

    // Let gc do its work
    for (int i = 0; i < size; i++)
       elementData[i] = null;   
       size = 0;
    }

The method is making the elements eligible for Garbage Collector.Not clearing them immediately. Once GC run's,They are gone.

edited Dec 27 '18 at 07:48

Agilanbu

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answered Aug 30 '13 at 08:19

Suresh Atta

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score 2 · Answer 4 · edited Dec 27 '18 at 09:20

The answer to your question, it will not FREE the memory. It will just remove all the object references. Now these objects may or may not be collected by garbage collector depending on how they are referenced at other places.

For instance,

Object 1 refers - > Object 2
ArrayList refers - > Object 2
ArrayList refers - > Object 3

In this case, after Arraylist.clear(), object 3 will be eligible for clean-up but not object 2.

Does ArrayList.clear() method frees memory?

4 Answers4