Question

For whatever reason, I wanted to see how many objects I could create and populate a LinkedList with. I used Runtime.getRuntime().freeMemory() to get the approximation of free memory in my JVM. I wrote this:

public static void main(String[] arg) {
    Scanner kb = new Scanner(System.in);
    List<Long> mem = new LinkedList<Long>();
    while (true) {
        System.out.println("Max memory: " + Runtime.getRuntime().maxMemory() + ". Available memory: " + Runtime.getRuntime().freeMemory() + " bytes. Press enter to use more.");
        String s = kb.nextLine();
        if (s.equals("m"))
        for (int i = 0; i < 1000000; i++) {
            mem.add(new Long(Long.MAX_VALUE));
        }
    }
}

If I write in m, the app adds a million Long objects to the list. You would think the more objects (to which we have references, so can’t be gc’ed), the less free memory. Running the code:

Max memory: 1897725952. Available memory: 127257696 bytes.
m
Max memory: 1897725952. Available memory: 108426520 bytes.
m
Max memory: 1897725952. Available memory: 139873296 bytes.
m
Max memory: 1897725952. Available memory: 210632232 bytes.
m
Max memory: 1897725952. Available memory: 137268792 bytes.
m
Max memory: 1897725952. Available memory: 239504784 bytes.
m
Max memory: 1897725952. Available memory: 169507792 bytes.
m
Max memory: 1897725952. Available memory: 259686128 bytes.
m
Max memory: 1897725952. Available memory: 189293488 bytes.
m
Max memory: 1897725952. Available memory: 387686544 bytes.

The available memory fluctuates. How does this happen? Is the GC cleaning up other things (what other things are there on the heap to really clean up?), is the freeMemory() method returning an approximation that’s way off? Am I missing something or am I crazy?

Answer 1

As you consume more memory, the heap grows. Note that freeMemory() reports usage relative to the current heap size, which can grow. Check totalMemory() instead of maxMemory(). If I’m recalling this right.

Or are you asking what is being allocated that can possibly be garbage collected?

• The Random you allocate in the loop
• Objects allocated during the process of reading from an OS stream, parsing into characters, then parsing into lines
• Possibly some stuff allocated during the call to check free memory itself