Memory usage by Array of Long, Array of Integer and ArrayList

Question

Whether I use Long[], Integer[] or ArrayList<Integer>, why do all of them return the same memory usage?

System.out.println("Memory Usage : " + Runtime.getRuntime().totalMemory()/(1024*1024));
System.out.println("Memory Usage : " + (Runtime.getRuntime().totalMemory() - Runtime.getRuntime().freeMemory())/(1024*1024));

//Long[] aa = new Long[70000000];
Integer[] bb = new Integer[70000000];
//ArrayList<Integer> a = new ArrayList<Integer>(70000000);

System.gc();

System.out.println("Memory Usage : " + Runtime.getRuntime().totalMemory()/(1024*1024));
System.out.println("Memory Usage : " + (Runtime.getRuntime().totalMemory() - Runtime.getRuntime().freeMemory())/(1024*1024));

Because your measurement methods are completely inaccurate. System.gc() is "Hey, can you maybe GC at some point in the vaguely near future?" and Runtime.getRuntime().totalMemory() is a very rough approximation that can be wildly off. — Louis Wasserman
– Louis Wasserman, Commented Nov 30, 2012 at 15:35

Jon Skeet · Accepted Answer · 2012-11-30 15:37:50Z

6

In the above statements Whether i use Long[], Integer[] or ArrayList, why all of them give same memory usage ?

It makes perfect sense. In all cases, you're basically allocating an array which is 70000000 * (size of reference on your JVM). The size of an Integer reference is the same as the size of a Long reference, which is the same as the size of an Object reference. In the ArrayList case you've got the very small additional overhead of the ArrayList object itself, but that's pretty small - and easily explained as per Louis's comment.

If you actually populated those arrays, creating instances of Long and Integer, then you'd see a difference. Likewise if you created a long[] vs an int[], you'd see a difference.

answered Nov 30, 2012 at 15:37

Jon Skeet

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