Java阅读Ram用法

Question

When i want to read ram usage from my program with

"long used = Runtime.getRuntime().totalMemory() -Runtime.getRuntime().freeMemory();"

method, it returns me the same value every time. For example my code

if(selectSort.equals("1")){

                //-------------------------BEST CASE MERGE---------------------------------------

                if(selectArray.equals("1"))
                    mergeSort.mergeSort(array1,0,array1.length - 1);

                else if(selectArray.equals("2"))
                    mergeSort.mergeSort(array2,0,array2.length - 1);

                else if(selectArray.equals("3"))
                    mergeSort.mergeSort(array3,0,array3.length - 1);

                else if(selectArray.equals("4"))
                    mergeSort.mergeSort(array4,0,array4.length - 1);
                else
                    System.out.println("Wrong Choice");

                long used = Runtime.getRuntime().totalMemory() -Runtime.getRuntime().freeMemory();
                String memoryString = format.format(used / (1024*1024));
                System.out.println("\nMemory Usage of : " + memoryString + " MB");

                //-----------------------------END---------------------------------------------------   
            }

In here there is no problem, ram values are 6 - 7 - 17 - 27 in an order BUT

if(selectSort.equals("2")){ 

                //-------------------------BEST CASE QUICK---------------------------------------

                if(selectArray.equals("1"))
                    quickSort.quickSort(array1, 0, array1.length - 1);
                else if(selectArray.equals("2"))
                    quickSort.quickSort(array2, 0, array2.length - 1);
                else if(selectArray.equals("3"))
                    quickSort.quickSort(array3, 0, array3.length - 1);
                else if(selectArray.equals("4"))
                    quickSort.quickSort(array4, 0, array4.length - 1);
                else
                    System.out.println("Wrong Choice");

                long used = Runtime.getRuntime().totalMemory() -Runtime.getRuntime().freeMemory();
                String memoryString = format.format(used / (1024*1024));
                System.out.println("\nMemory Usage of : " + memoryString + " MB");

In there ram values are the same, 6. I cannot understand why is it? Can you help me? Thanks.

Here is my program, just import it and start, there is a menu ;)

https://drive.google.com/open?id=0B-4t0pVRkFn_b3FJQkF0cmFIWlE

Answer 1

What these calls are measuring is the allocation of memory of the JVM. There is an important difference between the amount of memory that the OS has allocated for the JVM and the amount of that memory that the JVM actually uses to store data.

Furthermore, JVM internal memory is only freed after the garbage collector has run (GC). And after that the memory is usually not returned to the OS.