为什么Runtime.freeMemory（）在构造对象后显示更多内存？

Question

I am trying to figure the size of a hashmap in memory without a profiler. So I did the following:

HashMap<Integer, String> map = new HashMap<Integer, String>();
long start = System.currentTimeMillis();
lotsOfGC();
long freeMemoryBeforeConstruction = Runtime.getRuntime().freeMemory();
System.out.println("memory before = " + freeMemoryBeforeConstruction);
for(int i = 0; i < numbers.length; i++) {
    String value = "value"+ i;
    map.put(i, value);
}
long end = System.currentTimeMillis();
lotsOfGC();
long freeMemoryAfterConstruction = Runtime.getRuntime().freeMemory();
System.out.println("memory after= " + freeMemoryAfterConstruction );

Where lotsOfGC is just:

static void lotsOfGC() {
        for (int i = 0; i < 20; i++) {
            System.gc();
            try {
                Thread.sleep(100);
            } catch (InterruptedException e) {
                Thread.currentThread().interrupt();
            }
        }
    }

The result I get is:

memory before = 76083464
memory after = 722062528

Can someone please explain to me why the free memory after creating the hashmap is bigger?

Update: After reading the comment of @Patricia Shanahan I used the total memory and got:

memory before        =  76083464  
total memory before  =  96468992  
memory after         = 735235264  
total memory after   = 755367936

Answer 1

The currently allocated memory is the difference

Runtime.getRuntime().totalMemory() - Runtime.getRuntime().freeMemory()

Allocating an object, especially a large one, may require the JVM to obtain additional memory from the operating system. That is a relatively expensive operation, so it is more efficient for the JVM to request memory in large chunks. When it obtains more memory than the current allocation requires, both total memory and free memory will increase.