Java garbage collection

Question

Java automatically calls garbage collector, then why we need manual calls for garbage collection? When should use System.gc()

Answer 1

System.gc() is only a suggestion. But it does make sense in some situations.

Suppose you have class MyClass, and you're wondering how much memory does one instance take. What you can do is this (roughly speaking):

MyClass [] objects= new MyClass[100000];
System.gc();
long memoryNow = Runtime.getRuntime().freeMemory();
for (int i = 0; i < 100000; i ++) {
  objects[i] = new MyClass();
}
System.gc();
long memoryLater = Runtime.getRuntime().freeMemory();
int objectSize = (int)((memoryLater - memoryNow) / 100000);

There are other similar cases I've found System.gc() to be useful.