Why this happens in garbage collection injava

Question

As I know when I run garbage collection using System.gc() method it will collect all the unused and undestroyed objects from the heap and clean it. So when I run the System.gc(); the memory of the JVM should be increased. but in the below code when I run it, the result is confusing me.

Runtime rs = Runtime.getRuntime();
System.out.println("Free memory in JVM before Garbage Collection = " + 
                   rs.freeMemory());
rs.gc();
System.out.println("Free memory in JVM after Garbage Collection = " +
                   rs.freeMemory());

the output is

Free memory in JVM before Garbage Collection = 12184624
Free memory in JVM after Garbage Collection = 12184360    

see the before value is greater than after value.

Please correct me if I in a wrong concept or explain me why this happen.

Thank you.

Answer 1

Calling for garbage collection in Java is merely a suggestion to the JVM. The way that garbage is collected is technically 100% out of your control. Calling System.gc() politely suggests that the trash should be taken out, but Java is within its rights to ignore you completely like a disobedient child.

Although it will USUALLY do as you ask, it won't always and you certainly can't rely on it.