When JVM starts up, it creates a heap area which is known as runtime data area. This is where all the objects (instances of class) are stored. Since this area is limited, it is required to manage this area efficiently by removing the objects that are no longer in use. The process of removing unused objects from heap memory is known as Garbage collection and this is a part of memory management in Java.

Languages like C/C++ don’t support automatic garbage collection, however in java, the garbage collection is automatic.

Now we know that the garbage collection in java is automatic. Lets see when does java performs garbage collection.

When does java perform garbage collection?

1. When the object is no longer reachable:

BeginnersBook obj = new BeginnersBook();  
obj = null;

Here the reference obj was pointing to the object of class BeginnersBook but since we have assigned a null value to it, this is no longer pointing to that object, which makes the BeginnersBook object unreachable and thus unusable. Such objects are automatically available for garbage collection in Java.

Another example is:

char[] sayhello = { 'h', 'e', 'l', 'l', 'o'};
String str = new String(sayhello);
str = null;

Here the reference str of String class was pointing to a string “hello” in the heap memory but since we have assigned the null value to str, the object “hello” present in the heap memory is unusable.

2. When one reference is copied to another reference:

BeginnersBook obj1 = new BeginnersBook();
BeginnersBook obj2 = new BeginnersBook();
obj2 = obj1;

Here we have assigned the reference obj1 to obj2, which means the instance (object) pointed by (referenced by) obj2 is not reachable and available for garbage collection.