Q:

Destroy java object

We can delete object in C++ as
delete <object>

How can I delete object in java without inbuilt garbage collector.
If I perform
object = null

as per my knowledge it won't deallocate the memory. How can I free the space.

A:

An object in java is eligible for garbage collection when you  : 

Remove all the references to it.
Leave the block where it was declared

When it becomes eligible, care will be taken by the JVM to free up the used memory by that object at appropriate time.
Manually calling System.gc() is not a good idea at all. Any code that depends on it for correctness is certainly broken; any that rely on it for performance are most likely broken.
So, in short, there is no direct control of the programmer on the memory allocation/ deallocation.

A:

You don't need to. 
The Garbage Collector will do that any time it pleases. 
As long as there's no live reference to an object anymore it will be released ... then :-)

