Pointer to Pointer

[Ankur Srivastava]

Hi All,

I earlier had an understanding that when we create an object, the object is created on a heap and the reference we create, which points to the object is on the stack.

But recently I read something like: the reference we create does not directly point to the object in the heap, but it points to another memory location on the stack which has the reference to the object on heap.

I am a bit confused So please help !!

Thanks in advance

- Ankur

[Georgy Bolyuba]

Hi, Ankur.

The content of reference variable depends of current JVM. So, it might be a pointer to object or pointer to pointer. What's the diff?

George.

[ankur rathi]

Originally posted by Ankur Srivastava:

But recently I read something like: the reference we create does not directly point to the object in the heap, but it points to another memory location on the stack which has the reference to the object on heap.

I don't feel any advantage of having double reference, so logically it should not be like this. Could you post link or book name where you have read this...

[Steve Morrow]

I don't feel any advantage of having double reference, so logically it should not be like this.

The Java� Virtual Machine Specification:

In some of Sun's implementations of the Java virtual machine, a reference to a class instance is a pointer to a handle that is itself a pair of pointers: one to a table containing the methods of the object and a pointer to the Class object that represents the type of the object, and the other to the memory allocated from the heap for the object data.

[Ernest Friedman-Hill]

Originally posted by rathi ji:

I don't feel any advantage of having double reference

Double pointers like this (often called "handles") are a common feature of automated heap managers. I remember that the old Mac Toolbox used handles for everything. The useful thing about handles is that you can compact the heap by moving objects around to fill in empty space, and the pointer held by the application doesn't have to be changed -- only the "middle" pointer changes when a memory block is moved, while the application pointer still points to the "middle" pointer.

[Jeff Bosch]

It seems like a basic jump table to me. By using double-indirection, the JVM can optimize memory by moving objects around and it would only have to update the target side of the table.