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Taewan
Taewan

Reputation: 1227

Where does java reference variable stored?

How does Java's reference variable stored? Is that work similar to C pointer?

what I mean by reference variable is myDog in this code

Dog myDog = new Dog();

I understood about C pointer, it stores in the heap if global variable, and it stores in the stack if local variable. I wonder java works same way.

Upvotes: 12

Views: 12809

Answers (6)

Johnny
Johnny

Reputation: 15413

All the references to s1, s2, ob1, obj2 and obj3 will be stored in the Stack.

The objects data, will be stored in the Heap (and for String for example in can be stored in a special constant pool).

enter image description here

Upvotes: 4

nanofarad
nanofarad

Reputation: 41271

You need to understand a bit of the lower levels of Java memory organization. On the stack, primitives(int, double, boolean, etc) and object references pointing to the heap are stored.

Inside any object the same is true. It either contains references to other objects or primitives directly. Objects are always references in any context and those references are passed by value.

So we may have:

[ STACK ]                          [ HEAP ] 
int a: 10;                     ->  MyWrapperObject@21f03b70====||
double b: 10.4;                |   ||     int someField: 11    ||
MyWrapperObject@21f03b70 ------|   ||     String@10112222  ---------- 
......                             ||==========================||    |
                                                                     |
                                                                     |
                                    String@10112222============||<----
                                    || ...                     ||
                                    || ...                     ||
                                    }}=========================||

Note that use in some cases(as in via JVM internals) objects may be stored in non-heap memory.

Upvotes: 25

Ted Hopp
Ted Hopp

Reputation: 234795

Java works the same way. Be aware that no variable in Java has an object as a value; all object variables (and field) are references to objects. The objects themselves are maintained somewhere (usually on the heap) by the Java virtual machine. Java has automatic garbage collection, so (unlike in C) you don't need to worry about freeing the object. Once all live references to it are out of scope, it will eventually be swept up by the garbage collector.

For instance:

Dog myDog = new Dog();
someMethod(myDog);

passes to someMethod a reference to the dog object that is referenced by myDog. Changes to the object that might occur inside someMethod will be seen in myDog after the method returns.

Upvotes: 1

joews
joews

Reputation: 30310

In general local variables are stored on the stack. Instance variables are stored on the heap.

Upvotes: 3

necromancer
necromancer

Reputation: 24641

It is exactly the same as a C pointer, and almost always on the stack. In C the object itself is on the heap when you say new Dog(); but the Dog * myDogP pointer (4 or 8 bytes) need not be on the heap and is usually on the stack.

Upvotes: 0

tbodt
tbodt

Reputation: 16987

It does work similarly. What is actually happening is that the object itself is stored in the heap, and a reference to the object, which is just like a C pointer, is stored in a local variable.

Upvotes: 0

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