java - Pushing variables to Stack and Variables living in the Stack difference? -

so know there exists 2 memory areas: stack , heap.

i know if create local variable live in stack, not in heap. stack grow push data in:

now try pass confusion having you:

for example simple java code:

public class testclass {     public static void main(string[] args)  {         object foo = null;         object bar = null;     } } 

is translated byte code:

public static void main(java.lang.string[]);   code:    stack=1, locals=3, args_size=1    0:   aconst_null    1:   astore_1    2:   aconst_null    3:   astore_2    4:   return  linenumbertable:  line 5: 0 line 6: 2 line 7: 4  localvariabletable:  start  length  slot  name   signature 0      5      0    args       [ljava/lang/string; 2      3      1    foo       ljava/lang/object; 4      1      2    bar       ljava/lang/object; 

where definition acons_null is:

push null reference onto stack 

and astore_1 is:

store reference local variable 1 

the confusion having is, pushed foo stack, stored in stack again? meant storing reference in local variable? local variable live? same stack pushed foo or these seperate stacks?

now @ point, if call method on first object pushed stack, since stack pointer pointing last element pushed, how processed?

there exists 1 stack per thread in jvm. each stack composed of several frames: each method invocation creates new frame, , when method invocation done, frame destroyed.

within stack frame there 2 areas :

1. the operand stack (don't confuse word "stack" here jvm stack -- stack here denotes area last-in-first-out structure).
2. an array of local variables each variable has index (starting @ zero).

depending on jvm implementation, may or may not contiguous in memory. logically 2 separate sections of stack frame.

as explained in description of aconst_null, aconst_null instruction pushes null object reference onto operand stack.

and explained in description of astore_<n> (where n 0, 1, 2 or 3):

the <n> must index local variable array of current frame (§2.6). objectref on top of operand stack must of type returnaddress or of type reference. it popped operand stack, , value of local variable @ <n> set objectref.

so in example, statement object foo = null translates following:

1. push null (a special reference points "nothing") onto top of operand stack.

  operand stack    __________   |   null   | <-- null pushed on operand stack   |__________|   |          |   |__________|   |          |   |__________| 

2. pop reference operand stack , store in local variable @ index 1. local variable corresponds foo.

  operand stack                           local variables    __________      _______________ _______________ _______________ _______________   |          |    |      args     |   foo (null)  |               |               |   |__________|    |_______0_______|_______1_______|_______2_______|_______3_______|   |          |                    store null in lv#1    |__________|   |          |   |__________| 

same steps done object bar = null except null stored in local variable @ index 2.

source: java virtual machine specification (see this section).