When to create variables memory management

Question

You create a variable to store a value that you can refer to that variable in the future  I ve heard that you must set a variable to  null  once you re done using it so the garbage collector can get to it  if it s a field var    If I were to have a variable that I won t be referring to agaon  would removing the reference value vars I m using  and just using the numbers when needed  save memory  For example   int number   5  public void method         System out println number       Would that take more space than just plugging  5  into the println method   I have a few integers that I don t refer to in my code ever again  game loop   but I ve seen others use reference vars on things that really didn t need them  Been looking into memory management  so please let me know  along with any other advice you have to offer about managing memory

User · Accepted Answer

I ve heard that you must set a variable to  null  once you re done using it so the garbage collector can get to it  if it s a field var     This is very rarely a good idea   You only need to do this if the variable is a reference to an object which is going to live much longer than the object it refers to   Say you have an instance of Class A and it has a reference to an instance of Class B   Class B is very large and you don t need it for very long  a pretty rare situation   You might null out the reference to class B to allow it to be collected   A better way to handle objects which don t live very long is to hold them in local variables   These are naturally cleaned up when they drop out of scope      If I were to have a variable that I won t be referring to agaon  would removing the reference vars I m using  and just using the numbers when needed  save memory     You don t free the memory for a primitive until the object which contains it is cleaned up by the GC      Would that take more space than just plugging  5  into the println method    The JIT is smart enough to turn fields which don t change into constants      Been looking into memory management  so please let me know  along with any other advice you have to offer about managing memory   Use a memory profiler instead of chasing down 4 bytes of memory   Something like 4 million bytes might be worth chasing if you have a smart phone   If you have a PC  I wouldn t both with 4 million bytes

User · Answer

So notice variables are on the stack  the values they refer to are on the heap  So having variables is not too bad but yes they do create references to other entities  However in the simple case you describe it s not really any consequence  If it is never read again and within a contained scope  the compiler will probably strip it out before runtime  Even if it didn t the garbage collector will be able to safely remove it after the stack squashes  If you are running into issues where you have too many stack variables  it s usually because you have really deep stacks  The amount of stack space needed per thread is a better place to adjust than to make your code unreadable  The setting to null is also no longer needed

User · Answer

In your example number is a primitive  so will be stored as a value   If you want to use a reference then you should use one of the wrapper types  e g  Integer

User · Answer

Well  the JVM memory model works something like this  values are stored on one pile of memory stack and objects are stored on another pile of memory called the heap  The garbage collector looks for garbage by looking at a list of objects you ve made and seeing which ones aren t pointed at by anything  This is where setting an object to null comes in  all nonprimitive  think of classes  variables are really references that point to the object on the stack  so by setting the reference you have to null the garbage collector can see that there s nothing else pointing at the object and it can decide to garbage collect it  All Java objects are stored on the heap so they can be seen and collected by the garbage collector   Nonprimitive  ints  chars  doubles  those sort of things  values  however  aren t stored on the heap  They re created and stored temporarily as they re needed and there s not much you can do there  but thankfully the compilers nowadays are really efficient and will avoid needed to store them on the JVM stack unless they absolutely need to   On a bytecode level  that s basically how it works  The JVM is based on a stack-based machine  with a couple instructions to create allocate objects on the heap as well  and a ton of instructions to manipulate  push and pop values  off the stack  Local variables are stored on the stack  allocated variables on the heap   These are the heap and the stack I m referring to above  Here s a pretty good starting point if you want to get into the nitty gritty details    In the resulting compiled code  there s a bit of leeway in terms of implementing the heap and stack  Allocation s implemented as allocation  there s really not a way around doing so  Thus the virtual machine heap becomes an actual heap  and allocations in the bytecode are allocations in actual memory  But you can get around using a stack to some extent  since instead of storing the values on a stack  and accessing a ton of memory   you can stored them on registers on the CPU which can be up to a hundred times  maybe even a thousand  faster than storing it on memory  But there s cases where this isn t possible  look up register spilling for one example of when this may happen   and using a stack to implement a stack kind of makes a lot of sense    And quite frankly in your case a few integers probably won t matter  The compiler will probably optimize them out by itself in this case anyways  Optimization should always happen after you get it running and notice it s a tad slower than you d prefer it to be  Worry about making simple  elegant  working code first then later make it fast  and hopefully  simple  elegant  working code   Java s actually very nicely made so that you shouldn t have to worry about nulling variables very often  Whenever you stop needing to use something  it will usually incidentally be disappearing from the scope of your program  and thus becoming eligible for garbage collection   So I guess the real lesson here is to use local variables as often as you can    There s also a constant pool  a local variable pool  and a couple other things in memory but you have close to no control over the size of those things and I want to keep this fairly simple

User · Answer

It s really a matter of opinion  In your example  System out println 5  would be slightly more efficient  as you only refer to the number once and never change it  As was said in a comment  int is a primitive type and not a reference - thus it doesn t take up much space  However  you might want to set actual reference variables to null only if they are used in a very complicated method  All local reference variables are garbage collected when the method they are declared in returns

[java] When to create variables (memory management)

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