How can I use pointers in Java

Question

I know Java doesn t have pointers  but I heard that Java programs can be created with pointers and that this can be done by the few who are experts in java  Is it true

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Not really  no   Java doesn t have pointers   If you really wanted you could try to emulate them by building around something like reflection  but it would have all of the complexity of pointers with none of the benefits   Java doesn t have pointers because it doesn t need them   What kind of answers were you hoping for from this question  i e  deep down did you hope you could use them for something or was this just curiousity

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All objects in java are passed to functions by reference copy except primitives    In effect  this means that you are sending a copy of the pointer to the original object rather than a copy of the object itself   Please leave a comment if you want an example to understand this

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Java does not have pointers like C has  but it does allow you to create new objects on the heap which are  referenced  by variables  The lack of pointers is to stop Java programs from referencing memory locations illegally  and also enables Garbage Collection to be automatically carried out by the Java Virtual Machine

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Java reference types are not the same as C pointers as you can t have a pointer to a pointer in Java

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You can use addresses and pointers using the Unsafe class   However as the name suggests  these methods are UNSAFE and generally a bad idea  Incorrect usage can result in your JVM randomly dying  actually the same problem get using pointers incorrectly in C C     While you may be used to pointers and think you need them  because you don t know how to code any other way   you will find that you don t and you will be better off for it

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from the book named Decompiling Android by Godfrey Nolan  Security dictates that pointers aren   t used in Java so hackers can   t break out of an application and into the operating system  No pointers means that something else----in this case  the JVM----has to take care of the allocating and freeing memory  Memory leaks should also become a thing of the past  or so the theory goes  Some applications written in C and C   are notorious for leaking memory like a sieve because programmers don   t pay attention to freeing up unwanted memory at the appropriate time----not that anybody reading this would be guilty of such a sin  Garbage collection should also make programmers more productive  with less time spent on debugging memory problems

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All objects in Java are references and you can use them like pointers   abstract class Animal         class Lion extends Animal         class Tiger extends Animal      public Tiger         public void growl           Tiger first   null  Tiger second   new Tiger    Tiger third    Dereferencing a null   first growl        ERROR  first is null      third growl       ERROR  third has not been initialized    Aliasing Problem   third   new Tiger    first   third    Losing Cells   second   third     Possible ERROR  The old value of second is lost        You can make this safe by first assuring that there is no further need of the old value of second or assigning another pointer the value of second   first   second  second   third    OK   Note that giving second a value in other ways  NULL  new     is just as much a potential error and may result in losing the object that it points to   The Java system will throw an exception  OutOfMemoryError  when you call new and the allocator cannot allocate the requested cell  This is very rare and usually results from run-away recursion   Note that  from a language point of view  abandoning objects to the garbage collector are not errors at all  It is just something that the programmer needs to be aware of  The same variable can point to different objects at different times and old values will be reclaimed when no pointer references them  But if the logic of the program requires maintaining at least one reference to the object  It will cause an error   Novices often make the following error   Tiger tony   new Tiger    tony   third     Error  the new object allocated above is reclaimed     What you probably meant to say was   Tiger tony   null  tony   third     OK    Improper Casting   Lion leo   new Lion    Tiger tony    Tiger leo     Always illegal and caught by compiler    Animal whatever   new Lion       Legal  Tiger tony    Tiger whatever     Illegal  just as in previous example  Lion leo    Lion whatever     Legal  object whatever really is a Lion      Pointers in C   void main            int     x      Allocate the pointers x and y     int     y       but not the pointees       x   malloc sizeof int          Allocate an int pointee                                     and set x to point to it       x   42        Dereference x to store 42 in its pointee       y   13        CRASH -- y does not have a pointee yet      y   x          Pointer assignment sets y to point to x s pointee       y   13        Dereference y to store 13 in its  shared  pointee     Pointers in Java   class IntObj       public int value     public class Binky         public static void main String   args            IntObj  x      Allocate the pointers x and y         IntObj  y       but not the IntObj pointees           x   new IntObj         Allocate an IntObj pointee                                and set x to point to it          x value   42       Dereference x to store 42 in its pointee          y value   13       CRASH -- y does not have a pointee yet          y   x      Pointer assignment sets y to point to x s pointee          y value   13       Deference y to store 13 in its  shared  pointee            UPDATE  as suggested in the comments one must note that C has pointer arithmetic  However  we do not have that in Java

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As others have said  the short answer is  No    Sure  you could write JNI code that plays with Java pointers  Depending on what you re trying to accomplish  maybe that would get you somewhere and maybe it wouldn t   You could always simulate pointes by creating an array and working with indexes into the array  Again  depending on what you re trying to accomplish  that might or might not be useful

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There are pointers in Java  but you cannot manipulate them the way that you can in C   or C  When you pass an object  you are passing a pointer to that object  but not in the same sense as in C    That object cannot be dereferenced  If you set its values using its native accessors  it will change because Java knows its memory location through the pointer  But the pointer is immutable  When you attempt to set the pointer to a new location  you instead end up with a new local object with the same name as the other  The original object is unchanged  Here is a brief program to demonstrate the difference   import java util    import java lang    import java io     class Ideone        public static void main String   args  throws java lang Exception           System out println  Expected     0 1 2 2 1            Cat c   new Cat            c setClaws 0           System out println  Initial value is     c getClaws                prints 0 obviously         clawsAreOne c           System out println  Accessor changes value to     c getClaws                prints 1 because the value  referenced  by the  pointer  is changed using an accessor          makeNewCat c           System out println  Final value is     c getClaws                prints 1 because the pointer is not changed to  kitten   that would be a reference pass             public static void clawsAreOne Cat kitty            kitty setClaws 1              public static void makeNewCat Cat kitty            Cat kitten   new Cat            kitten setClaws 2           kitty   kitten          System out println  Value in makeNewCat scope of kitten     kitten getClaws               Prints 2  the value pointed to by  kitten  is 2         System out println  Value in makeNewcat scope of kitty     kitty getClaws               Prints 2  The local copy is being used within the scope of this method           class Cat        private int claws       public void setClaws int i            claws   i             public int getClaws             return claws            This can be run at Ideone com

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All java objects are pointer because a variable which holds address is called pointer and object hold address so object is pointer variable

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Java does have pointers  Any time you create an object in Java  you re actually creating a pointer to the object  this pointer could then be set to a different object or to null  and the original object will still exist  pending garbage collection    What you can t do in Java is pointer arithmetic  You can t dereference a specific memory address or increment a pointer   If you really want to get low-level  the only way to do it is with the Java Native Interface  and even then  the low-level part has to be done in C or C

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Technically  all Java objects are pointers   All primitive types are values though   There is no way to take manual control of those pointers  Java just internally uses pass-by-reference

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you can have pointers for literals as well  You have to implement them yourself  It is pretty basic for experts     Use an array of int object long byte and voila you have the basics for implementing pointers  Now any int value can be a pointer to that array int    You can increment the pointer  you can decrement the pointer  you can multiply the pointer  You indeed have pointer arithmetics  That s the only way to implements 1000 int attributes classes and have a generic method that applies to all attributes  You can also use a byte   array instead of an int    However I do wish Java would let you pass literal values by reference  Something along the lines        telling you it is a pointer  public void myMethod int  intValue

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As Java has no pointer data types  it is impossible to use pointers in Java  Even the few experts will not be able to use pointers in java   See also the last point in  The Java Language Environment

[java] How can I use pointers in Java?

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