Are arrays passed by value or passed by reference in Java

Question

Arrays are not a primitive type in Java  but they are not objects either  so are they passed by value or by reference  Does it depend on what the array contains  for example references or a primitive type

User · Answer

Everything in Java is passed by value    In the case of the array the reference is copied into a new reference  but remember that everything in Java is passed by value    Take a look at this interesting article for further information

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Your question is based on a false premise      Arrays are not a primitive type in Java  but they are not objects either         In fact  all arrays in Java are objects1   Every Java array type has java lang Object as its supertype  and inherits the implementation of all methods in the Object API           so are they passed by value or by reference  Does it depend on what the array contains  for example references or a primitive type     Short answers  1  pass by value  and 2  it makes no difference   Longer answer   Like all Java objects  arrays are passed by value     but the value is the reference to the array   So  when you assign something to a cell of the array in the called method  you will be assigning to the same array object that the caller sees   This is NOT pass-by-reference   Real pass-by-reference involves passing the address of a variable   With real pass-by-reference  the called method can assign to its local variable  and this causes the variable in the caller to be updated     But not in Java   In Java  the called method can update the contents of the array  and it can update its copy of the array reference  but it can t update the variable in the caller that holds the caller s array reference   Hence     what Java is providing is NOT pass-by-reference   Here are some links that explain the difference between pass-by-reference and pass-by-value   If you don t understand my explanations above  or if you feel inclined to disagree with the terminology  you should read them    http   publib boulder ibm com infocenter comphelp v8v101 topic com ibm xlcpp8a doc language ref cplr233 htm http   www cs fsu edu  myers c   notes references html   Related SO question    Is Java  quot pass-by-reference quot  or  quot pass-by-value quot     Historical background   The phrase  pass-by-reference  was originally  call-by-reference   and it was used to distinguish the argument passing semantics of FORTRAN  call-by-reference  from those of ALGOL-60  call-by-value and call-by-name     In call-by-value  the argument expression is evaluated to a value  and that value is copied to the called method  In call-by-reference  the argument expression is partially evaluated to an  lvalue   i e  the address of a variable or array element  that is passed to the calling method   The calling method can then directly read and update the variable   element  In call-by-name  the actual argument expression is passed to the calling method      which can evaluate it multiple times         This was complicated to implement  and could be used  abused  to write code that was very difficult to understand   Call-by-name was only ever used in Algol-60  thankfully      UPDATE  Actually  Algol-60 s call-by-name is similar to passing lambda expressions as parameters   The wrinkle is that these not-exactly-lambda-expressions  they were referred to as  thunks  at the implementation level  can indirectly modify the state of variables that are in scope in the calling procedure   function   That is part of what made them so hard to understand    See the Wikipedia page on Jensen s Device for example      1  Nothing in the linked Q amp A  Arrays in Java and how they are stored in memory  either states or implies that arrays are not objects

User · Answer

Everything in Java is passed by value  In case of an array  which is nothing but an Object   the array reference is passed by value  just like an object reference is passed by value   When you pass an array to other method  actually the reference to that array is copied   Any changes in the content of array through that reference will affect the original array  But changing the reference to point to a new array will not change the existing reference in original method   See this post  Is Java  quot pass-by-reference quot  or  quot pass-by-value quot   See this working example  public static void changeContent int   arr           If we change the content of arr     arr 0    10      Will change the content of array in main      public static void changeRef int   arr          If we change the reference    arr   new int 2       Will not change the array in main      arr 0    15     public static void main String   args        int    arr   new int 2       arr 0    4      arr 1    5       changeContent arr        System out println arr 0        Will print 10           changeRef arr        System out println arr 0        Will still print 10                                        Change the reference doesn t reflect change here

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No that is wrong  Arrays are special objects in Java  So it is like passing other objects where you pass the value of the reference  but not the reference itself  Meaning  changing the reference of an array in the called routine will not be reflected in the calling routine

User · Answer

Arrays are in fact objects  so a reference is passed  the reference itself is passed by value  confused yet    Quick example      assuming you allocated the list public void addItem Integer   list  int item        list 1    item      You will see the changes to the list from the calling code  However you can t change the reference itself  since it s passed by value      assuming you allocated the list public void changeArray Integer   list        list   null      If you pass a non-null list  it won t be null by the time the method returns

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The definitive discussion of arrays is at http   docs oracle com javase specs jls se5 0 html arrays html 27803    This makes clear that Java arrays are objects   The class of these objects is defined in 10 8   Section 8 4 1 of the language spec  http   docs oracle com javase specs jls se5 0 html classes html 40420   describe how arguments are passed to methods   Since Java syntax is derived from C and C    the behavior is similar   Primitive types are passed by value  as with C   When an object is passed  an object reference  pointer  is passed by value  mirroring the C syntax of passing a pointer by value   See 4 3 1  http   docs oracle com javase specs jls se5 0 html typesValues html 4 3    In practical terms  this means that modifying the contents of an array within a method is reflected in the array object in the calling scope  but reassigning a new value to the reference within the method has no effect on the reference in the calling scope  which is exactly the behavior you would expect of a pointer to a struct in C or an object in C     At least part of the confusion in terminology stems from the history of high level languages prior to the common use of C  In prior  popular  high level languages  directly referencing memory by address was something to be avoided to the extent possible  and it was considered the job of the language to provide a layer of abstraction   This made it necessary for the language to explicitly support a mechanism for returning values from subroutines  not necessarily functions    This mechanism is what is formally meant when referring to  pass by reference    When C was introduced  it came with a stripped down notion of procedure calling  where all arguments are input-only  and the only value returned to the caller is a function result   However  the purpose of passing references could be achieved through the explicit and broad use of pointers   Since it serves the same purpose  the practice of passing a pointer as a reference to a value is often colloquially referred to a passing by reference   If the semantics of a routine call for a parameter to be passed by reference  the syntax of C requires the programmer to explicitly pass a pointer   Passing a pointer by value is the design pattern for implementing pass by reference semantics in C   Since it can often seem like the sole purpose of raw pointers in C is to create crashing bugs  subsequent developments  especially Java  have sought to return to safer means to pass parameters   However  the dominance of C made it incumbent on the developers to mimic the familiar style of C coding   The result is references that are passed similarly to pointers  but are implemented with more protections to make them safer   An alternative would have been the rich syntax of a language like Ada  but this would have presented the appearance of an unwelcome learning curve  and lessened the likely adoption of Java   In short  the design of parameter passing for objects  including arrays  in Java is esentially to serve the semantic intent of pass by reference  but is imlemented with the syntax of passing a reference by value

User · Answer

Kind of a trick realty    Even references are passed by value in Java  hence a change to the reference itself being scoped at the called function level  The compiler and or JVM will often turn a value type into a reference

[java] Are arrays passed by value or passed by reference in Java?

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