Does Java have something like C s ref and out keywords

Question

Something like the following   ref example   void changeString ref String str        str    def      void main         String abc    abc       changeString ref abc       System out println abc     prints  def      out example   void changeString out String str        str    def      void main         String abc      changeString out abc       System out println abc     prints  def

User · Answer

java has no standard way of doing it  Most swaps will be made on the list that is packaged in the class  but there is an unofficial way to do it   package Example   import java lang reflect Field  import java util logging Level  import java util logging Logger      public class Test    private static  lt T gt  void SetValue T obj T value       try           Field f   obj getClass   getDeclaredField  value            f setAccessible true           f set obj value             catch  IllegalAccessException   IllegalArgumentException                NoSuchFieldException   SecurityException ex                Logger getLogger CautrucjavaCanBan class getName    log Level SEVERE          null  ex               private  static  void permutation Integer a Integer b       Integer tmp   new Integer a       SetValue a  b       SetValue b  tmp      private  static  void permutation String a String b       char   tmp   a toCharArray        SetValue a  b toCharArray         SetValue b  tmp     public static void main String   args                  Integer d   9          Integer e   8          HoanVi d  e           System out println d      e                       String d    tai nguyen           String e    Thai nguyen           permutation d  e           System out println d      e

User · Answer

Direct answer  No  But you can simulate reference with wrappers   And do the following   void changeString    lt String gt  str         str s  def       void testRef            lt String gt  abc   new   lt String gt   abc         changeString  abc         out println  abc       prints def     Out  void setString    lt String gt  ref         str s   def       void testOut          lt String gt  abc     lt String gt         setString  abc        out println abc      prints def     And basically any other type such as     lt Integer gt  one   new  lt Integer gt  1   addOneTo  one     out println  one       May print 2

User · Answer

Like many others  I needed to convert a C  project to Java  I did not find a complete solution on the web regarding out and ref modifiers  But  I was able to take the information I found  and expand upon it to create my own classes to fulfill the requirements  I wanted to make a distinction between ref and out parameters for code clarity  With the below classes  it is possible  May this information save others time and effort   An example is included in the code below                                                                                                   XOUT CLASS                                                                                               public class XOUT lt T gt        public XOBJ lt T gt  Obj   null       public XOUT T value                Obj   new XOBJ lt T gt  value              public XOUT               Obj   new XOBJ lt T gt                public XOUT lt T gt  Out                 return this              public XREF lt T gt  Ref                 return Obj Ref                                                                                                               XREF CLASS                                                                                                public class XREF lt T gt        public XOBJ lt T gt  Obj   null       public XREF T value                Obj   new XOBJ lt T gt  value              public XREF               Obj   new XOBJ lt T gt                public XOUT lt T gt  Out                 return Obj Out                public XREF lt T gt  Ref                 return this                                                                                                             XOBJ CLASS                                                                                                          author jsimms            XOBJ is the base object that houses the value  XREF and XOUT are classes that     internally use XOBJ  The classes XOBJ  XREF  and XOUT have methods that allow     the object to be used as XREF or XOUT parameter  This is important  because     objects of these types are interchangeable       See Method         XXX Ref          XXX Out        The below example shows how to use XOBJ  XREF  and XOUT                Reference parameter example            void AddToTotal int a  XREF lt Integer gt  Total               Total Obj Value    a                       out parameter example            void Add int a  int b  XOUT lt Integer gt  ParmOut               ParmOut Obj Value   a b                       XOBJ example            int XObjTest                XOBJ lt Integer gt  Total   new XOBJ lt  gt  0          Add 1  2  Total Out           Example of using out parameter        AddToTotal 1 Total Ref        Example of using ref parameter        return Total Value              public class XOBJ lt T gt         public T Value       public  XOBJ                 public XOBJ T value            this Value   value                       Method  Ref          Purpose  returns a Reference Parameter object using the XOBJ value            public XREF lt T gt  Ref                 XREF lt T gt  ref   new XREF lt T gt             ref Obj   this          return ref                        Method  Out          Purpose  returns an Out Parameter Object using the XOBJ value            public XOUT lt T gt  Out                 XOUT lt T gt  out   new XOUT lt T gt             out Obj   this          return out                        Method get          Purpose  returns the value        Note  Because this is combersome to edit in the code         the Value object has been made public            public T get             return Value                       Method get          Purpose  sets the value        Note  Because this is combersome to edit in the code         the Value object has been made public            public void set T anotherValue            Value   anotherValue              Override     public String toString             return Value toString                Override     public boolean equals Object obj            return Value equals obj               Override     public int hashCode             return Value hashCode

User · Answer

Three solutions not officially  explicitly mentioned   ArrayList lt String gt  doThings             void doThings ArrayList lt String gt  list            Pair lt String  String gt  doThings              For Pair  I would recommend  https   commons apache org proper commons-lang apidocs org apache commons lang3 tuple Pair html

User · Answer

Java passes parameters by value and doesn t have any mechanism to allow pass-by-reference  That means that whenever a parameter is passed  its value is copied into the stack frame handling the call   The term value as I use it here needs a little clarification  In Java we have two kinds of variables - primitives and objects  A value of a primitive is the primitive itself  and the value of an object is its reference  and not the state of the object being referenced   Therefore  any change to the value inside the method will only change the copy of the value in the stack  and will not be seen by the caller  For example  there isn t any way to implement a real swap method  that receives two references and swaps them  not their content

User · Answer

No  Java doesn t have something like C  s ref and out keywords for passing by reference   You can only pass by value in Java  Even references are passed by value  See Jon Skeet s page about parameter passing in Java for more details   To do something similar to ref or out you would have to wrap your parameters inside another object and pass that object reference in as a parameter

User · Answer

Actually there is neither ref nor out keyword equivalent in Java language as far as I know  However I ve just transformed a C  code into Java that uses out parameter and will advise what I ve just done  You should wrap whatever object into a wrapper class and pass the values wrapped in wrapper object instance as follows   A Simple Example For Using Wrapper  Here is the Wrapper Class   public class Wrapper       public Object ref1     use this as ref     public Object ref2     use this as out      public Wrapper Object ref1            this ref1   ref1            And here is the test code   public class Test        public static void main String   args            String abc    abc           changeString abc           System out println  Initial object      abc     wont print  def           Wrapper w   new Wrapper abc           changeStringWithWrapper w           System out println  Updated object      w ref1           System out println  Out     object      w ref2                 This won t work     public static void changeString String str            str    def                 This will work     public static void changeStringWithWrapper Wrapper w            w ref1    def           w ref2    And this should be used as out               A Real World Example  A C  NET method using out parameter  Here there is a C  NET method that is using out keyword   public bool Contains T value        BinaryTreeNode lt T gt  parent      return FindWithParent value  out parent     null     private BinaryTreeNode lt T gt  FindWithParent T value  out BinaryTreeNode lt T gt  parent        BinaryTreeNode lt T gt  current    head      parent   null       while current    null                int result   current CompareTo value            if  result  gt  0                        parent   current              current   current Left                    else if  result  lt  0                        parent   current              current   current Right                    else                       break                       return current      Java Equivalent of the C  code that is using the out parameter  And the Java equivalent of this method with the help of wrapper class is as follows   public boolean contains T value        BinaryTreeNodeGeneration lt T gt  result   findWithParent value        return  result    null      private BinaryTreeNodeGeneration lt T gt  findWithParent T value        BinaryTreeNode lt T gt  current   head      BinaryTreeNode lt T gt  parent   null      BinaryTreeNodeGeneration lt T gt  resultGeneration   new BinaryTreeNodeGeneration lt T gt         resultGeneration setParentNode null        while current    null            int result   current compareTo value            if result  gt 0                parent   current              current   current left            else if result  lt  0                parent   current              current   current right            else               break                       resultGeneration setChildNode current       resultGeneration setParentNode parent        return resultGeneration      Wrapper Class  And the wrapper class used in this Java code is as below   public class BinaryTreeNodeGeneration lt TNode extends Comparable lt TNode gt  gt          private BinaryTreeNode lt TNode gt    parentNode      private BinaryTreeNode lt TNode gt    childNode       public BinaryTreeNodeGeneration             this parentNode   null          this childNode   null             public BinaryTreeNode lt TNode gt  getParentNode             return parentNode             public void setParentNode BinaryTreeNode lt TNode gt  parentNode            this parentNode   parentNode             public BinaryTreeNode lt TNode gt  getChildNode             return childNode             public void setChildNode BinaryTreeNode lt TNode gt  childNode            this childNode   childNode

[c#] Does Java have something like C#'s ref and out keywords?

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