How to write a basic swap function in Java

Question

I am new to java  How to write the java equivalent of the following C code   void Swap int  p  int  q       int temp     temp    p      p    q      q   temp

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Java is pass by value  So the swap in the sense you mean is not possible  But you can swap contents of two objects or you do it inline

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Snippet-1  public int   swap1 int   values      if  values    null    values length    2      throw new IllegalArgumentException  parameter must be an array of size 2      int temp   values 0     values 0  values 1     values 1  temp    return values      Snippet-2  public Point swap2 java awt Point p      if  p    null      throw new NullPointerException      int temp   p x    p x   p y    p y   temp    return p      Usage   int   values   swap1 new int   x y    x   values 0   y   values 1    Point p   swap2 new Point x y    x   p x  y   p y

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Here s a method to swap two variables in java in just one line using bitwise XOR    operator   class Swap      public static void main  String   args             int x   5  y   10        x   x   y    y   x         System out println  New values of x and y are    x          y             Output   New values of x and y are 10  5

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Sorting two ints  The short answer is  you can t do that  java has no pointers   But here s something similar that you can do   public void swap AtomicInteger a  AtomicInteger b          look mom  no tmp variables needed     a set b getAndSet a get          You can do this with all kinds of container objects  like collections and arrays or custom objects with an int property   but just not with primitives and their wrappers  because they are all immutable   But the only way to make it a one-liner is with AtomicInteger  I guess   BTW  if your data happens to be a List  a better way to swap is to use Collections swap List  int  int    Swaps the elements at the specified positions in the specified list   If the specified positions are equal  invoking this method leaves the list unchanged    Parameters      list - The list in which to swap elements      i - the index of one element to be swapped      j - the index of the other element to be swapped       Sorting an int   array  apparently the real objective is to sort an array of ints  That s a one-liner with Arrays sort int      int   arr    2 3 1 378 19 25   Arrays sort arr     To check the output   System out println Arrays toString arr        1  2  3  19  25  378      And here is a simple helper function to swap two positions in an array of ints   public static void swap final int   arr  final int pos1  final int pos2       final int temp   arr pos1       arr pos1    arr pos2       arr pos2    temp

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There are no pointers in Java  However  every variable that  contains  an object is a reference to that object  To have output parameters  you would have to use objects  In your case  Integer objects   So you would have to make an object which contains an integer  and change that integer  You can not use the Integer class  since it is immutable  i e  its value cannot be changed    An alternative is to let the method return an array or pair of ints

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class Swap2Values      public static void main String   args          int a   20  b   10            before swaping        System out print  Before Swapping the values of a and b are  a     a    b     b             swapping        a   a   b         b   a - b         a   a - b            after swapping       System out print  After Swapping the values of a and b are  a     a    b     b

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In cases like that there is a quick and dirty solution using arrays with one element   public void swap int   a  int   b      int temp   a 0     a 0    b 0     b 0    temp      Of course your code has to work with these arrays too  which is inconvenient  The array trick is more useful if you want to modify a local final variable from an inner class   public void test       final int   a   int    42        new Thread new Runnable    public void run    a 0     10      start      while a 0     42        System out println  waiting                System out println a 0

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here is also another answer  class SwapDemo      static int a 1  b 2       public static void main String    args           Swap swp   new Swap            swp swaps x y           System out println    a  was 1 now is     a     b  was 2  now is     b           class Swap      void swaps int c  int d               SwapDemo f   new SwapDemo                f a   c              f a   d

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Use this one-liner for any primitive number class including double and float   a     b -  b   a      For example   double a   1 41  double b   0  a     b -  b   a    System out println  a       a      b       b     Output is a   0 0  b   1 41

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You have to do it inline  But you really don t need that swap in Java

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public class swaptemp       public static void main String   args            String s1  10           String s2  20           String temp          System out println s1           System out println s2            temp Integer toString Integer parseInt s1            s1 Integer toString Integer parseInt s2            s2 Integer toString Integer parseInt temp             System out println s1           System out println s2

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Java uses pass-by-value  It is not possible to swap two primitives or objects using a method    Although it is possible to swap two elements in an integer array

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You can swap variables with or without using a temporary variable    Here is an article that provides multiple methods to swap numbers without temp variable    http   topjavatutorial com java java-programs swap-two-numbers-without-a-temporary-variable-in-java

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Your swap function is essentially changing the values in two pieces of memory  Anything referencing those bits of memory will now get different values   In Java there aren t really pointers  so this won t work  Instead  references are held on objects  and you can only change stuff inside the objects  If you need to reference one object in two places  so that you can pass the same values around the system and have things react to them changing  try something like the repository pattern or dependency injection   We can only guess at why you needed this code in C  The only advice I can give is to think about the changes to the objects which you want to achieve  preferably add a method on the actual objects rather than pulling their internals out  and call that method instead  If this doesn t help you  try posting the calling code as we ll probably have a good idea of how to solve the real problem Java-style

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Swapping by using pointer is not possible in java  However  you can implement swapping by passing array containing two objects   Code goes like this   public class Swap       public static void swap String    a           String temp          temp   a 0           a 0    a 1           a 1    temp            public static void main String    args           String    foo   new String 2           foo 0     str1           foo 1     str2           swap foo           System out println  First value     foo 0            System out println  Second value     foo 1              Output   First value  str2 Second value  str1

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Here is one trick   public static int getItself int itself  int dummy        return itself     public static void main String   args        int a   10      int b   20       a   getItself b  b   a

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What about the mighty IntHolder  I just love any package with omg in the name   import org omg CORBA IntHolder   IntHolder a   new IntHolder p   IntHolder b   new IntHolder q    swap a  b    p   a value  q   b value   void swap IntHolder a  IntHolder b        int temp   a value      a value   b value      b value   temp

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You can easily write one yourself   given   int array    1 2     you do   int temp array 0   array 0  array 1   array 1  temp    And you re done  3 lines of code

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You cannot use references in Java  so a swap function is impossible  but you can use the following code snippet per each use of swap operations   T t   p p   q q   t   where T is the type of p and q  However  swapping mutable objects may be possible by rewriting properties   void swap Point a  Point b      int tx   a x  ty   a y    a x   b x  a y   b y    b x   t x  b y   t y

[java] How to write a basic swap function in Java

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