Java method to swap primitives

Question

how do I make my swap function in java if there is no method by which we can pass by reference   Could somebody give me a code   swap int a  int b         int temp   a       a   b       b   temp      But the changes wont be reflected back since java passes parameters by value

User · Answer

Try this magic  public static  lt T gt  void swap T a  T b        try           Field   fields   a getClass   getDeclaredFields            for  Field field   fields                field setAccessible true               Object temp   field get a               field set a  field get b                field set b  temp                   catch  IllegalAccessException e            e printStackTrace              And test it       System out println  a     a       System out println  b     b       swap a b       System out println  a     a       System out println  b     b

User · Answer

You can t create a method swap  so that after calling swap x y  the values of x and y will be swapped  You could create such a method for mutable classes by swapping their contents    but this would not change their object identity and you could not define a general method for this   You can however write a method that swaps two items in an array or list if that s what you want      For example you could create a swap method that takes two lists and after executing the method  list x will have the previous contents of list y and list y will have the previous contents of list x

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I think this is the closest you can get to a simple swap  but it does not have a straightforward usage pattern   int swap int a  int b        usage  y   swap x  x y      return a     y   swap x  x y     It relies on the fact that x will pass into swap before y is assigned to x  then x is returned and assigned to y   You can make it generic and swap any number of objects of the same type    lt T gt  T swap T    args         usage  z   swap a  a b  b c      y z       return args 0      c   swap a  a b  b c

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AFAIS  no one mentions of atomic reference    Integer  public void swap AtomicInteger a  AtomicInteger b       a set b getAndSet a get          String  public void swap AtomicReference lt String gt  a  AtomicReference lt String gt  b       a set b getAndSet a get

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For integer types  you can do  a    b  b    a  a    b    using the bit-wise xor operator    As all the other suggestions  you probably shouldn t use it in production code   For a reason I don t know  the single line version a    b    a    b doesn t work  maybe my Java compiler has a bug   The single line worked in C with all compilers I tried  However  two-line versions work   a    b    a  b    a    as well as  b    a  a    b    a    A proof that it works  Let a0 and b0 be the initial values for a and b  After the first line  a is a1   a0 xor b0  after the second line  b is b1   b0 xor a1   b0 xor  a0 xor b0    a0  After the third line  a is a2   a1 xor b1   a1 xor  b0 xor a1    b0

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Apparently I don t have enough reputation points to comment on Dansalmo s answer  but it is a good one  though mis-named  His answer is actually a K-combinator   int K  int a  int b         return a      The JLS is specific about argument evaluation when passing to methods ctors etc   Was this not so in older specs    Granted  this is a functional idiom  but it is clear enough to those who recognize it   If you don t understand code you find  don t mess with it    y   K x  x y       swap x and y   The K-combinator is specifically designed for this kind of thing  AFAIK there s no reason it shouldn t pass a code review   My  0 02

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I might do something like the following  Of course  with the wealth of Collection classes  i can t imagine ever needing to use this in any practical code   public class Shift     public static  lt T gt  T   left  final T    i        if  1  gt   i length          return i            final T t   i 0       int x   0      for    x  lt  i length - 1  x            i x    i x   1             i x    t      return i          Called with two arguments  it s a swap   It can be used as follows   int x   1  int y   2  Integer   yx   Shift left x y     Alternatively   Integer   yx    x y   Shift left yx     Then  x   yx 0   y   yx 1     Note  it auto-boxes primitives

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It depends on what you want to do  This code swaps two elements of an array   void swap int i  int j  int   arr      int t   arr i     arr i    arr j     arr j    t      Something like this swaps the content of two int   of equal length   void swap int   arr1  int   arr2      int   t   arr1 clone      System arraycopy arr2  0  arr1  0  t length     System arraycopy t  0  arr2  0  t length       Something like this swaps the content of two BitSet  using the XOR swap algorithm    void swap BitSet s1  BitSet s2      s1 xor s2     s2 xor s1     s1 xor s2       Something like this swaps the x and y fields of some Point class   void swapXY Point p      int t   p x    p x   p y    p y   t

[java] Java method to swap primitives

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