Compare every item to every other item in ArrayList

Question

I m having trouble with what I thought should be a pretty simple problem   I need to compare every item in an arrayList with every other item in the the list without comparing items to themselves  It s not as simple as calling an equals   comparison  it involves some custom logic that I ve omitted from my code below  Also the ArrayList should not be changed in any way    The problem I seem to be having is that once I get into the second loop  I don t know if I have another object to compare to  since its a variable sized list    for int i  0  i lt  list size    i            get first object to compare to     String a   list get i  getA         Iterator itr   list listIterator i   1       I don t know if i   1 is valid     while itr hasNext                compare A to all remaining items on list           I think I probably going about this the wrong way  I m open to suggestions or tips on how to do this better

User · Accepted Answer

for  int i   0  i  lt  list size    i        for  int j   i 1  j  lt  list size    j             compare list get i  and list get j

User · Answer

In some cases this is the best way because your code may have change something and j i 1 won t check that   for  int i   0  i  lt  list size    i           for  int j   0  j  lt  list size    j                      if i    j                     to do code here                     continue

User · Answer

The following code will compare each item with other list of items using contains   method Length of for loop must be bigger size   of bigger list then only it will compare all the values of both list   List lt String gt  str   new ArrayList lt String gt     str add  first    str add  second    str add  third    List lt String gt  str1   new ArrayList lt String gt     str1 add  first    str1 add  second    str1 add  third1    for  int i   0  i lt str1 size    i      System out println str contains str1 get i         Output is true true false

User · Answer

What s the problem with using for loop inside  just like outside   for  int j   i   1  j  lt  list size      j                In general  since Java 5  I used iterators only once or twice

User · Answer

This code helped me get this behaviour  With a list a b c  I should get compared ab  ac and bc  but any other pair would be excess   not needed   import java util    import static java lang System out      rl   rawList  lr   listReversed ArrayList lt String gt  rl   new ArrayList lt String gt     ArrayList lt String gt  lr   new ArrayList lt String gt     rl add  a    rl add  b    rl add  c    rl add  d    rl add  e    rl add  f     lr addAll rl   Collections reverse lr    for  String itemA   rl        lr remove lr size  -1           for  String itemZ   lr            System out println itemA   itemZ             The loop goes as like in this picture  Triangular comparison visual example  or as this          f    e    d    c    b   a    ------------------------------ a     af   ae   ad   ac   ab      b     bf   be   bd   bc         c     cf   ce   cd            d     df   de               e     ef                  f                         total comparisons is a triangular number  n   n-1  2

[java] Compare every item to every other item in ArrayList

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