java - An interchangeable key/value HashMap Set structure -

background

create series of sql join statements using 2 operands: primary , secondary. generic form of join statement is:

join primary primary on (secondary.id == primary.id) 

problem

the code iterates on list of primary , secondary operands, follows:

for( bundle primaryoperand : bundlecomparators ) {   for( bundle secondaryoperand : sortedbundles ) { 

the problem nested loop generates following:

join primary primary on (secondary.id == primary.id) join secondary secondary on (primary.id == secondary.id) 

the second join superfluous and, in case, causes error. duplication can eliminated following hypothetical data structure:

if( !interchangeablemap.contains( primaryoperand, secondaryoperand ) ) {     interchangeablemap.put( primaryoperand, secondaryoperand );      outputjoin( primaryoperand, secondaryoperand ); } 

where interchangeablemap.contains(...) return true if primaryoperand mapped secondaryoperand or secondaryoperand mapped primaryoperand.

questions

1. does such data structure exist in java libraries?
2. if not, data structures use implementation?

ideas

my first thought create class contains 2 hashmaps. checking containment queries 2 hashmaps see if 1 map contains primary , secondary operands or other map contains secondary , primary operands. insertions put 2 operand combinations respective hashmaps.

thank you!

here solution based on @roland's suggestion:

public final class pair {   private final object a;   private final object b;    public pair(object a, object b) {     this.a = a;      this.b = b;   }    @override public boolean equals(object o) {     if(o == null || !(o instanceof pair))        return false;      pair = (pair) o;     return this.a.equals(that.a) && this.b.equals(that.b)        || this.a.equals(that.b) && this.b.equals(that.a);   }    @override public int hashcode() {     return a.hashcode() ^ b.hashcode();   } } 

then:

set<pair> set = new hashset<pair>(); for(bundle primaryoperand : bundlecomparators) {   for(bundle secondaryoperand : sortedbundles) {     pair p = new pair(primaryoperand.id, secondaryoperand.id);     if(set.contains(p))        continue;      set.add(p);     outputjoin(primaryoperand, secondaryoperand);   } } 

a subtle point solution: must override hashcode() method (hash value must reflect equality relation), must in symmetric way, is: hash value of <a,b> must == of <b,a>