Java Cannot implement a function in Map ADT interface that takes Pair class as its generic parameter -

i created map interface has generic function entries().

// return iterable collection of key-value entries in map public arraylist<pair<keytype, valuetype>> entries(); 

the problem is, when try implement interface error in interface file @ entries() function: bound mismatch: type keytype not valid substitute bounded parameter <keytype extends comparable<keytype>> of type pair<keytype,valuetype>

my implementation of function shown below:

public arraylist<pair<keytype, valuetype>> entries(){     arraylist<pair<keytype, valuetype>> list = new arraylist<pair<keytype, valuetype>>();     preorderlist (root, list);     return list; } 

how can solve problem?

you left generic bound out of interface declaration. since pair requires keys comparable each other (keytype extends comparable<keytype>), map need reiterate bound in keytype declaration in order use pair keytype. may need bound valuetype if pair bounds it.

interface map <keytype extends comparable<keytype>, valuetype> { ... } 

under generic type erasure, pair's keytype replaced comparable. if not bound map's keytype, replaced object, not assignable comparable without narrowing cast can potentially fail.