java - Why can't I use .pollFirst() method for a Set that was initialized as a TreeSet? -

say i've intialized set new treeset:

    set<integer> foo = new treeset<integer>(); 

and want use .pollfirst() method, because i'm lazy , don't want have use temp variable because have handy method reduce otherwise 3-4 lines of code 1 line.

however, following code, way compile code call foo treeset.

    while (foo.size() > 1) {         int lower = foo.pollfirst();         int higher = foo.pollfirst();         foo.add(higher - lower);     } 

so why java doesn't realize foo treeset? should cast treeset if didn't want intialize foo treeset? or better intialize way?

further testing casting:

    while (foo.size() > 1) {         int lower = (int) ((treeset) foo).pollfirst();         int higher = (int) ((treeset) foo).pollfirst();         foo.add(higher - lower);     } 

this makes compile without explicitly calling treeset upon intialization.

that's because set doesn't define pollfirst() method, defined navigableset, implemented treeset. on other hand, answer question, 1 reason can think of java behave that, this, assume later in code this,

set<integer> foo = new treeset<integer>(); ... foo = externallibraryclass.getunknownsetimplementation(); // @ point, how compiler know 'foo' is? 

where externallibraryclass arbitrary class arbitrary library , getunknownsetimplementation() arbitrary method returns set implementation. important thing compiler not have access library's source code, therefore not know set implementation returned every time getunknownsetimplementation() gets called.

some more illustration

assume,

string whichset = externalwebservice.whichsetshouldiuse(); if(whichset.equals("hashset"))   foo = new hashset()<>; else if(whichset.equals("treeset"))   foo = new treeset()<>;  // @ point, how compiler know 'foo' is?