java - Why are iterators private? -

iterators nested classes exported clients. why declared private instead of public ? eg:

private abstract class hashiterator<e> implements iterator<e> {

 private final class entryiterator extends hashiterator<map.entry<k,v>> {         public map.entry<k,v> next() {             return nextentry();         }     } 

in general case, don't have private; i.e. if designing data structure, there nothing stopping declaring iterator class public.

however, private iterators "good design" if data structure intended string abstraction; i.e. 1 internal representation hidden client code.

one reason making iterator class private prevents undesirable coupling; i.e. stops external class depending on actual iterator code in way make future code changes harder.

another reason in cases extensible public iterator class couldn't instantiated anyway. or @ least, not without relaxing abstraction boundary of data structure.

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another way @ making iterator class public not achieve anything. caller typically doesn't need use apart methods in iterator api. , if does, solution extend iterator api (interface) provide additional methods.