oop - Java Method return Types from Inheritance -

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i'm writing method in class parent returns set of object a. however, have class child (inheriting form parent). want return set of object b (b inheriting a).

more specifically, methods looks (it throws compile errors right now).

parent class method:

public abstract <t extends a> set<t> getset();

child class (extends parent class) method:

public <t extends b> set<t> getset() {...}

is possible this, or not make sense?

first of all, let me explain why code not compile. basically, if have class {} , class b extends {} set<b> not sub-type of set<a>. if parent method returns set<a>, override must return same thing. set<b> completely different type.

likewise, set<t extends b> not sub-type of set<t extends a>. full explanation found in java docs.

the closest solution can think of, uses wildcards:

class {}  class b extends {}  abstract class parent {     abstract set<? extends a> getset(); }  class child extends parent {     set<? extends b> getset() {         return new hashset<b>();     } }

set<? extends b> is sub-type of set<? extends a>, , works because of covariant return types (appreciate comments lii & marco13 below).

depending on trying achieve exactly, might more limited expect, close gets.

perhaps similar achieved using inner classes, jude said, don't see how more convenient solution.


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