Strange compile error in java

Question

I'm trying to pass a variable of the type:

HashMap<Foo, HashSet<Bar>>

to a method, which expects:

Map<Foo, Set<Bar>>

I think it should work, but I'm getting the following compile error:

java: method setMenu in class com.xx.client.layout.Layout cannot be applied to given types;

  required: java.util.Map<com.xx.shared.model.UserType,java.util.Set<com.xx.shared.dto.model.MenuItemDTO>>

  found: java.util.HashMap<com.xx.shared.model.UserType,java.util.HashSet<com.xx.shared.dto.model.MenuItemDTO>>

  reason: actual argument java.util.HashMap<com.xx.shared.model.UserType,java.util.HashSet<com.xx.shared.dto.model.MenuItemDTO>> 

cannot be converted to 

java.util.Map<com.xx.shared.model.UserType,java.util.Set<com.xx.shared.dto.model.MenuItemDTO>> 

by method invocation conversion

The same way that a `List` is not a sub type of `List`, a `HashMap<..,HashSet<..>>` is not a sub type of a `Map<..,Set<..>>`. — Sotirios Delimanolis, Apr 09 '14 at 01:31
@SotiriosDelimanolis Its not a sub-type, but if a method expects a `Map`, then I should be able to pass a `HashMap` to it because `HashMap` implements `Map` — Ali, Apr 09 '14 at 01:32
No, type safety causes a failure in this case because the types are not compatible. — Sotirios Delimanolis, Apr 09 '14 at 01:34

score 2 · Accepted Answer · edited May 23 '17 at 12:13

To attempt to convince you, given a

HashMap<Foo, HashSet<Bar>> myMap;

you would expect to do

HashSet<Bar> aHashSet = myMap.get(aFoo);

But if you had

public void someMethod(Map<Foo, Set<Bar>> aMapParameter) {...}

and expected

someMethod(myMap);

to work, then someMethod could simply do

public void someMethod(Map<Foo, Set<Bar>> aMapParameter) {
    aMapParameter.put(aFoo, new TreeSet<>());
}

Your original

HashSet<Bar> aHashSet = myMap.get(aFoo);

would fail with a ClassCastException.

Compile time type safety has to guarantee that this doesn't happen. Therefore the compiler doesn't allow that method call.

Is List<Dog> a subclass of List<Animal>? Why aren't Java's generics implicitly polymorphic?

Strange compile error in java

1 Answers1