Called method of inner generic class doesn't recognize the generic type

Question

I struggle with the following inner class "inheriting" the generic parameter T from the outer class. The following lines are erroneous:

String string = "string";
new OuterClass<>()
    .inner(string)
    .call(s -> s.length());       // Cannot resolve method 'length' in 'Object'

Minimal and reproducible example of these classes:

public class OuterClass<T> {

    public InnerClass<T> inner(T object) {
        return new InnerClass<>(object);
    }

    @AllArgsConstructor(access = AccessLevel.PRIVATE)
    public static final class InnerClass<U> {

        U object;

        public final void call(Consumer<U> consumer) {
            consumer.accept(object);
        }
    }
}

Why does this happen? How am I supposed to specify the generic parameter through the creation of the inner class?

Answer 1

Essentially, this is because inference of type in the <> does not take into account the subsequent methods that you chain.

For example, this creates an ArrayList<Object> :

new ArrayList<>().add("");

If I insert the word String inside the <> , IntelliJ will not complain that the word String is redundant.

Similarly, this produces an error:

OuterClass<String>.InnerClass i = new OuterClass<>().inner(string);

Because the right hand side produces a OuterClass<Object>.Inner , while the left hand side is a OuterClass<String>.InnerClass .

To fix this, you need to specify the generic parameter:

new OuterClass<String>()
    .inner(string)
    .call(s -> s.length()); 

Regarding your edit, the edited inner method still depends on the generic type of Outer , which cannot be inferred correctly.

You can make the Inner object created not depend on the generic type of Outer by adding another generic parameter:

public <U> InnerClass<U> inner(U object) {
    return new InnerClass<>(object);
}