According to your example:

FirstFunctInterface<String> f = s -> s.length() > 10;

Interface definition should be like this:

@FunctionalInterface
public interface FirstFunctInterface<T> {
    public boolean firstMethod(T t); 
}

instead of:

@FunctionalInterface
public interface FirstFunctInterface<T> {
    public void firstMethod(T t); 
}

Please correct me if I am wrong.