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The Java Interview Handbook: 300+ Interview Questions

Wildcard

Explore the role of wildcards in Java generics to understand their effect on type safety, element insertion, and extraction limitations. Learn why you can only add null to a List<?> and how lower bounded wildcards can only serve as method arguments, not return types, enhancing your control over generic code behavior.

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1.

What is the ? used for in generics?

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Java
import java.util.ArrayList;
import java.util.Collection;
import java.util.List;
class Demonstration {
    public static void main( String args[] ) {
        Collection<Tiger> tigers = new ArrayList<>();
        tigers.add(new Tiger());
        printAnimal(tigers);
    }
  
    static void printAnimal(Collection<? extends Animal> animals) {
        for (Animal animal : animals)
            animal.speakUp();
    }
}
class Animal {
    void speakUp() {
        System.out.println("gibberish");
    }
}
class Elephant extends Animal {
    @Override
    public void speakUp() {
        System.out.println("Trumpets..");
    }
}
class Tiger extends Animal {
    @Override
    void speakUp() {
        System.out.println("Rooaaarrssss");
    }
}
1.

Will the following method have worked in the previous question for printing a list of type Animal?

    static <T extends Animal> void printAnimal(Collection<T> animals) {
        for (Animal animal : animals)
            animal.speakUp();
    }
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Technical Quiz
1.

What happens when we try to compile the following snippet?

        Collection<?> myColl = new ArrayList<Object>();
        myColl.add(new Object());
A.

Compiles

B.

Compiles with warnings

C.

Doesn’t compile

1 / 1
Java
import java.util.*;
class Demonstration {
    public static void main( String args[] ) {
        Collection<?> myColl = new ArrayList<>();
        // compile time error
        myColl.add(new Object());        
    }
}

The only element that you can ever insert into a ...