The Java Interview Handbook: 300+ Interview Questions/

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Wildcard

Wildcard

This lesson talks about how the wildcard can be used in generics.

We'll cover the following...
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

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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 ...