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The Java Interview Handbook: 300+ Interview Questions

Bounded Types

Explore how to apply bounded types in Java generics to restrict type parameters, ensuring they extend a specific class like Number. Understand how this technique enhances type safety and avoids runtime exceptions, allowing you to invoke methods such as intValue() without casting.

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1.

What are bounded type parameters?

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Java
import java.util.ArrayList;
import java.util.List;
class Demonstration {
    public static void main( String args[] ) {
        NumberCollection<Integer> myIntegerList = new NumberCollection<>();
        myIntegerList.add(5);
        myIntegerList.add(4);
        myIntegerList.printGreater(4);
    }
}
class NumberCollection<T> {
    List<T> list = new ArrayList<>();
    // Method compares integer portions of each
    // value stored in the list and prints those
    // that are greater than the method argument
    public void printGreater(T other) {
        for (T item : list) {
            // crude way to get integer portions
            int val1 = Double.valueOf(item.toString()).intValue();
            int val2 = Double.valueOf(other.toString()).intValue();
            if (val1 > val2)
                System.out.println(item);
        }
    }
    public void add(T item) {
        list.add(item);
    }
}

We can fix the above situation by bounding the type parameter T to only be a subclass of Number. This eliminates the possibility of runtime exceptions that can result if the ...