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The Java Interview Handbook: 300+ Interview Questions

Why Generics?

Understand the importance of generics in Java by learning how they label data structures to ensure type safety, prevent ClassCastException, and eliminate the need for explicit casting. Explore raw types versus generic types, type inference with diamond operators and var keyword, and how generics improve code robustness and readability.

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Generics act like labeled boxes. A raw data structure is an unlabeled box that can hold anything. This leads to issues at runtime if we extract an item expecting an integer but get a string instead. Generics allow us to label the box. The compiler then acts as a strict bouncer, rejecting incorrect types before the code even runs.

Let's explore the core mechanics of generics in Java and how we can apply these concepts to technical interview questions.

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What is the purpose of generics in Java?

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What is type safety?

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To see this in action, we can look at a method that sums a list of numbers using raw types.

Java
import java.util.ArrayList;
import java.util.List;
public class Demonstration {
    public static void main(String[] args) {
        TypeSafetyDemo demo = new TypeSafetyDemo();
        List myList = new ArrayList();
        myList.add("Hello");
        myList.add(2);
        demo.sum(myList);
    }
}
class TypeSafetyDemo {
    int sum(List list) {
        int total = 0;
        for (int i = 0; i < list.size(); i++) {
            total += (int) list.get(i);
        }
        return total;
    }
}

Note: This program compiles but throws a ClassCastException at runtime. It intentionally demonstrates the lack of type safety when using raw ...