Java Generics Interview Questions and Answers for freshers

1. What are Java Generics?

   • Answer: Java Generics allow you to write type-safe code by parameterizing types. Instead of using raw types (like `ArrayList`), you can specify the type of objects the collection will hold (e.g., `ArrayList`). This prevents runtime `ClassCastException` errors and improves code readability and maintainability.

2. What is the benefit of using Generics?

   • Answer: Generics offer type safety, improved code readability, reduced casting, and compile-time error checking, leading to more robust and maintainable code. They eliminate the need for many runtime checks associated with raw types.

3. Explain the concept of type parameters in Generics.

   • Answer: Type parameters are placeholders for actual types that are specified when you create an instance of a generic class or interface. For example, in `List`, `T` is a type parameter. When you create `List`, `String` replaces `T`.

4. What is a generic class? Give an example.

   • Answer: A generic class is a class that uses type parameters. Example: `public class Box { private T value; ... }`. This `Box` class can hold any type of object, specified when creating an instance (e.g., `Box`, `Box`).

5. What is a generic interface? Give an example.

   • Answer: A generic interface is similar to a generic class but defines a contract for methods. Example: `public interface DataStore { V get(K key); void put(K key, V value); }`. This interface uses two type parameters for keys and values.

6. What is a bounded type parameter?

   • Answer: A bounded type parameter restricts the types that can be used as arguments for the type parameter. Example: `public class NumberBox { ... }` This limits `T` to only types that extend the `Number` class (like `Integer`, `Double`, etc.).

7. Explain wildcard characters in Generics.

   • Answer: Wildcards provide flexibility when working with generics. `?` represents any type, `? extends Number` means any subtype of Number, and `? super Integer` means any supertype of Integer.

8. What is the difference between `? extends T` and `? super T`?

   • Answer: `? extends T` allows you to read objects of type T or its subtypes. `? super T` allows you to write objects of type T or its supertypes. `? extends Number` could receive an Integer, but `? super Integer` could receive an Object.

9. Can you explain the concept of Generics and inheritance?

   • Answer: Generics don't participate in inheritance directly. If you have a generic class `GenericClass`, you can't inherit from `GenericClass` to create `GenericClass`. However, you can create subclasses that use different type parameters.

10. How do generics help in avoiding ClassCastException?

    • Answer: Generics prevent `ClassCastException` at runtime by performing type checking at compile time. If you try to add an incompatible type to a generic collection, the compiler will generate an error.

11. What is type erasure in Generics?

    • Answer: Type erasure is the process where the compiler removes generic type information during compilation. At runtime, the generic type parameters are replaced with their raw types (e.g., `List` becomes `List`).

12. What are the limitations of Java Generics?

    • Answer: Generics cannot be used with primitive types (e.g., `int`, `float`). You must use wrapper classes (e.g., `Integer`, `Float`). Generic type information is erased at runtime; you can't get the type parameter at runtime using `instanceof`.

13. How can you use generics with methods?

    • Answer: You can define generic methods by adding type parameters to the method signature. Example: `public T returnGenericType(T input) { return input; }`

14. Explain the use of generics in collections.

    • Answer: Generics are extensively used with collections like `ArrayList`, `HashMap`, etc. to specify the type of elements they hold. This improves type safety and avoids runtime errors.

15. What is a raw type in Generics? Why should you avoid them?

    • Answer: A raw type is a generic type without type arguments. For example, `List` instead of `List`. They should be avoided because they lose the benefits of type safety offered by generics and can lead to `ClassCastException`s at runtime.

16. Give an example of using generics with multiple type parameters.

    • Answer: `public class Pair { private K key; private V value; ... }` This class uses two type parameters, `K` for key and `V` for value.

17. How can you create a generic method that works with multiple types?

    • Answer: Use multiple type parameters in the method signature: `public void process(T t, U u) { ... }`

18. Explain the use of generics in creating reusable components.

    • Answer: Generics allow you to write classes and methods that can work with various types without compromising type safety, making them highly reusable components.

19. How does generics help in improving code maintainability?

    • Answer: By enforcing type safety at compile time, generics reduce the risk of runtime errors, leading to easier debugging and maintenance. The code becomes more readable and understandable.

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