Java Generics Interview Questions and Answers for 10 years experience

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100 Java Generics Interview Questions & Answers

  1. What are Java Generics?

    • Answer: Java Generics allow you to write type-safe code that can work with various data types without losing type information at compile time. They enable you to define classes, interfaces, and methods that can operate on objects of different types while enforcing type constraints at compile time, preventing ClassCastException errors at runtime.

  2. Explain the benefits of using Generics.

    • Answer: Generics offer several key benefits: improved type safety (eliminating ClassCastException), increased code reusability (writing one generic class instead of multiple specific classes), better code readability (explicit type parameters clarify intent), and reduced code bloat (avoiding unnecessary boxing/unboxing).

  3. What is a generic class? Provide an example.

    • Answer: A generic class is a class that is parameterized by one or more type parameters. These parameters are placeholders for concrete types that are specified when the class is instantiated. Example: public class Pair<K, V> { private K key; private V value; ... }

  4. What is a generic interface? Provide an example.

    • Answer: A generic interface is similar to a generic class, but it defines a contract for methods rather than implementing them. Example: public interface DataProvider<T> { T getData(); }

  5. What is a generic method? Provide an example.

    • Answer: A generic method is a method that uses type parameters. It can operate on different types without requiring multiple method overloads. Example: public <T> void printArray(T[] array) { ... }

  6. Explain type erasure in Java Generics.

    • Answer: Type erasure is the process by which the Java compiler removes all generic type information during compilation. At runtime, generic types are treated as their raw types. This is done for backward compatibility with older Java versions that didn't support generics.

  7. What are unbounded wildcards? Give an example.

    • Answer: Unbounded wildcards, denoted by `?`, represent any type. They are useful when the type of an object doesn't matter. Example: List anyList; This list can hold elements of any type.

  8. Explain upper bounded wildcards. Give an example.

    • Answer: Upper bounded wildcards, denoted by `? extends T`, specify that the type parameter must be a subtype of T. Example: List<? extends Number> numberList; This list can hold objects of type Number or any of its subtypes (Integer, Double, etc.).

  9. Explain lower bounded wildcards. Give an example.

    • Answer: Lower bounded wildcards, denoted by `? super T`, specify that the type parameter must be a supertype of T. Example: List<? super Integer> integerList; This list can hold objects of type Integer or any of its supertypes (Number, Object).

  10. How do you handle exceptions within a generic method?

    • Answer: Exceptions are handled in generic methods the same way as in regular methods using try-catch blocks. The type of the exception doesn't depend on the generic type parameter.

  11. Can you instantiate a generic class with primitive types? If not, how can you work around this?

    • Answer: No, you cannot directly instantiate a generic class with primitive types. You must use their wrapper classes (Integer, Double, Boolean, etc.).

  12. Explain the difference between `List` and `List`.

    • Answer: `List` is a list that can only hold Strings. `List` is a list that can hold objects of any type, but you cannot add any elements to it without knowing the exact type at compile time because of type safety restrictions.

  13. What are the limitations of Java Generics?

    • Answer: Generics cannot be used with primitive types directly; type information is erased at runtime; generic arrays cannot be created; there are restrictions on using wildcards in certain situations (e.g., you cannot create an instance of a generic type with a wildcard).

  14. How do you create a generic method that can work with multiple types?

    • Answer: Use multiple type parameters in the method signature. For example, <T, U> void process(T t, U u) { ... }

  15. Explain how Generics help in preventing runtime exceptions.

    • Answer: Generics help prevent `ClassCastException` by enforcing type constraints at compile time. If you try to use a type that violates the generic constraints, the compiler will throw an error, preventing the error from manifesting at runtime.

  16. How do you use Generics with Collections?

    • Answer: Collections like ArrayList, HashMap, etc. can be parameterized with generic types. For example: `List stringList = new ArrayList<>();`

  17. Explain the concept of covariance and contravariance in Generics.

  18. Describe a situation where you would use a bounded wildcard.

    • Answer: You might use an upper bounded wildcard (`? extends Number`) when you have a method that needs to process a collection of numbers, but you don't need to add elements to the collection. This allows the method to accept Lists of Integer, Double, etc.

  19. How does Generics interact with inheritance?

    • Answer: Generics interact with inheritance through subtyping. If a class `B` extends a generic class `A`, then you can create `B` which inherits the generic type parameter from `A`. However, there are limitations related to covariance and contravariance.

  20. Explain how to use generics in a scenario involving multiple interfaces.

    • Answer: You can have a class implement multiple interfaces, each with its own generic type parameters, or a single interface with multiple type parameters. The implementation class must provide concrete types for each generic type parameter.

  21. How can you improve the performance of code using Generics?

    • Answer: While generics primarily improve type safety and code readability, performance impact is usually minimal. However, avoiding unnecessary boxing and unboxing of primitive types when using generics with wrapper classes can help improve performance.

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