Java Programming — Generics Questions & Answers

Q.41 Medium Generics

Which of the following will cause a compile-time error?

A List list = new ArrayList();

B List list = new ArrayList();

C List

D List list = new ArrayList();

Correct Answer: B. List list = new ArrayList();

EXPLANATION

Java does not allow covariance with generics. List<Integer> cannot be assigned to List<Number> because you could add a Double to the latter, breaking type safety.

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Q.42 Medium Generics

Consider the following code snippet:
public T processData(List

A T must be a subtype of the elements in the list

B The list can contain elements that are subtypes of T

C T and the wildcard bound are independent

D The list must contain exactly type T

Correct Answer: B. The list can contain elements that are subtypes of T

EXPLANATION

The wildcard '? extends T' means the list can contain any type that is a subtype of T. This allows reading from the list safely but restricts writing.

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Q.43 Easy Generics

What is the result of type erasure in Java generics?

A Generic type information is preserved at runtime

B All generic type information is removed at compile time, replaced with raw types or bounds

C Generic types are converted to Object only

D Type erasure only applies to wildcards

Correct Answer: B. All generic type information is removed at compile time, replaced with raw types or bounds

EXPLANATION

Type erasure removes all generic type parameters during compilation. For unbounded types, they're replaced with Object; for bounded types, they're replaced with the upper bound.

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Q.44 Easy Generics

Which of the following is a valid generic class declaration in Java?

A public class Box { }

B public class Box { }

C public class Box { }

D public class Box { }

Correct Answer: A. public class Box { }

EXPLANATION

Valid generic class declarations allow multiple type parameters with upper bounds. Option A is syntactically correct. Option C uses invalid 'super' keyword in class declaration. Option D has malformed syntax.

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Q.45 Easy Generics

What is the advantage of using generics over raw types in terms of 2024-25 Java standards?

A Compile-time type checking prevents ClassCastException

B Enables code reuse without sacrificing type safety

C Improves API clarity and documentation

D All of the above

Correct Answer: D. All of the above

EXPLANATION

Generics provide compile-time safety, code reusability through parameterization, and clearer API contracts. These are essential for modern Java development standards.

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Q.46 Medium Generics

How would you define a generic method that works with a pair of objects and returns the first one?

A T getFirst(T first, U second) { return first; }

B T getFirst(T first, T second) { return first; }

C public T getFirst(T first, U second) { return first; }

D Both A and C

Correct Answer: D. Both A and C

EXPLANATION

Both A and C define valid generic methods with multiple type parameters. C is more explicit with 'public' modifier, but both are functionally equivalent.

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Q.47 Medium Generics

What is the output?

List strings = new ArrayList();
List list = strings;
list.add(123);
String str = strings.get(0);
System.out.println(str);

A 123

B Compilation error

C Runtime ClassCastException

D null

Correct Answer: C. Runtime ClassCastException

EXPLANATION

Adding Integer 123 to raw type reference bypasses generics. When retrieving as String, a ClassCastException occurs at runtime.

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Q.48 Hard Generics

Which statement about generic constructors is TRUE?

A Generic constructors must be in generic classes only

B Non-generic classes can have generic constructors

C Constructor type parameters are independent of class type parameters

D Both B and C

Correct Answer: D. Both B and C

EXPLANATION

Generic constructors can exist in non-generic classes. Constructor's type parameters are separate from and independent of the class's type parameters.

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Q.49 Hard Generics

What is the PECS principle in generics?

A Producer Extends, Consumer Super

B Public Extends, Consumer Sets

C Parameter Encapsulation and Class Structure

D Polymorphic Extension and Class Signatures

Correct Answer: A. Producer Extends, Consumer Super

EXPLANATION

PECS (Producer Extends, Consumer Super) is a mnemonic for wildcard bounds: use '? extends' when reading (producing), use '? super' when writing (consuming).

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Q.50 Hard Generics

Can you create an instance of generic type parameter directly like: T obj = new T();?

A Yes, always possible

B No, because type information is erased at runtime

C Yes, only if T extends Serializable

D Only with reflection

Correct Answer: B. No, because type information is erased at runtime

EXPLANATION

Due to type erasure, T is replaced with Object at runtime, and you cannot instantiate Object with new T(). You need reflection or pass Class<T> as parameter.

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