What will be the result of executing: List<Integer> list = new ArrayList<String>();
Answer: A
This causes a compilation error due to type mismatch. List<Integer> cannot hold ArrayList<String>. Generics do not support implicit type conversion.
Q.42Medium
Which statement correctly defines a generic class with multiple type parameters and bounds?
Answer: C
Multiple type parameters with bounds must be comma-separated and each can have its own upper bound. Option C shows correct syntax with recursive bound on T and bound on K.
Q.43Medium
Consider: public static <T> T findMax(T a, T b) { return a; }. What is the issue with calling findMax(5, 3.5)?
Answer: A
The type parameter T cannot simultaneously be int and double. The compiler cannot infer a single type T that satisfies both arguments, resulting in a compilation error.
Q.44Medium
What does List<?> represent in Java generics?
Answer: D
List<?> is an unbounded wildcard that represents a list of unknown type. You can read from it (get Object), but cannot write to it (except null). Both A, B, and C are correct interpretations.
Q.45Medium
Which of the following will NOT compile?
Answer: D
Option D fails because List<Object> cannot reference ArrayList<String>. Generics are invariant. Options A, B, C are valid (covariance with extends, contravariance with super).
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Q.46Easy
What is the output of the following code?
List<Integer> list = new ArrayList<Integer>();
list.add(10);
Object obj = list.get(0);
System.out.println(obj.getClass().getName());
Answer: A
Although obj is declared as Object, it holds an Integer instance. getClass() returns the actual runtime type, which is Integer.
Q.47Medium
Consider a generic interface: interface Comparable<T> { int compareTo(T obj); }. How should a class implement this for String comparison?
Answer: D
Option B directly specifies String, while Option C makes the class generic. Both are valid approaches to implement a generic interface.
Q.48Medium
What happens when you use raw type List instead of List<String>?
Answer: D
Using raw types bypasses generic type checking, losing type safety. Elements are treated as Object at runtime, potentially causing ClassCastException when casting.
Q.49Hard
Which generic declaration allows a method to accept List of any type?
Answer: D
All three approaches allow processing lists of any type, though they differ: raw type (unsafe), unbounded wildcard (read-only), and type parameter (flexible).
Q.50Hard
What is the difference between <T extends Comparable<T>> and <T extends Comparable>?
Answer: B
<T extends Comparable<T>> is a recursive bound ensuring T implements Comparable of its own type. <T extends Comparable> uses raw type, losing type information.
Q.51Hard
Can you create an instance of generic type parameter directly like: T obj = new T();?
Answer: B
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.
Q.52Hard
What is the PECS principle in generics?
Answer: A
PECS (Producer Extends, Consumer Super) is a mnemonic for wildcard bounds: use '? extends' when reading (producing), use '? super' when writing (consuming).
Q.53Hard
Which statement about generic constructors is TRUE?
Answer: D
Generic constructors can exist in non-generic classes. Constructor's type parameters are separate from and independent of the class's type parameters.
Q.54Medium
What is the output?
List<String> strings = new ArrayList<String>();
List list = strings;
list.add(123);
String str = strings.get(0);
System.out.println(str);
Answer: C
Adding Integer 123 to raw type reference bypasses generics. When retrieving as String, a ClassCastException occurs at runtime.
Q.55Medium
How would you define a generic method that works with a pair of objects and returns the first one?
Answer: D
Both A and C define valid generic methods with multiple type parameters. C is more explicit with 'public' modifier, but both are functionally equivalent.
Q.56Easy
What is the advantage of using generics over raw types in terms of 2024-25 Java standards?
Answer: D
Generics provide compile-time safety, code reusability through parameterization, and clearer API contracts. These are essential for modern Java development standards.
Q.57Easy
Which of the following is a valid generic class declaration in Java?
Answer: A
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.
Q.58Easy
What is the result of type erasure in Java generics?
Answer: B
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.
Q.59Medium
Consider the following code snippet:
public <T> T processData(List<? extends T> list) { return list.get(0); }
What is the relationship between the wildcard and type parameter T?
Answer: B
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.
Q.60Medium
Which of the following will cause a compile-time error?
Answer: B
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.
