Which of these correctly demonstrates the Producer Extends Consumer Super (PECS) principle?
Answer: A
PECS principle: use 'extends' when reading from a collection (source), use 'super' when writing to it (dest). Option A reads from source and writes to dest correctly.
Q.2Hard
What will happen with this code?
List list = new ArrayList<String>();
list.add(123); // Adding Integer
String s = (String) list.get(0);
Answer: A
Raw type List accepts any object. At runtime, the Integer 123 cannot be cast to String, causing ClassCastException.
Q.3Hard
Consider this code:
public <T extends Comparable<T>> T getMax(T a, T b) {
return a.compareTo(b) > 0 ? a : b;
}
What is the benefit of this recursive bound <T extends Comparable<T>>?
Answer: A
Recursive bound <T extends Comparable<T>> ensures that T implements Comparable interface specifically for comparing with its own type, providing type safety.
Q.4Hard
Which statement is true about generic inheritance?
Answer: B
ArrayList<Integer> is a subtype of List<Integer> because ArrayList is a subclass of List with the same type parameter.
Q.5Hard
What will be the result of this code?
List<String> list = new ArrayList<>();
list.add("Hello");
List raw = list; // Unchecked assignment
raw.add(123); // Adding Integer to raw type
String s = list.get(1);
Answer: A
Raw type assignment bypasses generics. Integer 123 is added to the list. When casting to String at get(1), ClassCastException occurs.
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Q.6Hard
What will happen when you try to create an array of generic types like 'new ArrayList<String>[10]'?
Answer: A
Arrays of generic types are not allowed in Java due to type erasure and heap pollution prevention.
Q.7Hard
Consider: Map<String, ? extends List<?>> map = new HashMap<>();
What can you safely retrieve from this map?
Answer: A
The value type is '? extends List<?>', so you get a List with unknown element type. The exact subtype is unknown at compile-time.
Q.8Hard
What is the key difference between '? extends T' and '? super T' in practical usage?
Answer: A
Upper bounds (? extends T) are safe for reading because the compiler knows it's at least T. Lower bounds (? super T) are safe for writing because you can pass T or any superclass.
Q.9Hard
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.10Hard
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.11Hard
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.12Hard
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.13Hard
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.14Hard
Which statement correctly implements a generic factory method?
Answer: B
Cannot instantiate generic type T directly. Using Class<T>.newInstance() is a valid pattern (though deprecated in newer Java versions in favor of getConstructor()). Option A is invalid.
Q.15Hard
Which generic wildcard usage follows the consumer pattern correctly?
Answer: B
Consumer pattern uses '? super T' (lower bound). This allows adding elements safely. Option A (producer pattern) only allows reading, and Options C and D restrict type flexibility.
Q.16Hard
What is the compiled signature of this generic method?
public <T extends Number & Comparable<T>> void sort(T[] array)
Answer: D
With multiple bounds using '&', type erasure replaces T with the first bound. Here, Number is the first bound, so T is replaced with Number.
Q.17Hard
Which of the following represents valid bounded wildcard usage for a method that processes collections?
Answer: B
Only option B allows adding elements safely. '? super Integer' means the collection can hold Integer or any supertype. Option A prevents adding, Option C prevents adding, Option D restricts to exactly Number.
Q.18Hard
How would you correctly use generics in a recursive type bound scenario?
Answer: A
Option A demonstrates recursive type bound (also called F-bounded polymorphism), which allows a type to reference itself in its bound. This is commonly used in builder patterns and comparators.
Q.19Hard
What will happen when you compile and run this code?
java
List<String> strings = Arrays.asList("a", "b", "c");
List raw = strings;
raw.add(123);
String s = strings.get(3);
Answer: C
Assigning generic type to raw type suppresses type checking. The Integer added bypasses the type system, causing ClassCastException when retrieved as String.
Q.20Hard
Consider this declaration: public static <T> List<T> createList(). Which statement about type inference is correct?
Answer: B
Java's type inference deduces T from the assignment context. E.g., 'List<String> list = createList()' infers T as String.
