Java Programming — Generics Questions & Answers

Q.81 Easy Generics

In Java generics, what does the wildcard '?' represent?

A An unknown type that can be any class

B A mandatory type parameter

C A primitive data type only

D A deprecated feature in Java 8+

Correct Answer: A. An unknown type that can be any class

EXPLANATION

The wildcard '?' in generics represents an unknown type, allowing flexibility when the exact type is not known or not important.

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Q.82 Medium Generics

Which of these declarations would NOT cause a compilation warning or error?

A List list = new ArrayList();

B List list = new ArrayList();

C List list = new ArrayList();

D List list = new ArrayList();

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

EXPLANATION

Option A uses proper diamond syntax with matching type parameters on both sides. Options B, C produce unchecked warnings, and Option D is invalid due to invariance.

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Q.83 Hard Generics

What will be the result of this code?
List list = new ArrayList();
list.add("Hello");
List raw = list; // Unchecked assignment
raw.add(123); // Adding Integer to raw type
String s = list.get(1);

A ClassCastException when accessing list.get(1)

B Code executes without error

C Compilation error

D StringIndexOutOfBoundsException

Correct Answer: A. ClassCastException when accessing list.get(1)

EXPLANATION

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.84 Hard Generics

Which statement is true about generic inheritance?

A ArrayList is NOT a subtype of List due to invariance

B ArrayList IS a subtype of List

C ArrayList is a subtype of ArrayList

D Generic types support covariance by default

Correct Answer: B. ArrayList IS a subtype of List

EXPLANATION

ArrayList<Integer> is a subtype of List<Integer> because ArrayList is a subclass of List with the same type parameter.

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Q.85 Easy Generics

What is the output of this generic code?
class Pair {
public void display(K key, V value) {
System.out.println(key + ": " + value);
}
}
Pair p = new Pair();
p.display("Age", 25);

A Age: 25

B Compilation error - type mismatch

C Age: 25.0

D Runtime exception

Correct Answer: A. Age: 25

EXPLANATION

The generic class Pair with two type parameters K and V correctly accepts String and Integer. The display method prints the key-value pair.

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Q.86 Hard Generics

Consider this code:
public T getMax(T a, T b) {
return a.compareTo(b) > 0 ? a : b;
}
What is the benefit of this recursive bound ?

A Ensures type safety by guaranteeing T implements Comparable with itself

B Allows T to be compared with any type

C Improves runtime performance

D Eliminates the need for type erasure

Correct Answer: A. Ensures type safety by guaranteeing T implements Comparable with itself

EXPLANATION

Recursive bound <T extends Comparable<T>> ensures that T implements Comparable interface specifically for comparing with its own type, providing type safety.

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Q.87 Hard Generics

What will happen with this code?
List list = new ArrayList();
list.add(123); // Adding Integer
String s = (String) list.get(0);

A ClassCastException at runtime

B Compilation error

C Code executes successfully

D Unchecked warning at compile time only

Correct Answer: A. ClassCastException at runtime

EXPLANATION

Raw type List accepts any object. At runtime, the Integer 123 cannot be cast to String, causing ClassCastException.

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Q.88 Hard Generics

Which of these correctly demonstrates the Producer Extends Consumer Super (PECS) principle?

A public void process(List

B public void process(List

C public void process(List source, List dest) { }

D public void process(List source, List dest) { }

Correct Answer: A. public void process(List

EXPLANATION

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.

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Q.89 Medium Generics

What will be the output?
List list = new ArrayList();
list.add(10);
Object obj = list.get(0);
System.out.println(obj instanceof Integer);

A true

B false

C Compilation error

D Runtime exception

Correct Answer: A. true

EXPLANATION

The Integer value is stored in the list. At runtime, type erasure converts it to Object, but the actual stored value remains an Integer instance.

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Q.90 Medium Generics

Consider this generic interface:
interface Comparable { int compareTo(T o); }
Which class definition correctly implements this?

A class MyClass implements Comparable { public int compareTo(MyClass o) { return 0; } }

B class MyClass implements Comparable { public int compareTo(Object o) { return 0; } }

C class MyClass implements Comparable { public int compareTo(Object o) { return 0; } }

D class MyClass implements Comparable { public int compareTo(T o) { return 0; } }

Correct Answer: A. class MyClass implements Comparable { public int compareTo(MyClass o) { return 0; } }

EXPLANATION

Option A correctly implements the generic interface by specifying the concrete type parameter as MyClass in the implements clause.

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