Which method is used to stop a thread in Java?

The correct answer is B: interrupt(). The interrupt() method is the proper way to stop a thread in Java. The stop() method is deprecated and dangerous.

What is the purpose of a Predicate functional interface in Java?

The correct answer is C: To test a condition and return a boolean value. Predicate is a functional interface that takes a single input of type T and returns a boolean. It's commonly used for filtering operations in streams.

What will be the result of executing this code? PriorityQueue pq = new PriorityQueue(); pq.add(5); pq.add(3); pq.add(7); System.out.println(pq.peek());

The correct answer is B: 3. PriorityQueue orders elements based on their natural ordering (min-heap by default). peek() returns 3, the minimum element.

What is the output of the following code? FileReader fr = new FileReader("test.txt"); int data = fr.read(); System.out.println(data);

The correct answer is B: ASCII value of the character. FileReader.read() returns the ASCII/Unicode value of the character as an integer, not the character itself. It returns -1 when EOF is reached.

Consider this declaration: public static List createList(). Which statement about type inference is correct?

The correct answer is B: Type T is inferred from context where method is called. Java's type inference deduces T from the assignment context. E.g., 'List list = createList()' infers T as String.

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Java Programming
Generics

Java OOP, collections, multithreading

49 Q 10 Topics Take Mock Test

Difficulty: All Easy Medium Hard 11–20 of 49

Topics in Java Programming

All Basics & Syntax 100 OOP in Java 100 Collections Framework 100 Multithreading 100 Exception Handling 100 I/O Streams 100 JDBC 100 Generics 100 Lambda Expressions 100 Spring Framework Basics 58

Q.11 Medium Generics

What does the following generic declaration mean?
public T findMax(T a, T b)

A T must be Number or Comparable

B T must extend Number and implement Comparable

C T must be Number and a subclass of Comparable

D T can be either Number or Comparable

Correct Answer: B. T must extend Number and implement Comparable

EXPLANATION

Multiple bounds in generics use '&' to specify that the type must satisfy all constraints. T must extend Number AND implement Comparable<T>.

Test

Q.12 Medium Generics

In the context of generics, what does 'invariance' mean?

A A generic type List is a subtype of List

B A generic type List is not a subtype of List, despite Integer being a subtype of Number

C Invariance allows covariance and contravariance together

D Invariance means the type cannot change

Correct Answer: B. A generic type List is not a subtype of List, despite Integer being a subtype of Number

EXPLANATION

Generics are invariant in Java. List<Integer> is NOT a subtype of List<Number> to maintain type safety. This is a key feature preventing runtime errors.

Test

Q.13 Medium Generics

What happens when you try to cast a generic object?
List strings = (List) getSomeList();

A The cast is fully checked at compile time

B The cast is unchecked; generic type information is lost due to erasure

C The cast always succeeds because of type erasure

D A compile-time error is raised

Correct Answer: B. The cast is unchecked; generic type information is lost due to erasure

EXPLANATION

Due to type erasure, casts involving generic types are unchecked. The compiler warns about this but allows it. The actual cast check only verifies the raw type.

Test

Q.14 Medium Generics

Given the declaration: List listOfLists;
Which assignment is valid?

A listOfLists = new ArrayList();

B listOfLists = new ArrayList

C listOfLists = new ArrayList();

D listOfLists = new ArrayList();

Correct Answer: D. listOfLists = new ArrayList();

EXPLANATION

Nested generics must match exactly or use appropriate wildcards. Only option D provides exact type match. Option B would work with wildcards, but that's not listed correctly here.

Test

Q.15 Medium Generics

What is the correct way to declare a generic method that accepts a list and returns the maximum element?

A public T getMax(List list) { ... }

B public static T getMax(List

C public

D public T getMax(List list) { ... }

Correct Answer: D. public T getMax(List list) { ... }

EXPLANATION

Option D correctly ensures T is comparable with itself using F-bounded polymorphism. Option C has inverted logic with '? super'. Option B is unnecessarily complex.

Test

Q.16 Medium Generics

Consider:
public class Pair { }
Which declaration is invalid?

A Pair pair = new Pair();

B Pair pair = new Pair();

C Pair pair = new Pair();

D Pair

Correct Answer: D. Pair

EXPLANATION

Option D is invalid because we cannot instantiate a generic class with wildcard type parameters. Wildcards are only for variable declarations, not for instantiation with 'new'.

Test

Q.17 Medium Generics

What is the output of this code?
List strings = new ArrayList();
List raw = strings;
raw.add(123);
String str = strings.get(0);

A Compiles and runs without error

B Compile-time error: cannot add Integer to List

C Runtime ClassCastException when retrieving the element

D Compile-time error: raw type cannot be assigned

Correct Answer: C. Runtime ClassCastException when retrieving the element

EXPLANATION

The code compiles (with warnings about raw types) but throws ClassCastException at runtime when trying to cast Integer to String. Type safety is lost with raw types.

Test

Q.18 Medium Generics

Given: Map

A map.put("key", new ArrayList())

B List list = map.get("key"); list.add("value");

C List list = map.get("key");

D map.put("key", Arrays.asList(1, 2, 3))

Correct Answer: C. List list = map.get("key");

EXPLANATION

We can read from map.get() and assign to List<?>, but cannot add to List<?> or put into the map due to wildcard restrictions. Option C only reads the value.

Test

Q.19 Medium Generics

What does the following generic method signature indicate?
public static T findMax(T[] array)

A T must implement Comparable interface parameterized with itself

B T must be a Comparable type but not necessarily of itself

C Comparable is optional for T

D T is comparable only with Object types

Correct Answer: A. T must implement Comparable interface parameterized with itself

EXPLANATION

The bounded type parameter 'T extends Comparable<T>' enforces that T must implement Comparable and be comparable with its own type. This is the F-bounded polymorphism pattern.

Test

Q.20 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.

Test

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