Which of the following lambda expressions is INVALID in Java?

The correct answer is D: (x, y) -> return x + y;. In a lambda expression, if the body uses curly braces, 'return' keyword is required. But without curly braces, 'return' keyword cannot be used. Option D is invalid because it has 'return' without curly braces.

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

The correct answer is A: T must implement Comparable interface parameterized with itself. The bounded type parameter 'T extends Comparable' enforces that T must implement Comparable and be comparable with its own type. This is the F-bounded polymorphism pattern.

What is the primary purpose of the synchronized keyword in Java?

The correct answer is B: To prevent race conditions by ensuring mutual exclusion. The synchronized keyword ensures that only one thread can execute a critical section at a time, preventing race conditions and data inconsistency.

A batch update operation fails partially. How can you identify which statements failed in the batch?

The correct answer is B: Use the int array returned by executeBatch() to check individual update counts. executeBatch() returns an int array where each element represents the update count for the corresponding statement. A value of -3 (Statement.EXECUTE_FAILED) indicates failure for that statement.

Which DriverManager method is used to establish a database connection?

The correct answer is B: getConnection(). DriverManager.getConnection() is the standard method that takes URL, username, and password to establish a connection to the database.

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Java Programming
Generics

Java OOP, collections, multithreading

49 Q 10 Topics Take Mock Test

Difficulty: All Easy Medium Hard 41–49 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.41 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.

Test

Q.42 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.

Test

Q.43 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.

Test

Q.44 Medium Generics

What is the difference between List and List?

A List is read-only while List allows adding any object type

B List is read-only while List allows adding any type

C Both are equivalent

D List uses type erasure while List does not

Correct Answer: A. List is read-only while List allows adding any object type

EXPLANATION

List<?> accepts any type but is read-only (can't add). List<Object> explicitly accepts Object type and allows adding objects.

Test

Q.45 Medium Generics

Which of the following represents a bounded type parameter?

A class Box { }

B class Box { }

C class Box

D class Box { }

Correct Answer: A. class Box { }

EXPLANATION

Bounded type parameters restrict the types that can be used as arguments. T extends Number ensures T is Number or its subclass.

Test

Q.46 Medium Generics

Consider the code:
List

A No, compilation error - cannot add to ? extends

B Yes, it compiles successfully

C Only if Integer is used

D Only at runtime

Correct Answer: A. No, compilation error - cannot add to ? extends

EXPLANATION

Upper bounded wildcards (? extends Number) are read-only. You cannot add elements because the compiler doesn't know the exact subtype.

Test

Q.47 Medium Generics

What will happen when you try to create an array of generics?
List[] array = new List[10];

A Compilation error

B Runtime exception

C Code executes successfully

D Creates an unchecked warning only

Correct Answer: A. Compilation error

EXPLANATION

You cannot create arrays of generic types because of type erasure. The compiler prevents this with a compilation error.

Test

Q.48 Medium Generics

Which statement about wildcard generics (?) is correct?

A ? represents an unknown type and can be used with upper bounds (? extends T) or lower bounds (? super T)

B ? can only be used with extends keyword

C ? is used to create generic classes

D ? eliminates the need for type parameters

Correct Answer: A. ? represents an unknown type and can be used with upper bounds (? extends T) or lower bounds (? super T)

EXPLANATION

Wildcards (?) represent an unknown type. Upper bound (? extends T) accepts T or its subtypes. Lower bound (? super T) accepts T or its supertypes.

Test

Q.49 Medium Generics

What is type erasure in Java Generics?

A The process by which generic type information is removed at runtime

B Automatic deletion of generic collections

C Compiler optimization of generic code

D Runtime type checking for generics

Correct Answer: A. The process by which generic type information is removed at runtime

EXPLANATION

Type erasure is a mechanism where the Java compiler removes all generic type information during compilation, converting generic code to its raw type equivalent.

Test

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Generics