Java Generics - Practice Questions 2026

Master the complexities of Java Generics with this comprehensive practice exam course. Designed for developers who want to move beyond basic syntax and truly understand Type Safety, Polymorphism, and the nuances of the Java Collections Framework, this course provides a rigorous testing environment.

Why Serious Learners Choose These Practice Exams

Serious learners understand that watching a video is not the same as writing bug-free code. These practice exams are crafted to challenge your understanding of how the Java compiler handles generic types at compile-time and runtime. By engaging with these questions, you bridge the gap between theoretical knowledge and professional-level implementation. We focus on common pitfalls, edge cases, and the "why" behind every compilation error.

Course Structure

Our curriculum is organized into a progressive learning path to ensure you build a solid foundation before tackling complex logic.

• Basics / Foundations: This section focuses on the fundamental "Why" of Generics. You will be tested on type safety, the removal of explicit casting, and the basic syntax of generic classes and methods.

• Core Concepts: Here, we dive into Type Parameters and Type Erasure. You will explore how the JVM handles types and what happens to your generic code after compilation.

• Intermediate Concepts: This module covers Bounded Type Parameters. You will practice using the extends keyword to restrict the types that can be passed to a generic class or method.

• Advanced Concepts: This challenging section focuses on Wildcards. You will master Upper Bounded Wildcards, Lower Bounded Wildcards, and Unbounded Wildcards, learning exactly when to use ? extends T versus ? super T.

• Real-world Scenarios: These questions simulate actual production code. You will deal with generic interfaces, legacy code integration, and complex data structures.

• Mixed Revision / Final Test: A comprehensive cumulative exam that mixes all previous topics to ensure you are ready for technical interviews or professional certification.

Sample Practice Questions

Question 1

Which of the following code snippets will cause a compilation error?

• Option 1: List<Object> list = new ArrayList<String>();

• Option 2: List<?> list = new ArrayList<String>();

• Option 3: List<? extends Number> list = new ArrayList<Integer>();

• Option 4: List<? super Integer> list = new ArrayList<Number>();

• Option 5: List<String> list = new ArrayList<>();

Correct Answer: Option 1

Correct Answer Explanation:

In Java Generics, subtyping of the type arguments does not mean subtyping of the generic types. While String is a subtype of Object, List<String> is not a subtype of List<Object>. This is known as invariance. If this were allowed, you could potentially add an Integer into a List<Object> that was actually a List<String>, breaking type safety.

Wrong Answers Explanation:

• Option 2: This is valid because the unbounded wildcard <?> is the supertype of all specialized versions of the list.

• Option 3: This is valid. Integer is a subtype of Number, so the Upper Bounded Wildcard <? extends Number> accepts it.

• Option 4: This is valid. Number is a supertype of Integer, so the Lower Bounded Wildcard <? super Integer> accepts it.

• Option 5: This is valid. It uses the "diamond operator" introduced in Java 7, where the compiler infers the type.

Question 2

Given the principle of PECS (Producer Extends, Consumer Super), which declaration is most appropriate for a method that only reads data from a source list of numbers?

• Option 1: public void process(List<Number> list)

• Option 2: public void process(List<? super Number> list)

• Option 3: public void process(List<? extends Number> list)

• Option 4: public void process(List<?> list)

• Option 5: public void process(List<Object> list)

Correct Answer: Option 3

Correct Answer Explanation:

According to the PECS principle, if a parameterized collection is a "producer" (you are reading items from it), you should use ? extends. This allows the method to accept a List<Integer>, List<Double>, or any other list of types that inherit from Number, maximizing the flexibility of the method.

Wrong Answers Explanation:

• Option 1: This is too restrictive. It would only accept a List<Number> and would reject a List<Integer>.

• Option 2: This is used for "consumers" (where you write data into the list). It would not allow you to safely read items as Number.

• Option 4: An unbounded wildcard would only allow you to read items as Object, losing the specific Number functionality.

• Option 5: This would only accept a list specifically typed as Object, which is not compatible with a List<Number>.

Course Features

Welcome to the best practice exams to help you prepare for your Java Generics. Whether you are preparing for an OCP certification or a high-stakes job interview, this course provides:

• Unlimited Retakes: You can retake the exams as many times as you want to reinforce your learning.

• Original Question Bank: This is a huge original question bank designed to test deep understanding, not just rote memorization.

• Instructor Support: You get support from instructors if you have questions or need further clarification on a specific topic.

• Detailed Explanations: Each question has a detailed explanation so you understand the logic behind the correct answer.

• Mobile Access: Fully mobile-compatible with the Udemy app for learning on the go.

• Risk-Free: 30-days money-back guarantee if you are not satisfied with the course quality.

We hope that by now you are convinced! And there are a lot more questions inside the course.