Top 50 Java Interview Questions for Entry-Level Roles

As a recruiter, I screen dozens of entry-level Java developer candidates every week. The candidates who succeed don't just know Java syntax — they understand core concepts and communicate them clearly. Use this guide to prepare for your next Java interview with confidence.

I've organized these 50 questions into key topic areas that hiring managers and technical interviewers actually test. Bookmark this page and revisit it before every interview.

1. Java Basics

1. What is Java and why is it platform-independent?

   Java compiles source code into bytecode that runs on the Java Virtual Machine (JVM). Because the JVM is available on all major OS platforms, Java follows the principle of "write once, run anywhere."

2. What is the difference between JDK, JRE, and JVM?

   JVM executes bytecode. JRE includes the JVM plus standard libraries needed to run Java programs. JDK includes the JRE plus development tools like the compiler (javac) needed to write and build Java applications.

3. What are Java's eight primitive data types?

   byte, short, int, long, float, double, char, and boolean. Know their sizes — interviewers often ask about int (32-bit) vs long (64-bit).

4. What is the difference between == and .equals() in Java?

   == compares object references (memory addresses). .equals() compares the actual content/values. For String comparisons, always use .equals().

5. What is autoboxing and unboxing?

   Autoboxing automatically converts a primitive (e.g., int) to its wrapper class (Integer). Unboxing does the reverse. Java handles this conversion automatically since Java 5.

6. What is the difference between String, StringBuilder, and StringBuffer?

   String is immutable. StringBuilder is mutable and faster but not thread-safe. StringBuffer is mutable and thread-safe but slower. Use StringBuilder for single-threaded string manipulation.

7. What does the static keyword mean in Java?

   Static members belong to the class itself, not to any instance. You access them without creating an object. Static methods cannot access instance variables directly.

8. What is a constructor and how does it differ from a method?

   A constructor initializes an object when it's created. It has the same name as the class and no return type. A method performs an action and has a defined return type.

2. Object-Oriented Programming (OOP)

1. What are the four pillars of OOP?

   Encapsulation, Inheritance, Polymorphism, and Abstraction. Interviewers expect you to define each and give a real example from Java.

2. What is encapsulation?

   Encapsulation bundles data (fields) and methods within a class, hiding internal state using private access modifiers and exposing it via getters and setters.

3. What is inheritance and what keyword does Java use?

   Inheritance lets a child class acquire properties and methods of a parent class using the extends keyword. Java supports single inheritance for classes.

4. What is polymorphism? Give an example.

   Polymorphism means one interface, many implementations. Method overloading (compile-time) and method overriding (runtime) are the two types. Example: a Shape class with different area() implementations for Circle and Rectangle.

5. What is the difference between method overloading and overriding?

   Overloading defines multiple methods with the same name but different parameters in the same class. Overriding redefines a parent class method in a child class with the same signature.

6. What is an abstract class?

   An abstract class cannot be instantiated directly. It may have abstract methods (no body) that subclasses must implement, and concrete methods. Use it when classes share common behavior but need custom implementations.

7. What is an interface and how does it differ from an abstract class?

   An interface defines a contract — all methods are implicitly abstract (before Java 8). A class can implement multiple interfaces but extend only one class. Interfaces support full abstraction; abstract classes allow partial.

8. What is the super keyword used for?

   super refers to the immediate parent class. Use it to call a parent constructor (super()), access parent methods, or access parent fields that a child class has shadowed.

9. Can a constructor be inherited in Java?

   No. Constructors are not inherited. However, a child class can invoke a parent constructor explicitly using super() as the first line of its own constructor.

3. Exception Handling

1. What is the difference between checked and unchecked exceptions?

   Checked exceptions (e.g., IOException) must be declared or handled at compile time. Unchecked exceptions (e.g., NullPointerException) occur at runtime and do not require explicit handling.

2. What is the purpose of try-catch-finally?

   try contains risky code. catch handles specific exceptions. finally always executes — whether an exception occurs or not — making it ideal for closing resources.

3. What is the difference between throw and throws?

   throw explicitly raises an exception within a method. throws declares that a method may propagate specific exceptions to its caller, alerting the compiler and other developers.

4. What is a custom exception and when do you create one?

   A custom exception extends Exception or RuntimeException. You create one when built-in exceptions don't meaningfully describe a domain-specific error, improving code readability and error handling.

5. What is the try-with-resources statement?

   Introduced in Java 7, try-with-resources automatically closes resources (like file streams) that implement AutoCloseable, eliminating the need for explicit finally blocks for cleanup.

4. Collections Framework

1. What is the Java Collections Framework?

   It's a unified architecture of interfaces and classes (List, Set, Map, Queue) for storing and manipulating groups of objects. It provides ready-to-use data structures like ArrayList, HashMap, and LinkedList.

2. What is the difference between ArrayList and LinkedList?

   ArrayList uses a dynamic array — fast for random access (O(1)). LinkedList uses a doubly linked list — faster for frequent insertions/deletions. Choose based on your access pattern.

3. What is the difference between HashMap and Hashtable?

   HashMap is not synchronized and allows one null key. Hashtable is synchronized (thread-safe) and does not allow null keys or values. In modern Java, prefer ConcurrentHashMap over Hashtable.

4. What is the difference between HashSet, LinkedHashSet, and TreeSet?

   HashSet stores unique elements with no order. LinkedHashSet maintains insertion order. TreeSet stores elements in sorted order. All three reject duplicates.

5. What is the difference between Iterator and ListIterator?

   Iterator traverses a collection forward only. ListIterator works only on Lists and supports bidirectional traversal, element replacement, and index retrieval.

6. What is the difference between Comparable and Comparator?

   Comparable defines natural ordering via compareTo() inside the class. Comparator defines custom ordering via compare() externally, allowing multiple sorting strategies without modifying the class.

7. What does Collections.sort() do?

   It sorts a List using natural order (Comparable) or a custom Comparator. It uses a merge sort-based algorithm with O(n log n) time complexity.

5. Multithreading & Concurrency

1. What is a thread in Java?

   A thread is the smallest unit of execution within a process. Java supports multithreading natively, allowing multiple threads to run concurrently, improving CPU utilization and responsiveness.

2. How do you create a thread in Java?

   Two ways: extend the Thread class and override run(), or implement the Runnable interface and pass it to a Thread object. Implementing Runnable is preferred as it allows the class to extend another class.

3. What is the difference between start() and run()?

   start() creates a new thread and invokes run() in that new thread. Calling run() directly executes on the current thread — no new thread is created.

4. What is synchronization in Java?

   Synchronization controls thread access to shared resources, preventing race conditions. The synchronized keyword on a method or block allows only one thread to execute it at a time.

5. What is a deadlock?

   A deadlock occurs when two or more threads each hold a lock and wait indefinitely for the other's lock. Prevent it by acquiring locks in a consistent order or using tryLock() with timeouts.

6. What is the volatile keyword?

   volatile ensures a variable's value is always read from and written to main memory, not a thread's local cache. It guarantees visibility across threads but does not ensure atomicity.

6. Java 8+ Features

1. What are lambda expressions?

   Lambdas provide a concise way to implement functional interfaces (interfaces with one abstract method). Syntax: (parameters) -> expression. They reduce boilerplate in sorting, filtering, and event handling.

2. What is the Stream API?

   The Stream API processes collections functionally using filter(), map(), reduce(), and collect(). Streams are lazy — they compute only when a terminal operation is invoked.

3. What is Optional in Java 8?

   Optional is a container that may or may not hold a non-null value. It eliminates NullPointerExceptions by forcing explicit handling of absent values via isPresent(), orElse(), and ifPresent().

4. What are default methods in interfaces?

   Default methods let you add concrete implementations inside an interface using the default keyword. This adds new functionality to interfaces without breaking existing implementing classes.

5. What is a functional interface?

   A functional interface has exactly one abstract method and is annotated with @FunctionalInterface. Examples: Runnable, Callable, Comparator, Predicate. They are the foundation for lambda expressions.

7. Memory Management & JVM Internals

1. What is garbage collection in Java?

   Garbage collection automatically frees memory occupied by objects no longer referenced by the program. The JVM's GC runs in the background, preventing memory leaks without manual memory management.

2. What is the difference between heap and stack memory?

   Stack stores local variables and method call frames — it's thread-specific and fast. Heap stores objects — it's shared across threads and managed by garbage collection.

3. What causes a StackOverflowError?

   A StackOverflowError occurs when a program's call stack exceeds its limit, typically due to infinite or very deep recursion. The JVM throws it when there's no more stack space available.

4. What is the String pool in Java?

   The String pool is a special memory area in the heap that stores unique String literals. Java reuses existing instances from the pool, saving memory when you create String literals.

8. Miscellaneous Must-Know Topics

1. What is the difference between final, finally, and finalize?

   final is a keyword for constants, preventing method overriding or class inheritance. finally is a block that always runs after try-catch. finalize() is a deprecated method called by GC before destroying an object.

2. What is serialization in Java?

   Serialization converts an object into a byte stream for storage or transmission. A class must implement Serializable. Deserialization reverses the process, reconstructing the object from bytes.

3. What is the difference between deep copy and shallow copy?

   A shallow copy copies the object's reference — both copies share the same nested objects. A deep copy creates entirely new instances of nested objects, making the two completely independent.

4. What is a design pattern? Name three common ones.

   Design patterns are proven solutions to recurring software design problems. Common ones: Singleton (one instance per class), Factory (object creation abstraction), and Observer (event-driven notification).

5. What is the difference between an Array and an ArrayList?

   Arrays have a fixed size and can store primitives. ArrayLists are dynamically resizable, store only objects (with autoboxing for primitives), and provide built-in methods for add, remove, and search.

6. What are generics in Java?

   Generics allow classes, interfaces, and methods to operate on parameterized types (e.g., List<String>). They provide compile-time type safety, eliminate explicit casting, and reduce ClassCastException errors.

Final Recruiter Tips Before Your Interview

Practice out loud. Saying your answers is very different from reading them. Interviewers evaluate clarity of thought, not just correctness.

Write code by hand. Many entry-level interviews include whiteboard coding. Practice writing Java without an IDE.

Know your resume. If you list collections or multithreading on your resume, expect deep follow-up questions on those topics.

Ask questions back. Ask about the team's tech stack, code review culture, or mentorship programs. It signals genuine interest.

Good luck — you've got this.

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External Resources

Oracle Official Java Tutorials

Java 8 API Documentation