Getting your first Java developer job is exciting — but the interview can feel overwhelming if you don't know what to expect. The good news? Most companies ask a very similar set of core Java questions in the first round. Here are the top 20 questions you must prepare for in 2026.
1. What is the difference between JDK, JRE, and JVM?
- JVM (Java Virtual Machine): Runs Java bytecode on any platform — it's what makes Java "write once, run anywhere."
- JRE (Java Runtime Environment): Contains the JVM plus the standard libraries needed to run Java applications.
- JDK (Java Development Kit): Contains everything in the JRE plus compilers and debugging tools needed to write and compile Java programs.
2. What are the 4 pillars of Object-Oriented Programming in Java?
- Encapsulation – Bundling data and methods inside a class and restricting direct access using access modifiers.
- Inheritance – A subclass inheriting properties and methods from a parent class using
extends. - Polymorphism – The ability of one interface to be used for different data types (method overloading and overriding).
- Abstraction – Hiding implementation details and showing only essential features using abstract classes or interfaces.
3. What is the difference between == and .equals() in Java?
==compares references (memory addresses)..equals()compares content/values.
For example, two String objects with the same value are .equals() to each other but may not be == if they are created with new String(...).
4. What is the difference between ArrayList and LinkedList?
| Feature | ArrayList | LinkedList |
|---|---|---|
| Internal structure | Dynamic array | Doubly linked list |
| Access speed | Fast (O(1) for index) | Slow (O(n) for index) |
| Insert/Delete | Slow (shifting needed) | Fast (O(1) at known position) |
| Memory | Less | More (stores node pointers) |
Use ArrayList when you need frequent reads; LinkedList when you need frequent insertions/deletions.
5. What is a constructor? How is it different from a method?
A constructor is a special block of code that runs when an object is created. It has the same name as the class and no return type. Unlike methods, you cannot call a constructor explicitly after object creation.
6. What are access modifiers in Java?
- public – Accessible from anywhere.
- private – Accessible only within the same class.
- protected – Accessible within the same package and subclasses.
- default (no keyword) – Accessible only within the same package.
7. What is the difference between abstract class and interface?
- An abstract class can have both abstract and concrete methods; can have constructors and instance variables.
- An interface can only have abstract methods (before Java 8). From Java 8+, interfaces can have
defaultandstaticmethods. A class can implement multiple interfaces but extend only one abstract class.
8. What is method overloading vs method overriding?
- Overloading: Same method name, different parameters — resolved at compile time (static polymorphism).
- Overriding: Subclass provides a new implementation of a method inherited from the parent — resolved at runtime (dynamic polymorphism).
9. What is the final keyword in Java?
- Applied to a variable: Value cannot be changed (constant).
- Applied to a method: Cannot be overridden.
- Applied to a class: Cannot be subclassed/extended.
10. What is the difference between String, StringBuilder, and StringBuffer?
- String: Immutable. Every modification creates a new object.
- StringBuilder: Mutable. Not thread-safe but faster.
- StringBuffer: Mutable. Thread-safe but slower due to synchronization.
Use StringBuilder for single-threaded string manipulation; StringBuffer for multi-threaded.
11–20: Quick-Fire Round
- What is a NullPointerException? Thrown when you try to use a reference that points to null.
- What is autoboxing? Automatic conversion between primitive types and their wrapper classes (e.g., int → Integer).
- What is a static method? A method that belongs to the class, not instances. Called with ClassName.method().
- What is the
thiskeyword? Refers to the current object inside a class. - What is exception handling? Managing runtime errors using try-catch-finally blocks.
- Checked vs Unchecked exceptions? Checked: must be declared or handled (e.g., IOException). Unchecked: runtime exceptions (e.g., NullPointerException).
- What is a HashMap? A key-value data structure that uses hashing. Does not maintain insertion order.
- What is the difference between HashMap and Hashtable? HashMap is not synchronized (faster); Hashtable is synchronized (thread-safe).
- What is a thread in Java? A lightweight subprocess. Created by extending Thread or implementing Runnable.
- What is garbage collection? Automatic memory management in Java — the JVM automatically reclaims memory from objects no longer in use.
How to Prepare
Practice each concept by writing small programs — don't just memorize definitions. Interviewers often follow up with "can you show me an example?" Use platforms like LeetCode, GeeksForGeeks, and HackerRank to practice. Read "Head First Java" or "Effective Java" for deeper understanding.
Good luck with your interview! 🎉