If you are a student to become a Software Engineer in 2026, you already know the importance of Java. Despite the rise of Python and JavaScript, Java remains the backbone of enterprise software. Companies like Amazon, Google, and TCS still rely heavily on it.
Cracking a Java interview isn't just about memorizing definitions; it's about understanding the "Why" and "How". In this guide, I have compiled the top Java interview questions that are being asked right now, categorized for easy revision.
Part 1: The Core Fundamentals (The Basics)
1. What is the difference between JDK, JRE, and JVM?
This is the most common opener. Don't get confused!
- JVM (Java Virtual Machine): It is an abstract machine that executes Java bytecode. It provides the runtime environment.
- JRE (Java Runtime Environment): It contains the JVM + Libraries (jar files) required to run Java applications.
- JDK (Java Development Kit): It contains the JRE + Development tools (javac compiler, debugger).
Formula: JDK = JRE + Development Tools
2. Why is Java not considered 100% Object-Oriented?
Java is not fully object-oriented because it supports Primitive Data Types (like int, char, boolean, float) which are not objects. A purely object-oriented language treats everything as an object (like Smalltalk).
3. Explain "public static void main(String args[])" in simple terms.
- public: Access modifier so the JVM can execute the method from anywhere.
- static: Allows the JVM to call this method without creating an object of the class.
- void: The main method does not return any value.
- String args[]: Used for command-line arguments.
Part 2: OOPS Concepts (The Real Test)
4. What is the difference between Overloading and Overriding?
This question tests your understanding of Polymorphism.
- Method Overloading (Compile-time Polymorphism): Same method name, but different parameters (number or type) within the same class.
- Method Overriding (Runtime Polymorphism): Same method name and same parameters, but in a Parent-Child (Inheritance) relationship.
static methods can be overloaded but NOT overridden.
5. Can we execute a program without a main() method?
Prior to JDK 7, it was possible using a static block. However, in JDK 1.7 and later versions (including Java 21 in 2026), it is impossible to execute a Java class without the main method. The JVM checks for the main method before initializing the class.
Part 3: String Handling & Collections
6. String vs. StringBuilder vs. StringBuffer
| Feature | String | StringBuffer | StringBuilder |
|---|---|---|---|
| Storage | String Constant Pool | Heap | Heap |
| Mutability | Immutable (Cannot change) | Mutable | Mutable |
| Thread Safe? | Yes | Yes (Synchronized) | No (Faster) |
7. How does HashMap work internally?
This is a favorite question for experienced candidates. HashMap works on the principle of Hashing.
- It uses the
hashCode()method to calculate the index (bucket). - It uses the
equals()method to check if the key already exists. - Collision Handling: Before Java 8, it used a LinkedList to store multiple nodes in the same bucket. Since Java 8, if the list grows beyond a threshold (8 nodes), it converts the LinkedList into a Balanced Tree (Red-Black Tree) for faster performance (O(log n)).
Part 4: Advanced & Java 8 Features
8. What are the key features introduced in Java 8?
Even in 2026, Java 8 features are crucial because they changed how we code.
- Lambda Expressions: For functional programming.
- Stream API: For processing collections of objects (filter, map, reduce).
- Default Methods: Methods with a body inside interfaces.
- Optional Class: To handle NullPointerException gracefully.
9. What is a "Marker Interface"?
A Marker Interface is an interface that has no methods or constants. It provides run-time type information to the JVM.
Examples: Serializable, Cloneable, Remote.
Conclusion
Preparing for a Java interview can feel overwhelming, but focusing on these core concepts will give you a strong foundation. Remember, the interviewer is looking for clarity in concepts rather than just syntax.
Are you preparing for an interview soon? Let me know in the comments if you want a Part 2 covering Multithreading and System Design!
Good luck, future Engineers!