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Table of contents
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1. How is Java Platform Independent? |
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2. What Are the Key Features of Java? |
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3. What is JVM (Java Virtual Machine)? |
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4. What is JIT Compiler in Java? |
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5. What is a ClassLoader in Java? |
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6. What is a JVM, JRE and JDK in Java? |
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7. Key Differences Between Java and C++ |
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8. What Happens If We Don’t Declare main() as static? |
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9. What are Packages in Java? |
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10. When a byte datatype is used? |
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11. Conclusion |
Java remains the backbone of modern software development, powering everything from enterprise systems to Mobile apps, such as those for Android. Its "write once, run anywhere" philosophy and robust ecosystem make it a staple in tech giants like Amazon, Netflix, and Uber. For freshers, mastering core Java interview questions is the first step toward landing a role in these top-tier companies.
This guide curates 25 essential Java interview questions and answers, focused on real-world scenarios that freshers face during technical rounds. We’ll break down complex topics like OOPs, Java Collections Interview Questions, and exception handling into digestible explanations.
Whether you’re preparing for your first interview or looking to strengthen fundamentals through Java Practice Programs, this list ensures you’re ready to impress. You can pair these questions with hands-on coding from our Java course in Pune to seamlessly bridge theory and practical application.
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How is Java Platform Independent?
Short Answer:
Yes, Java is platform-independent because it compiles code into bytecode (.class files) that runs on any system with a Java Virtual Machine (JVM).
Detailed Explanation:
Java code is compiled into bytecode, not machine code, by the javac compiler. This bytecode is universal—it doesn’t depend on OS or hardware. The JVM (platform-specific) interprets bytecode into machine-native instructions. While the JVM itself is platform-dependent, the same bytecode works everywhere. For example, you can compile a .java file on Windows and run the resulting .class file on Linux or macOS without needing to recompile
Pro Tip: Mention how this contrasts with languages like C++ (which are platform-dependent) to showcase its depth.
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What Are the Key Features of Java?
Java dominates modern software development due to its powerful, developer-friendly features. Here's why companies from startups to tech giants rely on it:
Core Features Explained
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Platform Independence: "Write once, run anywhere": Java compiles to bytecode, executable on any system with a JVM. Example: A .class file created on Windows runs seamlessly on Linux.
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Object-Oriented: Built on OOP principles (encapsulation, inheritance, polymorphism, abstraction). Real-world use: Modular code for scalable enterprise applications.
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Robust & Secure: Automatic garbage collection prevents memory leaks. Bytecode verification and sandboxing enhance security.
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High Performance: Just-In-Time (JIT) compilation optimizes bytecode to near-native speed.
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Multithreaded: Concurrent thread execution improves performance in applications like servers.
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Distributed: APIs, such as RMI and networking libraries, enable distributed computing.
Interviewers ask this to test your understanding of Java’s competitive edge over languages like Python and C++. They expect examples (e.g., how platform independence simplifies deployment).
Pro Tip: Link features to real-world scenarios: "Platform independence reduces DevOps overhead in cross-environment deployments."
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What is JVM (Java Virtual Machine)?
JVM is the engine that executes Java bytecode, making "Write Once, Run Anywhere" possible. It’s platform-dependent but enables Java’s platform independence.
JVM does the following things:
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Loads bytecode (.class files)
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Verifies code for security (sandboxing)
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Executes via interpretation or JIT compilation
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Manages memory (garbage collection)
JVM itself is OS-specific (e.g., Windows JVM ≠ Mac JVM). Also, Bytecode is universal – it works on any JVM. JVM enables Java’s core promise: compile once, run anywhere, and provides a runtime environment for features like multithreading.
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What is JIT Compiler in Java?
JIT (Just-In-Time) compiler is a performance booster in JVM that converts bytecode to native machine code at runtime, making Java applications faster.
How JIT Works - Step by Step:
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Java source code (.java) → javac → Bytecode (.class)
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JVM loads bytecode
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JIT identifies "hot spots" (frequently executed code)
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Compiles these hot spots to native machine code
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Subsequent calls execute native code directly
Key advantages of JIT complier are the performance as it avoids re-interpreting the same bytecode repeatedly, adaptive optimization as it focuses on critical code paths, seed as it can match native C/C++ performance for hot code.
Pro Tip: Mention that JIT is why Java can outperform purely interpreted languages in long-running applications.
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What is a ClassLoader in Java?
A ClassLoader is a core component of the JRE that dynamically loads Java classes and interfaces into the JVM at runtime. It abstracts away file system dependencies, enabling Java's "write once, run anywhere" capability.
The class loader loads .class files only when they are first referenced during execution, which enables features such as on-demand loading and runtime extensibility. Also, the key responsibilities of this are: Loading → Linking, → Initialization.
It maintains namespaces to prevent class conflicts, and also implements security by separating system vs. application classes and classes load only when needed → reduces startup memory footprint.
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What is a JVM, JRE and JDK in Java?
JVM (Java Virtual Machine)
This is the execution engine that runs Java bytecode. It is platform-dependent (needs OS-specific implementation) and also provides memory management, garbage collection, and security
JRE (Java Runtime Environment)
In a JRE, the, the minimum environment required to run Java applications. It contains JVM + core libraries (but no development tools). End-users only need the JRE to run Java programs
JDK (Java Development Kit)
Full-featured SDK for Java development, includes JRE + compiler (javac), debugger, docs, and development tools
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Component |
Purpose |
Contains |
Needed For |
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JVM |
Executes bytecode |
Bytecode interpreter, memory manager |
Running Java programs |
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JRE |
Provides runtime environment |
JVM + Core Libraries (java.lang, java.util, etc.) |
Executing pre-compiled Java applications |
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JDK |
Development toolkit |
JRE + Compiler (javac), Debugger, Docs |
Developing and compiling Java applications |
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Key Differences Between Java and C++
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Basis |
C++ |
Java |
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Platform |
Platform dependent (compiles to machine code) |
Platform independent (runs on JVM) |
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Primary Use |
System programming, game development, embedded systems |
Enterprise applications, Android apps, web services |
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Hardware Access |
Direct hardware interaction possible |
Limited hardware access (runs in JVM sandbox) |
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Memory Management |
Manual (new/delete) |
Automatic (Garbage Collection) |
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Global Variables |
Supports global scope |
No global variables (everything belongs to classes) |
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Pointers |
Supports pointers |
No pointer support (uses references) |
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Multiple Inheritance |
Supported through classes |
Not supported (only through interfaces) |
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Runtime Features |
No built-in thread/dynamic loading support |
Built-in thread support, dynamic class loading |
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Root Hierarchy |
No single root hierarchy |
Single root hierarchy (java.lang.Object) |
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Exception Handling |
No mandatory exception handling |
Enforced exception handling (try-catch) |
Interview Tip: Be prepared to explain how these differences impact real-world development choices (e.g., why Android uses Java instead of C++ for most apps)
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What Happens If We Don’t Declare main() as static?
If you declare main() without static, the code will compile, but the JVM cannot execute it as the program’s entry point. You’ll get a runtime error:
Error: Main method is not static in class Test, please define the main method as: public static void main(String[] args)
Why static is Mandatory for main()
The JVM needs to call main() without creating an instance of the class. Static allows this by making main() class-level (not object-level). At startup, JVM looks for public static void main(String[]) before any objects exist. Non-static main() would require an object, creating a chicken-and-egg problem.
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What are Packages in Java?
Packages in Java are containers that organize related classes/interfaces into namespaces, preventing naming conflicts and enabling access control (like package-private visibility). They mirror directory structures (e.g., com.example.util) and are essential for modular, maintainable code. Example: java.util groups collections and utilities.
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When a byte datatype is used?
Use byte in Java when working with small integers (-128 to 127) to save memory, especially in large arrays or low-level file/network I/O handling raw binary data.
Conclusion:
Mastering Java Interview Questions and Answers—from core concepts to Java Collections interview questions—gives you a competitive edge in technical interviews. Regular Java Practice Programs strengthen problem-solving skills, while tackling Java coding questions sharpens your logical thinking. For structured learning, consider a Java Course in Pune to gain hands-on experience and industry-relevant training. Keep coding, keep learning, and ace those interviews!
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