Difference Between JDK, JRE, and JVM in Java
Scenario 1: Can not Compile Your First Java Program?
You are a new Java learner. You downloads and installs only the JRE on your laptop. You then tries to compile a Java program using the javac command but gets an error saying 'javac' is not recognized as an internal or external command.
Interview Question
Why did you encounter this error, and how can you resolve it?
Answer:
I encountered this error because the JRE (Java Runtime Environment) does not include development tools like the Java compiler (javac). The JRE is designed solely for running Java applications, not for compiling them. To resolve this, i should install the JDK (Java Development Kit), which provides both the compiler and other development utilities required for Java programming.
Tip: Interviewers love when you show awareness of tools. Mentioning the difference between JDK and JRE here shows you're hands-on.
Scenario 2: Running a Java Application on Different Platforms
You compiles your Java application on Windows and shares the .class files with your friend , who uses a Mac. Your friend runs the program using the java command, and it works perfectly.
Interview Question
How is it possible for the same compiled Java code to run on both Windows and Mac without modification ?
Answer:
Answer:
This cross-platform compatibility is possible because Java source code is compiled into platform-independent bytecode (.class files). The JVM (Java Virtual Machine) on each operating system interprets or compiles this bytecode into native machine code specific to that OS. As long as a compatible JVM is installed, the same Java bytecode can run on any platform, embodying Java’s “write once, run anywhere” principle.
Tip: Use this phrase in your answer it is a Java classic and makes you sound interview-ready.
Scenario 3: Memory Management in Java
During performance testing, you notice your Java app is using a lot of memory
than expected but you never wrote any code to free memory. Should not that cause problems?
than expected but you never wrote any code to free memory. Should not that cause problems?
Interview Question
Who manages memory in a Java application, and how is unused memory reclaimed?
Answer:
Memory management in Java is handled by the JVM. The JVM includes an automatic garbage collector that tracks and removes objects that are no longer referenced by the application. This process helps prevent memory leaks and ensures efficient use of system resources, so developers do not need to manually free memory as they would in languages like C or C++.
Scenario 4: Choosing Between JDK and JRE
Tip: If asked to explain Java's memory model, mention heap, stack, and GC basics. Interviewers love that.
Scenario 4: Choosing Between JDK and JRE
A company wants to deploy a Java-based desktop application to its employees. They are unsure whether to install the JDK or the JRE on each user’s machine.
Interview Question
Which should they choose, and why?
Answer:
For end-users who only need to run Java applications, installing the JRE is sufficient. The JRE contains the JVM and necessary libraries to execute Java programs. The JDK is only required on machines where Java development, compilation, or debugging will occur. For most employees, the JRE will keep installations lighter and more secure.
Tip: Emphasize “use case” — interviewers want to see that you choose tools based on real-world needs.
Scenario 5: JVM Version Compatibility
An application was compiled using JDK 17, but the server where it is deployed has only JVM 8 installed. The application fails to start.
Interview Question
Why did this happen, and how can you prevent it?
Answer:
Java bytecode compiled with a newer JDK (e.g., JDK 17) may use features not supported by older JVM versions (e.g., JVM 8). This leads to compatibility issues and runtime errors.
Tip: If you know the
--release flag, mention it. It's a newer best practice interviewers appreciate.