Interview Questions of jdk-vs-jre-vs-jvm

Scenario 1: Setting Up a Java Development Environment

Scenario:

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.

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.

Scenario 2: Running a Java Application on Different Platforms

Scenario:

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.

Question:

How is it possible for the same compiled Java code to run on both Windows and Mac without modification?

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.

Scenario 3: Memory Management in Java

Scenario:

During performance testing, a you notices that your Java application is using more memory than expected, but they never explicitly free up memory in their code.

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

Scenario:

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.

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.

Scenario 5: JVM Version Compatibility

Scenario:

An application was compiled using JDK 17, but the server where it’s deployed has only JVM 8 installed. The application fails to start.

Question:

Why did this happen, and how can it be prevented?

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. To prevent this, always ensure that the target environment’s JVM version is equal to or newer than the JDK version used for compilation, or use the -target and -source flags during compilation to generate compatible bytecode.