In Java, what is the difference between PATH and CLASSPATH environment settings and what does each one control?

Introduction / Context:
Understanding the difference between PATH and CLASSPATH is fundamental for any Java developer, especially when working outside an Integrated Development Environment. Interviewers often ask this question to check whether you can run and configure Java applications from the command line and troubleshoot issues related to missing classes or tools.

Given Data / Assumptions:

• Java is installed on the system.
• There are Java tools such as javac and java that must be executed.
• There are compiled classes and JAR files that must be found at runtime.
• Environment variables can be configured at the operating system level.

Concept / Approach:
PATH is an operating system level environment variable. It tells the shell or command prompt where to look for executable programs, such as javac.exe or java.exe on Windows, or javac and java on Linux. CLASSPATH is specific to Java. It tells the Java compiler and the JVM where to look for user defined classes, packages, and JAR files when compiling or running a program. Confusing these two variables is a common source of errors.

Step-by-Step Solution:
1. PATH is configured so that when you type javac or java at the command line, the operating system can locate these executables without requiring a full path. 2. If PATH does not include the Java bin directory, you must type the full path to the executable each time or you receive a "command not found" type error. 3. CLASSPATH is used by the Java compiler and JVM to locate .class files and JARs that are not in the default locations. 4. If CLASSPATH does not include a directory or JAR that contains a required class, you may get a ClassNotFoundException or NoClassDefFoundError when running the program. 5. You can also override or set CLASSPATH at runtime using the -cp or -classpath command line options, which is often preferred over relying on a global environment variable.

Verification / Alternative check:
You can verify the role of PATH by temporarily removing the Java bin directory from PATH and observing that javac is no longer recognized. To verify CLASSPATH, you can place a class in a non default directory and run java with and without specifying that directory in the classpath. The program will fail to find the class when it is not in CLASSPATH and will run correctly when it is included.

Why Other Options Are Wrong:
Option B reverses the responsibilities of PATH and CLASSPATH and is therefore incorrect. Option C is wrong because PATH and CLASSPATH serve different purposes and cannot be used interchangeably. Option D is incorrect because both PATH and CLASSPATH exist and are used on multiple operating systems; they are not tied to a specific platform in the way described.

Common Pitfalls:
A frequent problem is setting CLASSPATH as a global environment variable and forgetting its contents, which can cause conflicts between projects. Another pitfall is confusing CLASSPATH with the module system or with the path to the Java installation itself. Modern build tools such as Maven and Gradle manage classpaths automatically, but understanding these concepts is still vital for debugging and working with legacy systems.

Final Answer:
PATH tells the operating system where to find Java tools like javac and java, while CLASSPATH tells the Java runtime where to find classes and JAR files.