In Java, what is the difference between PATH and CLASSPATH environment variables?

Introduction / Context:
When setting up a Java development environment, developers often configure both PATH and CLASSPATH. Although the names look similar, they serve different purposes. Confusing them can lead to common errors such as commands not found or classes not found. This question focuses on distinguishing the roles of PATH and CLASSPATH in relation to the Java Development Kit and the Java Virtual Machine.

Given Data / Assumptions:
- We are working on a typical operating system such as Windows, Linux or macOS.
- Java commands like java and javac are provided by the JDK or JRE installation.
- Our applications and libraries are packaged as .class files or JAR files.
- The environment variables PATH and CLASSPATH are potentially set in the system or user configuration.

Concept / Approach:
PATH is an operating system level environment variable. It tells the shell or command prompt where to look for executable programs when a user types a command. Adding the JDK bin directory to PATH allows you to run java, javac and other tools from any directory. CLASSPATH is used by the Java runtime to locate classes and JARs that applications depend on. If required classes are not found on the CLASSPATH or specified with -cp at runtime, the program may fail with class loading errors. Recognizing this distinction between OS level command lookup and Java level class lookup is the key to answering the question.

Step-by-Step Solution:
Step 1: Recall that PATH is consulted by the operating system when resolving a command like java typed into a terminal. Step 2: Remember that CLASSPATH is consulted by the Java Virtual Machine when it needs to load application or library classes. Step 3: Understand that missing PATH entries lead to errors like command not found, while missing CLASSPATH entries lead to ClassNotFoundException. Step 4: Observe that PATH has nothing to do with heap size or threads, and CLASSPATH does not help the OS find shell commands. Step 5: Choose the option that clearly states that PATH is for executables and CLASSPATH is for classes and JARs.

Verification / Alternative check:
Imagine you have installed the JDK but did not add its bin directory to PATH. Typing javac at the command line results in an error because the operating system does not know where to find the executable. Once PATH is set correctly, the command is recognized. Separately, if you run a Java program that depends on an external library that is not on CLASSPATH, the program starts but then fails when it tries to load a class from that library. These two distinct failure modes confirm the different roles described in option A.

Why Other Options Are Wrong:
Option B reverses the roles, claiming PATH is used only inside Java programs and CLASSPATH only by the operating system, which is incorrect.
Option C says PATH and CLASSPATH are identical and interchangeable, which ignores their very different responsibilities.
Option D introduces heap size and thread count, which are controlled by JVM options, not by PATH or CLASSPATH environment variables.

Common Pitfalls:
Developers new to Java often incorrectly set CLASSPATH globally, which can create conflicts between projects. A better practice is to use explicit -cp or -classpath options for each application. Another pitfall is assuming that once PATH is set, all class loading problems will disappear, which is not true because CLASSPATH is a separate concern. Understanding the clear separation between command resolution and class resolution helps debug environment issues quickly.

Final Answer:
The correct explanation is PATH tells the operating system where to find Java executables like java and javac, while CLASSPATH tells the Java runtime where to find classes and JAR libraries.