Learning About the Environment

OCP Exam Topic — Understand the Java development and execution environment: JDK components, compilation, and execution flow. Covered in Chapter 1 of OCP Java SE 21 Developer Study Guide (Selikoff & Boyarsky).

The Java Platform

Java code does not run directly on an operating system. Instead, it runs on the Java Virtual Machine (JVM), a software layer that translates compiled bytecode into native machine instructions. This design is what makes Java portable: write once, run anywhere.

The two main distributions you need to know for the exam are:

As of Java 11, Oracle no longer ships a standalone JRE. The JDK itself is used for both development and deployment.

Compiling with javac

The javac command compiles .java source files into .class bytecode files.

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# Compile a single file
javac HelloWorld.java

# Compile multiple files at once
javac HelloWorld.java Greeter.java

# Specify an output directory for .class files
javac -d out HelloWorld.java

The output .class file contains platform-independent bytecode, not native machine code.

What javac Produces

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HelloWorld.java  →  javac  →  HelloWorld.class

If the source file defines a public class, the filename must exactly match the class name (including capitalisation). A file named HelloWorld.java must declare public class HelloWorld.

Running with java

The java command launches the JVM and executes a compiled class:

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# Run a compiled class (no .class extension!)
java HelloWorld

# Pass arguments to the program
java HelloWorld Alice 42

# Run with a classpath
java -cp out HelloWorld

The argument to java is the class name, not the filename. Do not include .class.

The Classpath

The classpath tells the JVM where to look for .class files. It can be a directory, a JAR file, or a list of both.

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# Single directory
java -cp out HelloWorld

# Multiple entries (colon on Unix/macOS, semicolon on Windows)
java -cp out:lib/utils.jar HelloWorld

# Using the CLASSPATH environment variable (not recommended for production)
export CLASSPATH=out:lib/utils.jar
java HelloWorld

Common classpath flags

Source Launcher (Java 11+)

Since Java 11, you can run a single-file program directly without an explicit compilation step:

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# No prior javac needed
java HelloWorld.java

The JVM compiles the file in memory and executes it immediately. This mode is useful for scripts and learning but is not used in multi-file projects. The exam expects you to know both workflows.

Java Versions and LTS

Java follows a six-month release cadence. Every three years a Long-Term Support (LTS) release is designated; Java 21 is an LTS release. The OCP exam tracks LTS versions, which is why this course targets Java 21.

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Java 17 (LTS)  →  18  →  19  →  20  →  Java 21 (LTS)  →  22  →  ...

Key Points to Remember

• The JDK contains both the compiler (javac) and the runtime (java); the JRE contains only the runtime.
• Source files end in .java; compiled bytecode files end in .class.
• The java command takes a class name, not a file path.
• The classpath (-cp) controls where the JVM looks for compiled classes.
• Java 11+ supports running a single .java file directly with java FileName.java.
• null is not a keyword you need here, but understanding that classes live on the heap matters from the very first lesson.