How to Run a Compiled Java .class File from the Command Line

How can I execute a Java .class file from the command line without an IDE?

java Echo "hello"

Executing a Java .class file from the command line can be straightforward. However, certain conditions must be met to avoid common errors like 'NoClassDefFoundError.' This guide will walk you through the process and common pitfalls.

javac Echo.java
java Echo "hello"

Causes

• The class file is not in the specified directory.
• There are issues with the Java classpath settings.
• The class file was not compiled correctly.

Solutions

• Make sure you are in the same directory as the Echo.class file by using the `cd` command.
• Use `java -cp . Echo "hello"` to specify the current directory as the classpath.
• Ensure your Java class file is compiled properly before running it.

Mistake: Not in the correct directory where Echo.class is located.

Solution: Use the `cd` command to navigate to the directory that contains your Echo.class file.

Mistake: Missing the classpath when running the class.

Solution: Include `-cp .` to indicate that the current directory should be searched for class files.

Mistake: Forgetting to compile the .java file before execution.

Solution: Ensure you compile the Java file using `javac Echo.java` before trying to run it.