How to compile and run Java programs manually from the command line.

Without Package Declaration

If the Java file does not declare a package.

For example, Test.java in directory MyProject:

public class Test {
  public static void main(String[] args) {
    System.out.println("Hello World");
  }
}

Compiling

From MyProject directory

javac Test.java

Creates Test.class in MyProject.

From another directory

javac path/MyProject/Test.java

Creates Test.class in MyProject (same directory as Java file, not current working directory).

Running

From MyProject directory

java Test

From another directory

Necessary to set classpath.

java -cp path/MyProject Test

With Package Declaration

If the Java file does declare a package.

For example, Test.java in directory MyProject:

package com.example.test;

public class Test {
  public static void main(String[] args) {
    System.out.println("Hello World");
  }
}

Compiling

Must use the javac -d option to set the BASE directory of the directory structure for the class files.

Note: class files using a package declaration must be located in a directory structure reflecting their package name. Otherwise, these class files cannot be executed. Source files (Java files), on the other hand, can be located in any directory.

javac -d BASE Test.java

BASE must be an existing directory.

Creates the directory structure BASE/com/example/test/ containing Test.class.

Running

If the Java file declares a package name, then the class must be referenced with the fully qualified class name, that is, including the package name.

Without stating the package name on the command line, the class file cannot be executed in any case.

From the BASE directory

java com.example.test.Test

From another directory

Necessary to set the classpath to the BASE directory.

java -cp path/BASE com.example.test.Test