javac -d out: compile multiple Java files with packages and run them

To compile a Java program made of several files and packages, pass all source files to javac with -d out: javac writes each class into out/ in folders that match its package (out/com/example/App.class). Run it with java -cp out com.example.App, using the fully qualified class name and the output folder as the class path.

Compile and run a two-package program

mkdir -p src/com/example/app src/com/example/util
cat > src/com/example/util/Greeter.java <<'EOF'
package com.example.util;

public class Greeter {
    public static String greet(String name) { return "Hello, " + name; }
}
EOF
cat > src/com/example/app/App.java <<'EOF'
package com.example.app;

import com.example.util.Greeter;

public class App {
    public static void main(String[] args) {
        System.out.println(Greeter.greet(args.length > 0 ? args[0] : "javac"));
    }
}
EOF
find src -name '*.java' > sources.txt
javac -d out @sources.txt
find out -name '*.class' | sort
java -cp out com.example.app.App
java -cp out com.example.app.App packages

out/com/example/app/App.class
out/com/example/util/Greeter.class
Hello, javac
Hello, packages

@sources.txt is an argument file: javac reads the file list from it, which avoids "argument list too long" on big projects and works the same on every shell.

Options

Pitfalls

• Running with a path instead of a class name. java out/com/example/app/App or java App fails with Could not find or load main class. Pass the class path with -cp out and the name with dots: com.example.app.App.
• Forgetting a file. If javac only receives App.java and cannot find Greeter on the class path or source path, it reports cannot find symbol or package com.example.util does not exist.
• Stale classes. javac does not delete old .class files; a class you removed from the sources still exists in out/. Delete out/ before a clean build.
• Windows globs. src/**/*.java is not expanded by cmd.exe; use an argument file or dir /s /b src\*.java > sources.txt.
• Single-file shortcut. For a program in one file, java App.java compiles in memory and runs it without javac: see running a source file.

Confirm it worked

out/ contains one .class file per class in package folders, and java -cp out PACKAGE.CLASS runs main. For anything larger than a few files, a build tool (Maven or Gradle) does the same with dependency management.

FAQ