Error: Could not find or load main class

Error: Could not find or load main class X means the JVM searched the class path for a class named X and found no matching .class file. Almost always the name or the class path is wrong: the class sits in a package and must be run by its fully qualified name (java com.example.Hello) from the root of the package tree, or with -cp pointing at that root. The Caused by: line says why: ClassNotFoundException when no file matches, NoClassDefFoundError ... (wrong name: ...) when a file was found but declares another fully qualified name.

Reproduce it

mkdir -p src/com/example
cat > src/com/example/Hello.java <<'EOF'
package com.example;
public class Hello {
    public static void main(String[] args) { System.out.println("hello"); }
}
EOF
javac -d out src/com/example/Hello.java
java -cp out Hello

Error: Could not find or load main class Hello
Caused by: java.lang.ClassNotFoundException: Hello

The class was compiled to out/com/example/Hello.class. Its name is com.example.Hello, so Hello alone matches nothing on the class path out.

Fix: fully qualified name and the right class path

mkdir -p src/com/example
cat > src/com/example/Hello.java <<'EOF'
package com.example;
public class Hello {
    public static void main(String[] args) { System.out.println("hello"); }
}
EOF
javac -d out src/com/example/Hello.java
java -cp out com.example.Hello

hello

Wrong-name variant

Running from inside the package folder finds the file but not the right class:

mkdir -p src/com/example
cat > src/com/example/Hello.java <<'EOF'
package com.example;
public class Hello {
    public static void main(String[] args) { System.out.println("hello"); }
}
EOF
javac -d out src/com/example/Hello.java
cd out/com/example
java Hello

Error: Could not find or load main class Hello
Caused by: java.lang.NoClassDefFoundError: Hello (wrong name: com/example/Hello)

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