I can execute Main.java file but not Main.class file-CodePudding

i am learning java and i have some difficulties with the package mechanism. I have different classes in a package.I compiled it right but when i execute the Main file i have a different behavior.Music3 is the file where is the main method.

andrea@andrea:~/Documenti/java$ java -Xdiag  -cp class/ source/Music3
Errore: impossibile trovare o caricare la classe principale source.Music3
Causato da: java.lang.ClassNotFoundException: source.Music3
java.lang.ClassNotFoundException: source.Music3
    at java.base/jdk.internal.loader.BuiltinClassLoader.loadClass(BuiltinClassLoader.java:581)
    at java.base/jdk.internal.loader.ClassLoaders$AppClassLoader.loadClass(ClassLoaders.java:178)
    at java.base/java.lang.ClassLoader.loadClass(ClassLoader.java:522)
    at java.base/java.lang.Class.forName0(Native Method)
    at java.base/java.lang.Class.forName(Class.java:398)
    at java.base/sun.launcher.LauncherHelper.loadMainClass(LauncherHelper.java:791)
    at java.base/sun.launcher.LauncherHelper.checkAndLoadMain(LauncherHelper.java:686)
andrea@andrea:~/Documenti/java$ java -Xdiag  -cp class/ source/Music3.java
Wind.play() C_Sharp
Percussion.play() C_Sharp
Stringed.play() C_Sharp

` As you can see i have the right output when i execute Music3.java. What is wrong?

CodePudding user response：

It looks like you are trying to execute a .class file by running java -Xdiag -cp class/ source/Music3.class, but you are getting a ClassNotFoundException. This error indicates that the java command is unable to find the class you are trying to run.

There are a few possible reasons for this error. One reason might be that you are not specifying the correct classpath when running the java command. The classpath tells the java command where to look for class files that it needs to load. In your case, you are using -cp class/, which means that the java command will only look for class files in the class directory. If the Music3.class file is not in the class directory, then the java command will not be able to find it.

Another possible reason for the error is that the Music3.class file is not in the correct package. In Java, classes are organized into packages, and each class belongs to a single package. When you compile a Java program, the compiler creates a .class file for each class in the program. The .class file for a class is placed in a directory that corresponds to the package that the class belongs to. For example, if the Music3 class belongs to the source package, then the Music3.class file should be in the source directory.

To fix the error, you will need to make sure that the Music3.class file is in the correct directory, and that you are specifying the correct classpath when running the java command. For example, if the Music3.class file is in the class/source directory, then you would need to run the java command like this:

java -Xdiag -cp class/source Music3

This command tells the java command to look for class files in the class/source directory, which is where the Music3.class file should be located.

CodePudding user response：

The java command takes a class name, not a file name: It's java -Xdiag -cp class source.Music3.

Note that 'source' is an extremely unfortunate package name, pick something else. Say it's 'sposito', then your source file should be in project/src/sposito/Music3.java, have package sposito; compile it someplace so that the class file ends up in project/class/sposito/Music3.class, run it from the project dir with java -cp class sposito.Music3.