IntelliJ: Why can't I use a class from external jar

Question

When I created the jar file I wrote the following java file:

package myjar;

public class MyClass {

    public static void hello() {
        System.out.println("hello");
    }

    public static void main(String[] args) {
        MyClass.hello();
    }
}

1. I named the file MyClass.java.
2. I created a class file by typing "javac src/main/java/myjar/MyClass.java"
3. I generated a jar file by the command: "jar cvf myjar.jar src/main/java/myjar/MyClass.class"

Now, I took the jar file and added it to Project Structure. The name of the jar is 'myjar':

and in the IDE I wrote "import myjar.MyClass" and the word 'myjar' was marked in red.

When I use 'MyClass.hello()' in the project, I get an error:

no main manifest

Do you know what I can to to load it successfully ?

Answer 1

You can only import some classes from your libraries on classpath into a Java source file, not a contents of a whole Java archive (JAR) file. So if you have inside myjar.jar file class Foo.class inside package (folder) bar, you can do

import bar.Foo;

Answer 2

After adding the .jar as a library in IntelliJ, you still need to add it to the module.

Then the library will appear in the module's list of dependencies.