Current Directory: Desktop
Create New Directory: mylib
Current Directory: mylib
Create New File: MyString.java
package myjava;
public class MyString {
    private String str; // default value is null
    
    public MyString () {
    }
    
    public MyString (String str) {
        this.str = str;
    }
    
    public String toString () {
        return str;
    }
    
    public int indexOf (String find) {
        return str.indexOf (find);
    }
}
Command Prompt
cd Desktop\myjava
javac MyString.java
cd ..
jar cvf mylib.jar myjava/*.class
Now, current dir is Desktop
Create New File: MyApp.java
import myjava.MyString;

public class MyApp {
    public static void main (String args[]) {
        MyString ms = new MyString (“i am a string”);
        System.out.print (ms.toString ());
    }
}
set CLASSPATH=mylib.jar;
javac MyApp.java
java MyApp