Could not find or load main class mainClass ใน java

import javax.swing.JFrame;
public class mainClass {

    public static void main(String[] args) {
        frame1 f = new frame1();
        f.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);;
        f.setSize(200,100);
        f.setVisible(true);

    }

}

แล้วพอเรารันมันก้ขึ้นว่า
save could not be completed. try File > Save As...if the problem persists

reason

some characters cannot be mapped using "MS874" character encoding.
Either change the encoding or remove the characters which are not supported by the "MS874" character encoding
เป็นเพราะอะไรคะ