Error in here , can't find it by myself .

If someone will find it ,please let me know.
package flow.layout;
import java.awt.*;
import javax.swing.*;
public class Flow {
JPanel windowContent;
JLabel label1;
JLabel label2;
JLabel label3;
Flow () {
windowContent = new JPanel();
FlowLayout fl = new FlowLayout();
windowContent.setLayout(fl);
label1 = new JLabel(" test 1");
label2 = new JLabel (" test 2");
label3 = new JLabel (" test 3 ");
windowContent.add(label1);
windowContent.add(label2);
windowContent.add(label3);
JFrame frame = new JFrame("My test" );
frame.setContentPane(windowContent);
frame.pack();
frame.setVisible(true);
}
public static void main(String[] args){
new Flow() ;
}
}