unknown error - really need help

i'm working on a gui that's part of a system for a project. have been compiling again and again to see how things are taking shape....added some code, took it away using ctrl+z a few times, now when i try to compile it i'm getting the following error below. i don't think i've undone anything but i can't figure out what's up. would really appreciate some help. error and code below:

Code:

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Exception in thread "main" java.lang.IllegalArgumentException: horizontalAlignment
        at javax.swing.JLabel.checkHorizontalKey(Unknown Source)
        at javax.swing.JLabel.setHorizontalAlignment(Unknown Source)
        at javax.swing.JLabel.<init>(Unknown Source)
        at javax.swing.JLabel.<init>(Unknown Source)
        at employeeSide.EnterDates.<init>(EnterDates.java:103)
        at employeeSide.EnterDates.main(EnterDates.java:164)

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Code:

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package employeeSide;

import java.awt.*;
import java.awt.event.ActionListener;
import javax.swing.*;
import javax.swing.GroupLayout.Alignment;

public class EnterDates extends JFrame //implements ActionListener
{
        //declare and initialise variables for the width and height of the window and buttons
        private static final int FRAME_WIDTH = 600;
        private static final int FRAME_HEIGHT = 400;
        private static final int BUTTON_WIDTH = 100;
        private static final int BUTTON_HEIGHT = 40;
       
        //declare submit and cancel buttons
        private JButton submitButton;
        private JButton cancelButton;
       
        //declare title, end date and start date labels
        private JLabel startDateLabel;
        private JLabel endDateLabel;
        private JLabel title;
       
        private JComboBox dayBoxStart;
        private JComboBox monthBoxStart;
        private JComboBox yearBoxStart;
        private JComboBox dayBoxEnd;
        private JComboBox monthBoxEnd;
        private JComboBox yearBoxEnd;
       
        private String[] dayListStart = {"1","2","3","4","5","6","7","8","9","10",
                                                                        "11","12","13","14","15","16","17","18","19",
                                                                        "20","21","22","23","24","25","26","27","28","29",
                                                                        "30","31"};
       
        private String[] dayListEnd = {"1","2","3","4","5","6","7","8","9","10",
                                                                  "11","12","13","14","15","16","17","18","19",
                                                                  "20","21","22","23","24","25","26","27","28","29",
                                                                  "30","31"};
       
        private String[] monthListStart = {"JAN","FEB","MAR","APR","MAY","JUN",
                                                                              "JUL","AUG","SEP","OCT","NOV","DEC"};
       
        private String[] monthListEnd = {"JAN","FEB","MAR","APR","MAY","JUN",
                                                                            "JUL","AUG","SEP","OCT","NOV","DEC"};
       
        private String[] yearListStart = {"2011","2012","2013","2014","2015","2016","2017","2018","2019","2020",
                                                                            "2021","2022","2023","2024","2025","2026","2027","2028","2029","2030",
                                                                            "2031","2032","2033","2034","2035","2036","2037","2038","2039","2040",
                                                                            "2041","2042","2043","2044","2045","2046","2047","2048","2049","2050",
                                                                            "2051","2052","2053","2054","2055","2056","2057","2058","2059","2060",
                                                                            "2061","2062","2063","2064","2065","2066","2067","2068","2069","2070",
                                                                            "2071","2072","2073","2074","2075","2076","2077","2078","2079","2080",
                                                                            "2081","2082","2083","2084","2085","2086","2087","2088","2089","2090",
                                                                            "2091","2092","2093","2094","2095","2096","2097","2098","2098","2099"};
       
        private String[] yearListEnd = {"2011","2012","2013","2014","2015","2016","2017","2018","2019","2020",
                                                                        "2021","2022","2023","2024","2025","2026","2027","2028","2029","2030",
                                                                        "2031","2032","2033","2034","2035","2036","2037","2038","2039","2040",
                                                                        "2041","2042","2043","2044","2045","2046","2047","2048","2049","2050",
                                                                        "2051","2052","2053","2054","2055","2056","2057","2058","2059","2060",
                                                                        "2061","2062","2063","2064","2065","2066","2067","2068","2069","2070",
                                                                        "2071","2072","2073","2074","2075","2076","2077","2078","2079","2080",
                                                                        "2081","2082","2083","2084","2085","2086","2087","2088","2089","2090",
                                                                        "2091","2092","2093","2094","2095","2096","2097","2098","2098","2099"};
       
        //declare three panels to attach components
        private JPanel top;
        private JPanel middle;
        private JPanel bottom;
       
        public EnterDates()
        {
                //set size, title, what happens on exit and window resizing privileges
                setSize(FRAME_WIDTH, FRAME_HEIGHT);
                setTitle("Online Roster System");
                setDefaultCloseOperation(EXIT_ON_CLOSE);
                setResizable(false);
               
                //initialise content pane, set its colour and define layout
                Container cPane = this.getContentPane();
                cPane.setBackground(Color.LIGHT_GRAY);
                setLayout(new GridLayout(3,1));
               
                submitButton = new JButton("Submit");
                submitButton.setSize(BUTTON_WIDTH,BUTTON_HEIGHT);
                //submitButton.addActionListener(this);
               
                cancelButton = new JButton("Cancel");
                cancelButton.setSize(BUTTON_WIDTH,BUTTON_HEIGHT);
                //cancelButton.addActionListener(this);
               
                title = new JLabel("SELECT DATES TO VIEW BOOKED SHIFTS", SwingConstants.CENTER);
                title.setFont(new Font("Arial", Font.BOLD, 24));
                title.setSize(250, 50);
                Box box = Box.createHorizontalBox();
                box.add(Box.createHorizontalGlue());
                box.add(title);
                box.add(Box.createHorizontalGlue());
               
                startDateLabel = new JLabel("Select start date", SwingConstants.NORTH);
                startDateLabel.setFont(new Font("Arial", Font.BOLD,16));
                startDateLabel.setSize(40, 40);
               
                endDateLabel = new JLabel("Select finish date", SwingConstants.CENTER);
                endDateLabel.setFont(new Font("Arial", Font.BOLD,16));
                endDateLabel.setSize(40, 40);
               
                dayBoxStart = new JComboBox(dayListStart);
                dayBoxStart.setSelectedIndex(0);
                //dayBoxStart.addActionListener(this);
               
                dayBoxEnd = new JComboBox(dayListEnd);
                dayBoxEnd.setSelectedIndex(0);
                //dayBoxEnd.addActionListener(this);
               
                monthBoxStart = new JComboBox(monthListStart);
                monthBoxStart.setSelectedIndex(0);
                //monthBoxStart.addActionListener(this);
               
                monthBoxEnd = new JComboBox(monthListEnd);
                monthBoxEnd.setSelectedIndex(0);
                //monthBoxEnd.addActionListener(this);
               
                yearBoxStart = new JComboBox(yearListStart);
                yearBoxStart.setSelectedIndex(0);
                //yearBoxStart.addActionListener(this);
               
                yearBoxEnd = new JComboBox(yearListEnd);
                yearBoxEnd.setSelectedIndex(0);
                //yearBoxEnd.addActionListener(this);
               
                top = new JPanel();
                top.setBorder(BorderFactory.createLineBorder(Color.black));
                top.setBackground(Color.LIGHT_GRAY);
               
                middle = new JPanel(new FlowLayout());
                middle.setBorder(BorderFactory.createLineBorder(Color.black));
                middle.setBackground(Color.LIGHT_GRAY);
               
                bottom = new JPanel();
                bottom.setBorder(BorderFactory.createLineBorder(Color.black));
               
                top.add(title);
               
                middle.add(startDateLabel);
                middle.add(dayBoxStart);
                middle.add(monthBoxStart);
                middle.add(yearBoxStart);
                middle.add(endDateLabel);
                middle.add(dayBoxEnd);
                middle.add(monthBoxEnd);
                middle.add(yearBoxEnd);
               
                setLayout(new GridLayout(3,1));
                cPane.add(title);
                cPane.add(middle);
                cPane.add(bottom);
        }
        public static void main(String[]args)
        {
                EnterDates dates = new EnterDates();
                dates.setVisible(true);
        }
}

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thanks.

i just removed the SwingConstants on the first the JLabels... bit confusing but it's compiling again

SwingConstants is an Interface, I think it means that if you instantiate SwingConstants you have to call all of its methods/fields. I had a similar problem before when I tried to instantiate List e.g you can't do this:

Code:

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import java.util.List;
List list = new List();

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without shadowing all the methods in List