unknown error - really need help

[smallmos1]

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:

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)

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);
	}
}

thanks.

[smallmos1]

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

[ozzyman]

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:

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

without shadowing all the methods in List