How to put error checking on a code

Question

hi im creating a daily dairy prototype i have almost finished the code it works but there no error checking and i dont know how to write a code to help me validate my prototype here is the code:
import java.applet.Applet;
import java.awt.BorderLayout;
import java.awt.GridLayout;
import java.awt.Label;
import java.awt.Panel;
import java.awt.TextField;
import java.awt.Component;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import java.text.ParseException;
import java.awt.List;
import javax.swing.JButton;
import javax.swing.JComboBox;
import javax.swing.JTextField;
     public class DailyDairy extends Applet implements ActionListener {
          DailyDairy panel1, panel2, panel3, panel4;
          Component[][] rowData;
          private JComboBox[] jcboxShapeCombo = new JComboBox[7];     
          private String [] shapeName={"Meeting", "Lunch", "Holiday", "Sickness", "Preparing report", "Administrative work", "Emails", "Query"};
          public void init(){
               setLayout(new BorderLayout());
               setBounds (20, 30, 300, 180);
               Panel headerPanel = new Panel();
               headerPanel.setLayout(new GridLayout(1,4));
               //JButton button1 = new JButton("SUBMIT");
               //add(button1);
               Label lblHeader = new Label("Start Time");
               headerPanel.add(lblHeader);
               lblHeader = new Label();
               headerPanel.add(lblHeader);
               add(headerPanel, BorderLayout.NORTH);
               Label lblHeader1 = new Label("End Time");
               headerPanel.add(lblHeader1);
               lblHeader1 = new Label();
               headerPanel.add(lblHeader1);
               add(headerPanel, BorderLayout.NORTH);
               Label lblHeader2 = new Label("Category of Task");
               headerPanel.add(lblHeader2);
               lblHeader2 = new Label();
               headerPanel.add(lblHeader2);
               add(headerPanel, BorderLayout.WEST);
               Label lblHeader3 = new Label("Comment");
               headerPanel.add(lblHeader3);
               lblHeader3 = new Label();
               headerPanel.add(lblHeader3);
               add(headerPanel, BorderLayout.NORTH);
               Label lblHeader4 = new Label("Total Time(Minutes)");
               headerPanel.add(lblHeader4);
               lblHeader4 = new Label();
               headerPanel.add(lblHeader4);
               add(headerPanel, BorderLayout.NORTH);
               Panel data = new Panel();
               data.setLayout(new GridLayout(7, 4));
               rowData = new Component[ 7 ][];
               // One row
               for(int row = 0; row < 7; row++) {
                    rowData[ row ] = new Component[ 5 ];
                         rowData[ row ][ 0 ] = new TextField(10);
                         rowData[ row ][ 1 ] = new TextField(10);
                         ((TextField)rowData[ row ][ 1 ]).addActionListener(this);
                         rowData[ row ][ 2 ]= new JComboBox (shapeName);
                         ((JComboBox)rowData[ row ][ 2 ]).addActionListener(this);
                         //((TextField)rowData[ row ][ 2 ]).addActionListener(this);
                         rowData[ row ][ 3 ] = new TextField(10) ;
                         ((TextField)rowData[ row ][ 3 ]).addActionListener(this);
                         //JComboBox jcboxShapeCombo;
                         //System.out.println(rowData[row][2]);                    
                         //jcboxShapeCombo[2] = new JComboBox (shapeName);
                         //rowData[ row ][ 3 ] = new TextField(10);
                         rowData[ row ][ 4 ] = new TextField(10);
                         ((TextField)rowData[ row ][ 4 ]).addActionListener(this);
                         java.text.SimpleDateFormat sdf = new java.text.SimpleDateFormat("h:mm");
                         ((TextField) rowData[ row ][ 0 ]).setText(sdf.format(new java.util.Date()));
                         data.add(rowData[ row ][ 0 ]);
                         data.add(rowData[ row ][ 1 ]);
                         data.add(rowData[ row ][ 2 ]);
                         data.add(rowData[ row ][ 3 ]);
                         data.add(rowData[ row ][ 4 ]);
               add(data, BorderLayout.CENTER);
          public void actionPerformed(ActionEvent event) {
               java.text.SimpleDateFormat sdf = new java.text.SimpleDateFormat("h:mm");
               java.util.Date start = new java.util.Date();
               try {
                    start = sdf.parse(((TextField)rowData[ 0 ][ 0 ]).getText());
               } catch (ParseException e) {
                    // TODO Auto-generated catch block
                    e.printStackTrace();
               java.util.Date end = new java.util.Date();
               try {
                    end = sdf.parse(((TextField)rowData[ 0 ][ 1 ]).getText());
                    //System.out.println("end" +end.getTime());
                    //System.out.println("start" +start.getTime());
                    if (end.getTime() > 0) {
                         long difference = (end.getTime() - start.getTime()) / 60000L;
                         ((TextField)rowData[ 0 ][ 4 ]).setText(Long.toString(difference));
               } catch (ParseException e) {
                    // TODO Auto-generated catch block
                    e.printStackTrace();
               //long difference = (end.getTime() - start.getTime()) / 60000;
               //java.util.Date difference = new java.util.Date();
               java.util.Date start1 = new java.util.Date();
               try {
                    start1 = sdf.parse(((TextField)rowData[ 1 ][ 0 ]).getText());
               } catch (ParseException e) {
                    // TODO Auto-generated catch block
                    e.printStackTrace();
               java.util.Date end1 = new java.util.Date();
               try {
                    end1 = sdf.parse(((TextField)rowData[ 1 ][ 1 ]).getText());
                    if (end1.getTime() > 0) {
                         long difference1 = (end1.getTime() - start1.getTime()) / 60000;
                         //java.util.Date difference = new java.util.Date();
                         ((TextField)rowData[ 1 ][ 4 ]).setText(Long.toString(difference1));
               } catch (ParseException e) {
                    // TODO Auto-generated catch block
                    e.printStackTrace();
               java.util.Date start2 = new java.util.Date();
               try {
                    start2 = sdf.parse(((TextField)rowData[ 2 ][ 0 ]).getText());
               } catch (ParseException e) {
                    // TODO Auto-generated catch block
                    e.printStackTrace();
               java.util.Date end2 = new java.util.Date();
               try {
                    end2 = sdf.parse(((TextField)rowData[ 2 ][ 1 ]).getText());
                    if (end2.getTime() > 0) {
                         long difference2 = (end2.getTime() - start2.getTime()) / 60000;
                         //java.util.Date difference = new java.util.Date();
                         ((TextField)rowData[ 2 ][ 4 ]).setText(Long.toString(difference2));
               } catch (ParseException e) {
                    // TODO Auto-generated catch block
                    e.printStackTrace();
               //long difference2 = (end2.getTime() - start2.getTime()) / 60000;
               //java.util.Date difference = new java.util.Date();
               //((TextField)rowData[ 2 ][ 4 ]).setText(Long.toString(difference2));
               java.util.Date start3 = new java.util.Date();
               try {
                    start3 = sdf.parse(((TextField)rowData[ 3 ][ 0 ]).getText());
               } catch (ParseException e) {
                    // TODO Auto-generated catch block
                    e.printStackTrace();
               java.util.Date end3 = new java.util.Date();
               try {
                    end3 = sdf.parse(((TextField)rowData[ 3 ][ 1 ]).getText());
                    if (end3.getTime() > 0) {
                         long difference3 = (end3.getTime() - start3.getTime()) / 60000;
                         ((TextField)rowData[ 3 ][ 4 ]).setText(Long.toString(difference3));
               } catch (ParseException e) {
                    // TODO Auto-generated catch block
                    e.printStackTrace();
               //long difference3 = (end3.getTime() - start3.getTime()) / 60000;
               //java.util.Date difference = new java.util.Date();
               java.util.Date start4 = new java.util.Date();
               try {
                    start4 = sdf.parse(((TextField)rowData[ 4 ][ 0 ]).getText());
               } catch (ParseException e) {
                    // TODO Auto-generated catch block
                    e.printStackTrace();
               java.util.Date end4 = new java.util.Date();
               try {
                    end4 = sdf.parse(((TextField)rowData[ 4 ][ 1 ]).getText());
                    if (end4.getTime() > 0) {
                         long difference4 = (end4.getTime() - start4.getTime()) / 60000;
                         ((TextField)rowData[ 4 ][ 4 ]).setText(Long.toString(difference4));
               } catch (ParseException e) {
                    // TODO Auto-generated catch block
                    e.printStackTrace();
               //long difference4 = (end4.getTime() - start4.getTime()) / 60000;
               //java.util.Date difference = new java.util.Date();
               //JTextField aField= (JTextField)event.getSource();
               //if (aField)== rowData[ 0 ][ 1 ])calc row(0, 4);
               //if (aField)== rowData[ 1 ][ 1 ])calc row(1, 4);
}

Answer 1

edit your post to have [ c o d e ] tags around your code,
then maybe someone will have a look.
And try to better explain your goals and your problem. Let us know what you have tried, what did you expect and what have you got.
What is a "prototype" and on what specs should it be validaded?