Question

I have created a class which name is Manager and I have a Frame which name is BirthList and this frame has a table.I work with MySQL and I have entered some data in the “birthtable” in MySQL.and i want to add those data from MySQL table in to the table which is in my frame .
HINT: birthList is a list of Birth objects.
but I will find this exception why?
Please help me:(

Manager class:

  public Manager class{
  Logger logger = Logger.getLogger(this.getClass().getName());
  private static Connection conn = DBManager.getConnection();
  private static Admin admin;

  public static void addToBirthListFromMySQL() throws SQLException {



    try{
      Statement  stmt = conn.createStatement();



     ResultSet rs= stmt.executeQuery("SELECT * FROM birthtable");

       Birth list1;

        while (rs.next()) {
            String s1 = rs.getString(2);
            if (rs.wasNull()) {
                s1 = null;
            }
            String s2 = rs.getString(3);
            if (rs.wasNull()) {
                s2 = null;
            }
            String s3 = rs.getString(4);
            if (rs.wasNull()) {
                s3 = null;
            }
            String s4 = rs.getString(5);
            if (rs.wasNull()) {
                s4 = null;
            }
            String s5 = rs.getString(6);
            if (rs.wasNull()) {
                s5 = null;
            }
            String s6 = rs.getString(7);
            if (rs.wasNull()) {
                s6 = null;
            }


          list1 = new Birth(s1, s2, s3, s4, s5, s6);
          admin.birthList.add(list1);


        }




    }
    catch(SQLException e){





    }

}

My Frame:

public class BirthList extends javax.swing.JFrame {

    private Admin admin;

    /** Creates new form BirthList */
    public BirthList(Admin admin) {
        initComponents();
        this.admin = admin;
        try {
            Manager.addToBirthListFromMySQL();
        } catch (SQLException ex) {
            Logger.getLogger(BirthList.class.getName()).log(Level.SEVERE, null, ex);
        }
        fillTable();
    }


public void fillTable() {

        String[] columNames = {"name", "family", "father's name", "mother's name", "date of birth", "place of birth"};
        List<Birth> birth = admin.getBirthList();
        Object[][] data = new Object[birth.size()][columNames.length];
        for (int i = 0; i < data.length; i++) {
            Birth birth1 = birth.get(i);
            data[i][0] = birth1.getName();
            data[i][1] = birth1.getFamily();
            data[i][2] = birth1.getFatherName();
            data[i][3] = birth1.getMotherName();
            data[i][4] = birth1.getDateOfBirth();
            data[i][5] = birth1.getPlaceOfBirth();


        }
        DefaultTableModel model = new DefaultTableModel(data, columNames);
        jTable1.setModel(model);
    }

    public boolean isCellEditable(int row, int col) {
        return true;
    }
}

stacktrace:
Exception in thread “AWT-EventQueue-0” java.lang.NullPointerException
at database.Manager.addToBirthListFromMySQL(Manager.java:272)
at AdminGUI.BirthList.(BirthList.java:35)
at AdminGUI.BirthFrame.newButton1ActionPerformed(BirthFrame.java:127)
at AdminGUI.BirthFrame.access$000(BirthFrame.java:21)
at AdminGUI.BirthFrame$1.actionPerformed(BirthFrame.java:58)
at javax.swing.AbstractButton.fireActionPerformed(AbstractButton.java:1995)
at javax.swing.AbstractButton$Handler.actionPerformed(AbstractButton.java:2318)
at javax.swing.DefaultButtonModel.fireActionPerformed(DefaultButtonModel.java:387)
at javax.swing.DefaultButtonModel.setPressed(DefaultButtonModel.java:242)
at javax.swing.plaf.basic.BasicButtonListener.mouseReleased(BasicButtonListener.java:236)
at java.awt.Component.processMouseEvent(Component.java:6038)
at javax.swing.JComponent.processMouseEvent(JComponent.java:3265)
at java.awt.Component.processEvent(Component.java:5803)
at java.awt.Container.processEvent(Container.java:2058)
at java.awt.Component.dispatchEventImpl(Component.java:4410)
at java.awt.Container.dispatchEventImpl(Container.java:2116)
at java.awt.Component.dispatchEvent(Component.java:4240)
at java.awt.LightweightDispatcher.retargetMouseEvent(Container.java:4322)
at java.awt.LightweightDispatcher.processMouseEvent(Container.java:3986)
at java.awt.LightweightDispatcher.dispatchEvent(Container.java:3916)
at java.awt.Container.dispatchEventImpl(Container.java:2102)
at java.awt.Window.dispatchEventImpl(Window.java:2429)
at java.awt.Component.dispatchEvent(Component.java:4240)
at java.awt.EventQueue.dispatchEvent(EventQueue.java:599)
at java.awt.EventDispatchThread.pumpOneEventForFilters(EventDispatchThread.java:273)
at java.awt.EventDispatchThread.pumpEventsForFilter(EventDispatchThread.java:183)
at java.awt.EventDispatchThread.pumpEventsForHierarchy(EventDispatchThread.java:173)
at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:168)
at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:160)
at java.awt.EventDispatchThread.run(EventDispatchThread.java:121)

line 272 is :admin.birthList.add(list1);

**I also debug my project and the result was :

debug:
Listening on javadebug
User program running
Debugger stopped on uncompilable source code.

*I also print my object admin with system.out.println(admin) and the result was:

classes.Admin@20be79*

Answer 1

Instead of hunting the root cause in the dark, I think it’s better to explain how and why NPE’s are caused and how they can be avoided, so that the OP can apply the newly gained knowledge to hunt his/her own trivial problem.

Look, object references (the variables) can contain either a fullworthy Object or simply nothing, which is null.

SomeObject someObject1 = new SomeObject(); // References something.
SomeObject someObject2 = null; // References nothing.

Now, if you’re trying to access nothing (null), then you will undoubtely get a NullPointerException, simply because null doesn’t have any variables or methods.

someObject1.doSomething(); // Works fine.
someObject2.doSomething(); // Throws NullPointerException.

Workarounding this is fairly simple. It can be done in basically two ways: either by instantiating it or just by ignoring it.

if (someObject2 == null) {
    someObject2 = new SomeObject();
}
someObject2.doSomething(); // No NPE anymore!

or

if (someObject2 != null) {
    someObject2.doSomething(); // No NPE anymore!
}

In case of a NPE, the first line number of the stacktrace points the exact line where the it is been caused. You told literally “line 272 is admin.birthList.add(list1);“. This line contains two places where object references are been accessed/invoked (using the dot . operator). The first being admin.birthList and the second being birthList.add(list1). It’s up to you to find out if one or both caused the NPE. If it is the first invocation, then admin is simply null. If it is the second invocation, then birthList is simply null. You can fix it by instantiating it with a fullworthy object.

Edit: If you have a hard time in determining the root cause (as turns out from comments), then you need to learn debugging. Run a debugger or just do “poor man’s debugging” with help of a System.out.println() of every variable before accessing/invoking them. First look at line where the NPE is caused. If this is for example

admin.birthList.add(list1);

then you need to change it as follows to nail down the root cause:

System.out.println("admin: " + admin);
List<Birth> birthList = admin.birthList;
System.out.println("birthList: " + birthList);
birthList.add(list1);

check if any of them prints null. Alternatively you can also do:

if (admin == null) throw new NullPointerException("admin is null!");
List<Birth> birthList = admin.birthList;
if (birthList == null) throw new NullPointerException("birthList is null!");
birthList.add(list1);

you can also separate the individual invocations over separate lines so that you have enough to the line number to know which reference is null.

List<Birth> birthList = admin.birthList; // If NPE line points here, then admin is null.
birthList.add(list1); // If NPE line points here, then birthList is null.