NullPointerException: Cannot invoke "JComboBox.getSelectedIndex()" because "this.this-CodePudding

My Combo Box Always Returns Null Value.

In my Project "Attendance Management System" ,I need to validate the selected Item in the Combo box. But it returns null. How can I get it to return the item selected from the Combo Box. I use Eclipse IDE to run my project. Here is My Code:

import java.awt.*;
import javax.swing.*;
import javax.swing.border.BevelBorder;
public class admin extends javax.swing.JFrame {
private static final long serialVersionUID=1L;
@SuppressWarnings({ "unchecked", "rawtypes" })
admin(){
//setResizable(false);  
JFrame newFrame=new JFrame("Admin");
newFrame.getContentPane().setForeground(new Color(0, 0, 0));
newFrame.getContentPane().setFont(new Font("Arial Black", Font.BOLD, 15));
newFrame.getContentPane().setBackground(new Color(0, 191, 255));
newFrame.setSize(1000,600);
newFrame.setVisible(true);
newFrame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);

JLabel lblNewLabel = new JLabel("Welcome Admin");
lblNewLabel.setBounds(416, 75, 185, 28);
lblNewLabel.setFont(new Font("Arial Black", Font.BOLD, 18));
newFrame.getContentPane().setLayout(null);
newFrame.getContentPane().add(lblNewLabel);
JButton btnNewButton = new JButton("Go");
btnNewButton.setBorder(new BevelBorder(BevelBorder.RAISED, new Color(0, 0, 255), null, 
Color.BLUE, null));
btnNewButton.setFont(new Font("Tahoma", Font.BOLD, 13));
btnNewButton.setBounds(471, 343, 85, 35);
btnNewButton.addActionListener(new java.awt.event.ActionListener() {
public void actionPerformed(java.awt.event.ActionEvent evt) {
    if(comboBox_1.getSelectedItem().toString().equals("Student"))
    {

   JOptionPane.showMessageDialog(null, "You can  Login !","", JOptionPane.ERROR_MESSAGE);
   //new Register_Student().setVisible(true); 
   }
}
});
newFrame.getContentPane().add(btnNewButton);
JComboBox comboBox_1 = new JComboBox();
comboBox_1.setBorder(new BevelBorder(BevelBorder.LOWERED, new Color(0, 0, 255), null, new 
 Color(0, 0, 255), null));
 comboBox_1.setFont(new Font("Arial Black", Font.BOLD, 14));
 comboBox_1.setModel(new DefaultComboBoxModel(new String[] {"Student", "Teacher", "Parent"}));

 comboBox_1.setBounds(359, 179, 291, 80);
 newFrame.getContentPane().add(comboBox_1);
  pack();
  }
public static void main(String args[]) {

try {
    for (javax.swing.UIManager.LookAndFeelInfo info : javax.swing.UIManager.getInstalledLookAndFeels()) {
        if ("Nimbus".equals(info.getName())) {
            javax.swing.UIManager.setLookAndFeel(info.getClassName());
            break;
        }
    }
} catch (ClassNotFoundException ex) {
    java.util.logging.Logger.getLogger(admin.class.getName()).log(java.util.logging.Level.SEVERE, null, ex);
} catch (InstantiationException ex) {
    java.util.logging.Logger.getLogger(admin.class.getName()).log(java.util.logging.Level.SEVERE, null, ex);
} catch (IllegalAccessException ex) {
    java.util.logging.Logger.getLogger(admin.class.getName()).log(java.util.logging.Level.SEVERE, null, ex);
} catch (javax.swing.UnsupportedLookAndFeelException ex) {
    java.util.logging.Logger.getLogger(admin.class.getName()).log(java.util.logging.Level.SEVERE, null, ex);
}

java.awt.EventQueue.invokeLater(new Runnable() {
    public void run() {
        new admin().setVisible(true);
        
    }
});
}
 private javax.swing.JComboBox<String> comboBox_1;
 }

Immediate help will be appreciated. Thank you in advance:)

CodePudding user response：

You have created two combo box objects. Please remove the ComboBox declaration on the top or JComboBox comboBox_1 = to comboBox_1 = in the constructor.

import java.awt.*;
import javax.swing.*;
import javax.swing.border.BevelBorder;
public class admin extends javax.swing.JFrame {
    private static final long serialVersionUID=1L;
    @SuppressWarnings({ "unchecked", "rawtypes" })
    admin(){
//setResizable(false);
        JFrame newFrame=new JFrame("Admin");
        newFrame.getContentPane().setForeground(new Color(0, 0, 0));
        newFrame.getContentPane().setFont(new Font("Arial Black", Font.BOLD, 15));
        newFrame.getContentPane().setBackground(new Color(0, 191, 255));
        newFrame.setSize(1000,600);
        newFrame.setVisible(true);
        newFrame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);

        JLabel lblNewLabel = new JLabel("Welcome Admin");
        lblNewLabel.setBounds(416, 75, 185, 28);
        lblNewLabel.setFont(new Font("Arial Black", Font.BOLD, 18));
        newFrame.getContentPane().setLayout(null);
        newFrame.getContentPane().add(lblNewLabel);
        JComboBox comboBox_1 = new JComboBox();
        comboBox_1.setBorder(new BevelBorder(BevelBorder.LOWERED, new Color(0, 0, 255), null, new
                Color(0, 0, 255), null));
        comboBox_1.setFont(new Font("Arial Black", Font.BOLD, 14));
        comboBox_1.setModel(new DefaultComboBoxModel(new String[] {"Student", "Teacher", "Parent"}));

        comboBox_1.setBounds(359, 179, 291, 80);
        JButton btnNewButton = new JButton("Go");
        btnNewButton.setBorder(new BevelBorder(BevelBorder.RAISED, new Color(0, 0, 255), null,
                Color.BLUE, null));
        btnNewButton.setFont(new Font("Tahoma", Font.BOLD, 13));
        btnNewButton.setBounds(471, 343, 85, 35);
        btnNewButton.addActionListener(new java.awt.event.ActionListener() {
            public void actionPerformed(java.awt.event.ActionEvent evt) {
                if(comboBox_1.getSelectedItem().toString().equals("Student"))
                {

                    JOptionPane.showMessageDialog(null, "You can  Login !","", JOptionPane.ERROR_MESSAGE);
                    //new Register_Student().setVisible(true);
                }
            }
        });
        newFrame.getContentPane().add(btnNewButton);

        newFrame.getContentPane().add(comboBox_1);
        pack();
    }
    public static void main(String args[]) {

        try {
            for (javax.swing.UIManager.LookAndFeelInfo info : javax.swing.UIManager.getInstalledLookAndFeels()) {
                if ("Nimbus".equals(info.getName())) {
                    javax.swing.UIManager.setLookAndFeel(info.getClassName());
                    break;
                }
            }
        } catch (ClassNotFoundException ex) {
            java.util.logging.Logger.getLogger(admin.class.getName()).log(java.util.logging.Level.SEVERE, null, ex);
        } catch (InstantiationException ex) {
            java.util.logging.Logger.getLogger(admin.class.getName()).log(java.util.logging.Level.SEVERE, null, ex);
        } catch (IllegalAccessException ex) {
            java.util.logging.Logger.getLogger(admin.class.getName()).log(java.util.logging.Level.SEVERE, null, ex);
        } catch (javax.swing.UnsupportedLookAndFeelException ex) {
            java.util.logging.Logger.getLogger(admin.class.getName()).log(java.util.logging.Level.SEVERE, null, ex);
        }

        java.awt.EventQueue.invokeLater(new Runnable() {
            public void run() {
                new admin().setVisible(true);

            }
        });
    }
    private javax.swing.JComboBox<String> comboBox_1;
}

CodePudding user response：

You're shadowing your variables. That is, you've declared comboBox_1 twice, once as an instance field and once as a local variable...

public class admin extends javax.swing.JFrame {
    admin() {
        //...
        JComboBox comboBox_1 = new JComboBox();
        //...
    }
    //...
    private javax.swing.JComboBox<String> comboBox_1;
}

Get rid of the local decleration

public class admin extends javax.swing.JFrame {
    admin() {
        //...
        comboBox_1 = new JComboBox();
        //...
    }

Now, having said that, you have another issue. admin extends from JFrame...

public class admin extends javax.swing.JFrame {

But then you create a second JFrame in the constructor...

public class admin extends javax.swing.JFrame {
    private static final long serialVersionUID = 1L;

    @SuppressWarnings({"unchecked", "rawtypes"})
    admin() {
        JFrame newFrame = new JFrame("Admin");

So, when I do new admin().setVisble(true) which frame is getting displayed?

As a general recommendation, you should avoid extending directly from top level containers like JFrame, you're not adding any new functionality to the class and you're restricting yourself to a single use case.

And, you really, really, really want to learn how to use layout managers, see Laying Out Components Within a Container for more details

import java.awt.Color;
import java.awt.Dimension;
import java.awt.EventQueue;
import javax.swing.JFrame;
import java.awt.Font;
import java.awt.GridBagConstraints;
import java.awt.GridBagLayout;
import java.awt.Insets;
import javax.swing.DefaultComboBoxModel;
import javax.swing.JButton;
import javax.swing.JComboBox;
import javax.swing.JLabel;
import javax.swing.JOptionPane;
import javax.swing.JPanel;
import javax.swing.border.BevelBorder;

public class Main {

    public static void main(String[] args) {
        new Main();
    }

    public Main() {
        EventQueue.invokeLater(new Runnable() {
            @Override
            public void run() {
                JFrame frame = new JFrame();
                frame.add(new Admin());
                frame.pack();
                frame.setLocationRelativeTo(null);
                frame.setVisible(true);
            }
        });
    }

    public class Admin extends JPanel {
        private static final long serialVersionUID = 1L;

        @SuppressWarnings({"unchecked", "rawtypes"})
        Admin() {
            setForeground(new Color(0, 0, 0));
            setBackground(new Color(0, 191, 255));

            setLayout(new GridBagLayout());
            GridBagConstraints gbc = new GridBagConstraints();
            gbc.gridwidth = GridBagConstraints.REMAINDER;
            gbc.insets = new Insets(50, 0, 50, 0);

            JLabel lblNewLabel = new JLabel("Welcome Admin");
            lblNewLabel.setFont(new Font("Arial Black", Font.BOLD, 18));
            add(lblNewLabel, gbc);

            comboBox_1 = new JComboBox();
            comboBox_1.setBorder(new BevelBorder(BevelBorder.LOWERED, new Color(0, 0, 255), null, new Color(0, 0, 255), null));
            comboBox_1.setFont(new Font("Arial Black", Font.BOLD, 14));
            comboBox_1.setModel(new DefaultComboBoxModel(new String[]{"Student", "Teacher", "Parent"}));
            gbc.ipady = 100;
            gbc.insets = new Insets(0, 0, 0, 0);
            add(comboBox_1, gbc);

            JButton btnNewButton = new JButton("Go");
            btnNewButton.setBorder(new BevelBorder(BevelBorder.RAISED, new Color(0, 0, 255), null, Color.BLUE, null));
            btnNewButton.setFont(new Font("Tahoma", Font.BOLD, 13));
            btnNewButton.addActionListener(new java.awt.event.ActionListener() {
                public void actionPerformed(java.awt.event.ActionEvent evt) {
                    if (comboBox_1.getSelectedItem().toString().equals("Student")) {

                        JOptionPane.showMessageDialog(null, "You can  Login !", "", JOptionPane.ERROR_MESSAGE);
                        //new Register_Student().setVisible(true); 
                    }
                }
            });
            gbc.insets = new Insets(50, 0, 50, 0);
            gbc.ipady = 0;
            add(btnNewButton, gbc);
        }

        @Override
        public Dimension getPreferredSize() {
            return new Dimension(1000, 600);
        }

        private javax.swing.JComboBox<String> comboBox_1;
    }
}