NullPointerException: Cannot invoke "JComboBox.getSelectedIndex()" because "this.this$0.comboBox_1" is null

Question

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;
}

Answer 1

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;
}