How JComboBox in Java Works

Introduction

This article explains how JComboBox of Swing works and explain how to search records of a user from the database by their name of the Swing GUI in Java. The NetBeans IDE is used for the sample examples.

Search Employee Records

To create this app that searches Employee records from the database in a Window Forms application depending on their name, we need the following files:

1. Java file (JComboBoxSearchApp.java)
2. SQL table (emp.sql)

1. SwingSearchApp.java

This Java file consists of all the logic. First of all we initialize JFrame components using a constructor then create a database connection and finally set the database value to the textfield. If the given name is not found in the database then it displays an error message and displays it by running the constructor.

2. emp.sql

For fetching records we need a database table; for that we create an "emp" table in our database.

Syntax

emp.sql

create table emp
 (
    uname varchar2(20), umail varchar2(30),
    upass varchar2(20), ucountry varchar2(20)
 );

Insert some rows in it

1. insert into emp values ('sandeep', '[email protected]', 'welcome', 'India');

2. insert into emp values ('rahul', '[email protected]' , '123', 'India');

Now let's start creating this app. Use the following procedure to do that in the NetBeans IDE.

Step 1

Open the NetBeans IDE.

Step 2

Choose "Java" -> "Java application" as shown below.

Step 3

Type your project name as "JComboBoxSearchApp" as in the following.

Step 4

Select server and version wizard as in the following.

Step 5

Create a new Java Class "JComboBoxSearchApp.java" and write the following code in the "JComboBoxSearchApp.java" file.

JComboBoxSearchApp.java

import javax.swing.*;

import java.awt.event.*;

import java.awt.*;

import java.sql.*;

 

public class JComboBoxSearchApp extends JFrame implements ActionListener {

 

//Initializing Components

    JLabel lb, lb1, lb2, lb3, lb4, lb5;

    JTextField tf1, tf2, tf3, tf4, tf5;

    JButton btn;

    JComboBox bx;

 

    //Creating Constructor for initializing JFrame components

    JComboBoxSearchApp() {

        //Providing Title

        super("Fetching Student Information");

        lb5 = new JLabel("Select Name:");

        lb5.setBounds(20, 20, 100, 20);

 

        //adding a combox box and assign some name.

        bx = new JComboBox();

        bx.addItem("sandeep");

        bx.addItem("rahul");

        bx.addItem("Other");

 

        //add button for searching records from database

        bx.setBounds(130, 20, 200, 20);

        btn = new JButton("Submit");

        btn.setBounds(50, 50, 100, 20);

        btn.addActionListener(this);

 

        lb = new JLabel("Fetching Student Information From Database");

        lb.setBounds(30, 80, 450, 30);

        lb.setForeground(Color.red);

        lb.setFont(new Font("Serif", Font.BOLD, 20));

        setVisible(true);

        setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);

        setSize(500, 500);

 

        //adding lable and textfields for displaying emp records

        lb1 = new JLabel("U_Name:");

        lb1.setBounds(20, 120, 100, 20);

        tf1 = new JTextField(50);

        tf1.setBounds(130, 120, 200, 20);

        lb2 = new JLabel("U_Mail:");

        lb2.setBounds(20, 150, 100, 20);

        tf2 = new JTextField(100);

        tf2.setBounds(130, 150, 200, 20);

        lb3 = new JLabel("U_Pass:");

        lb3.setBounds(20, 180, 100, 20);

        tf3 = new JTextField(50);

        tf3.setBounds(130, 180, 200, 20);

        lb4 = new JLabel("U_Country:");

        lb4.setBounds(20, 210, 100, 20);

        tf4 = new JTextField(50);

        tf4.setBounds(130, 210, 100, 20);

        setLayout(null);

 

        //Add components to the JFrame

        add(lb5);

        add(bx);

        add(btn);

        add(lb);

        add(lb1);

        add(tf1);

        add(lb2);

        add(tf2);

        add(lb3);

        add(tf3);

        add(lb4);

        add(tf4);

 

        //Set TextField Editable False

        tf1.setEditable(false);

        tf2.setEditable(false);

        tf3.setEditable(false);

        tf4.setEditable(false);

    }

 

    public void actionPerformed(ActionEvent e) {

 

        //Create DataBase Coonection and Fetching Records

        try {

            //get combobox items and passed in a string

            String str = (String) bx.getSelectedItem();

 

            Class.forName("oracle.jdbc.driver.OracleDriver");

            Connection con = DriverManager.getConnection("jdbc:oracle:thin:@mcndesktop07:1521", "sandeep", "welcome");

            PreparedStatement st = con.prepareStatement("select * from emp where uname=?");

            st.setString(1, str);

 

            //Excuting Query

            ResultSet rs = st.executeQuery();

 

            if (rs.next()) {

                String s = rs.getString(1);

                String s1 = rs.getString(2);

                String s2 = rs.getString(3);

                String s3 = rs.getString(4);

 

                //Sets Records in TextFields.

                tf1.setText(s);

                tf2.setText(s1);

                tf3.setText(s2);

                tf4.setText(s3);

            } else {

                JOptionPane.showMessageDialog(null, "Name not Found");

            }

 

            //Create Exception Handler

        } catch (Exception ex) {

            System.out.println(ex);

        }

    }

 

    //Running Constructor

    public static void main(String args[]) {

        new JComboBoxSearchApp();

    }

} 

Step 6

Now our project is ready to run. Right-click on the "JComboBoxSearchApp.java" file, then choose "Run". The following output will be generated.

Step 7

Now select a given employee name in the Combo-Box for searching records.

Case 1:

Select a first name then click on "submit".

Case 2:

Select the asecond name as "rahul".

Case 3:

Searching records for "other".

Note: This is not a name so it generates a warning message. For this type of error we use:

JOptionPane.showMessageDialog(null, "Name not Found");

We use this dialogue box for generating the error message. So a new dialogue box appears with the message.