This is a very simple GUI application in the swing library written in Java which computes the Net Amount based on the choices received from the user.

GUI.java

import javax.swing.*;
import java.awt.*;
import java.awt.event.*;
public class netAmount_Calculator extends JFrame
{
    private Label billAmount,modeOfPayment,Discount,netAmount;
    private Button discountButton,netAmountButton,Stop;
    private TextField billAmount_tf,modeOfPaymnet_tf,Discount_tf,netAmount_tf;
    private JComboBox cb;
    String mode_of_payment[]={"Cash","Cheque","Credit Card"};
    public netAmount_Calculator()
    {
        super("Glamour Garments");
        setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        setVisible(true);
        setSize(350,200);
        setResizable(false);
        setLayout(new GridBagLayout());
        GridBagConstraints c=new GridBagConstraints();
        Label billAmount=new Label("Bill Amount");
        c.gridx=2;
        c.gridy=1;
        add(billAmount,c);
        TextField billAmount_tf=new TextField(10);
        c.gridx=3;
        c.gridy=1;
        add(billAmount_tf,c);
        Label modeOfPayment=new Label("Mode of Payment:");
        c.gridx=2;
        c.gridy=2;
        add(modeOfPayment,c);
        JComboBox cb=new JComboBox(mode_of_payment);
        c.gridx=3;
        c.gridy=2;
        add(cb,c);
        Button discountButton=new Button("Calculate Discount");
        c.gridx=2;
        c.gridy=4;
        add(discountButton,c);
        Button netAmountButton=new Button("Calculate Net Amount");
        c.gridx=3;
        c.gridy=4;
        add(netAmountButton,c);
        Label Discount=new Label("Discount:");
        c.gridx=3;
        c.gridy=5;
        add(Discount,c);
        TextField Discount_tf=new TextField(8);
        Discount_tf.setEditable(false);
        c.gridx=4;
        c.gridy=5;
        add(Discount_tf,c);
        Label netAmount=new Label("Net Amount:");
        c.gridx=3;
        c.gridy=6;
        add(netAmount,c);
        TextField netAmount_tf=new TextField(8);
        netAmount_tf.setEditable(false);
        c.gridx=4;
        c.gridy=6;
        add(netAmount_tf,c);
        Button Stop=new Button("STOP");
        c.gridx=4;
        c.gridy=7;
        add(Stop,c);
        event e=new event();
        discountButton.addActionListener(e);
        netAmountButton.addActionListener(e);
        Stop.addActionListener(e);
    }
    public class event implements ActionListener
    {
        public void actionPerformed(ActionEvent e)
        {
            String op,s;
            Double disc=0.0,bm,nm=0.0;
            op=e.getActionCommand();
            bm=Double.parseDouble(billAmount_tf.getText());
            if(op.equals("Calculate Discount"))
            {
               s=cb.getSelectedItem().toString(); 
               if(s.equals("Cash"))
               disc=0.08*bm;
               else if(s.equals("Cheque"))
               disc=0.07*bm;
               else if(s.equals("Credit Card"))
               disc=0.0;
               Discount_tf.setText("" + disc);
            }
            else if(op.equals("Calculate Net Amount"))
            {
                nm=bm-disc;
                netAmount_tf.setText("" + nm);
            }
            else if(op.equals("STOP"))
            {
                System.exit(0);
            }
        }
    }
           public static void main(String[]args)
           {
               netAmount_Calculator gui=new netAmount_Calculator();
            }
            
}