Thread: Help with removing objects

Hello. I'm new to Java and these forums and wondered if anybody could assist with this. The aim of my code is to get the user to input a number of taxis they would like to draw and this number is drawn. That works, but if the user then enters a new number it draws that amount too but is supposed to remove the old amount. For example, user enters 2 and 2 taxis are drawn - if they then enter 4 it should remove the existing 2 and draw 4 but at present it just draws 4 in addition to the existing 2. Please see code below:

import java.awt.*;
import java.awt.event.*;
import javax.swing.*;
 
public class TaxiJFrame extends JFrame implements ActionListener 
 
{
   private TaxiCanvas tc;
 
 
    JTextField numTaxiField = new JTextField(8);
    JPanel jPanel = new JPanel();
    JLabel enterText = new JLabel();
    JLabel numTaxis = new JLabel();
 
   public TaxiJFrame(){   
 
        super("Taxi Express");
        setSize(800, 600);
        setLocation(100, 100);
        getContentPane().setBackground(Color.CYAN);
        setDefaultCloseOperation(EXIT_ON_CLOSE);
 
        jPanel.setBackground(Color.YELLOW);
        jPanel.add(numTaxis);
        numTaxis.setText("Number of taxis");
        jPanel.add(numTaxiField);
        numTaxiField.addActionListener(this);
 
        enterText.setText("Type a number and press [Enter]");
        jPanel.add(enterText);
        getContentPane().add("South", jPanel);
        setVisible(true);
    }
 
    public void actionPerformed(ActionEvent pressEnter){
        try {
 
           int inputNum = Integer.parseInt(numTaxiField.getText());
           getContentPane().add("Center", new TaxiCanvas(inputNum));
           setVisible(true);
        } // end try
 
        catch (NumberFormatException nfe){
 
           int inputNum = Integer.parseInt(numTaxiField.getText());
           getContentPane().remove(tc);
           getContentPane().add("Center", new TaxiCanvas(inputNum));
           setVisible(true);
        }
    }
 
    public static void main(String[] args)
    {
      new TaxiJFrame();      
    }
}

import java.awt.Graphics;
import javax.swing.JComponent;
import java.util.ArrayList;
 
public class TaxiCanvas extends JComponent{
    private ArrayList<Taxi> taxis = new ArrayList<Taxi>();
 
    public TaxiCanvas(int numTaxi){
        for (int i=1; i<=numTaxi; i++){
            taxis.add(new Taxi(i));
        }
    }
 
    public void paint(Graphics g){
        final int TAXI_WIDTH = 80;
 
        for (Taxi b: taxis){
            int x = randomInt(5, getWidth()/100*TAXI_WIDTH);
            int y = randomInt(5, getHeight()/100*TAXI_WIDTH);
            b.paint(g, x, y);
        }
 
    }
 
    private int randomInt(int min, int max){
        return (int)(Math.random() * (max - min + 1) + min);
 
    }
}

import java.awt.Color;
import java.awt.Graphics;
 
public class Taxi {
    private int num;
 
    public Taxi(int num){ 
        this.num = num;
    }    
    public void paint(Graphics g, int x, int y){
        final int TAXI_WIDTH = 80;
        final int UNIT = TAXI_WIDTH/12;
 
 
        g.setColor(Color.YELLOW);
        g.fillRect(x + 3*UNIT, y, 5*UNIT ,3*UNIT);
        //Top of the body
        g.setColor(Color.YELLOW);
        g.fillRect(x, y + 3*UNIT, 12*UNIT, 4*UNIT);
        //Main part of the body
        g.setColor(Color.WHITE);
        g.fillRect(x + 4*UNIT, y + 1*UNIT , 3*UNIT, 2* UNIT);
        //Window
        g.setColor(Color.BLACK);
        //Colour of wheels
        g.fillOval(x + 1*UNIT, y + 5*UNIT, 3*UNIT, 3*UNIT);
        //Left wheel
        g.fillOval(x + 8*UNIT, y + 5*UNIT, 3*UNIT, 3*UNIT);
        //Right wheel
        g.drawString(""+ num, x + 5*UNIT, y + 6* UNIT);
        //Position of number, offset to be placed inside taxi
 
    }
}

With my limited knowledge using try/catch and: getContentPane.remove(tc);

Firts of all you should override paintComponent() when doing custom painting in Swing. A call to super.paintComponent() will repaint the JComponent, i.e. removing old taxis.

Read this tutorial: Lesson: Performing Custom Painting (The Java™ Tutorials > Creating a GUI With JFC/Swing)