about ItemEvent in java GUI - java

I'm doing a small program with Java GUI
here is mu code:
// DebugFourteen3
// User selects pizza topping and sees price
import javax.swing.*;
import java.awt.*;
import java.awt.event.*;
//use correct spelling of class name
public class DebugFourteen3 extends JFrame
{
FlowLayout flow = new FlowLayout();
JComboBox pizzaBox = new JComboBox();
JLabel toppingList = new JLabel("Topping List");
JLabel aLabel = new JLabel("Paulos's American Pie");
JTextField totPrice = new JTextField(10);
int[] pizzaPrice = {7,10,10,8,8,8,8};
int totalPrice = 0;
String output;
int pizzaNum;
public DebugFourteen3()
{
super("Pizza List");
setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
setLayout(flow);
add(toppingList);
pizzaBox.addItem("cheese");
pizzaBox.addItem("sausage");
pizzaBox.addItem("pepperoni");
pizzaBox.addItem("onion");
pizzaBox.addItem("green pepper");
pizzaBox.addItem("green olive");
pizzaBox.addItem("black olive");
add(pizzaBox);
add(aLabel);
add(totPrice);
itemStateChanged(this);
}
public static void main(String[] arguments)
{
JFrame frame = new DebugFourteen3();
frame.setSize(200, 150);
frame.setVisible(true);
}
public void itemStateChanged(ItemEvent[] list)
{
Object source = list.getSource();
if(source == pizzaBox)
{
int pizzaNum = pizzaBox.getSelectedIndex();
totalPrice = pizzaPrice[pizzaNum];
output = "Pizza Price $" + totalPrice;
totPrice.setText(output);
}
}
}
the compiler gets me error on line 35, it says itemStateChanged() should receive a argument which type is ItemEvent[] but I'm passing "this"(the class it self)
can anyone explain how the itemStateChanged works with the JComboBox?
thanks

You have to add the listener on the combo box instead of calling the itemStateChanged method directly. Adding the listener will enable the callbacks when the click event happens, and jvm will call the itemStateChanged method automatically as combo box is registered for the event. Replace this line
itemStateChanged(this);
with
pizzaBox.addItemListener(this);

At first, you need to implements ItemListener with JFrame. Than add following line with pizzaBox, this will notify overrided public void itemStateChanged(ItemEvent ev) method ItemListener.
pizzaBox.addItemListener(this);
For details, see this tutorial.

Problem :
1.you are not registering itemListener for JComboBox
2.itemStateChanged event takes ItemEvent as argument not ItemEvent[] array
1.Replace this :
itemStateChanged(this);
with this:
pizzaBox.addItemListener(this);
2.Replace This:
public void itemStateChanged(ItemEvent[] list)
With this:
public void itemStateChanged(ItemEvent list)
Complete Code:
// DebugFourteen3
// User selects pizza topping and sees price
import javax.swing.*;
import java.awt.*;
import java.awt.event.*;
//use correct spelling of class name
public class DebugFourteen3 extends JFrame
{
FlowLayout flow = new FlowLayout();
JComboBox pizzaBox = new JComboBox();
JLabel toppingList = new JLabel("Topping List");
JLabel aLabel = new JLabel("Paulos's American Pie");
JTextField totPrice = new JTextField(10);
int[] pizzaPrice = {7,10,10,8,8,8,8};
int totalPrice = 0;
String output;
int pizzaNum;
public DebugFourteen3()
{
super("Pizza List");
setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
setLayout(flow);
add(toppingList);
pizzaBox.addItem("cheese");
pizzaBox.addItem("sausage");
pizzaBox.addItem("pepperoni");
pizzaBox.addItem("onion");
pizzaBox.addItem("green pepper");
pizzaBox.addItem("green olive");
pizzaBox.addItem("black olive");
add(pizzaBox);
add(aLabel);
add(totPrice);
pizzaBox.addItemListener(this);
}
public static void main(String[] arguments)
{
JFrame frame = new DebugFourteen3();
frame.setSize(200, 150);
frame.setVisible(true);
}
public void itemStateChanged(ItemEvent list)
{
Object source = list.getSource();
if(source == pizzaBox)
{
int pizzaNum = pizzaBox.getSelectedIndex();
totalPrice = pizzaPrice[pizzaNum];
output = "Pizza Price $" + totalPrice;
totPrice.setText(output);
}
}
}

Here this i.e DebugFourteen3 is not a type ItemEvent. Thats why
itemStateChanged(ItemEvent[] list)
gives error.
If you want to handle itemStateChanged then you need to implement ItemLisntener and add actionListner to it as
pizzaBox.addItemListener(this);
also add Handler method of ItemLisntener as
public void itemStateChanged(....)
{
//Handling Code goes here
}
Or you can implement ActionListner also
For more information on how to use combobox you can visit here

Related

Static variable and actionePerformed method?

I have a class called Windows. The class extends JFrame and adds GUI components to the JFrame container. One of those components is a JTextfield. I am trying to set the text in the JTextfield through the actionPerformed() when generator JButton is clicked. The actionPerformed() is a class called EvenHandler. This is the eventHandler:
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import java.util.Random;
public class EventHandler implements ActionListener {
int x = 0;
PassWordGenerator password;
Window Window; // It works only when static Window Window.
public void start() {
Window = new Window();
}
#Override
public void actionPerformed(ActionEvent e) {
password = new PassWordGenerator(3,3,3,3);
Window.setGeneratedPswd(password.getPswd());
x += 1;
System.out.println(x);
}
public static void main(String[] args) {
EventHandler x = new EventHandler();
x.start();
}
}
the window class if you want to know how the GUI looks like. The button is the one calling actionePerfome().
import javax.swing.*;
import java.awt.*;
public class Window extends JFrame {
Label passwordLength;
Label labelGnPswd;
JTextField psdLength;
JCheckBox upperCase_letters;
JCheckBox lowerCase_letters;
JCheckBox numbers;
JCheckBox symbols;
JTextField generatedPswd;
EventHandler event = new EventHandler();
JButton generetor;
public Window() {
setLayout(new FlowLayout());
setTitle("PasswordGenerator");
setDefaultCloseOperation(WindowConstants.EXIT_ON_CLOSE);
setLocationRelativeTo(null);
setResizable(false);
//Password Length
passwordLength = new Label("Password Length");
add(passwordLength);
//Input text
psdLength = new JTextField("0", 5);
add(psdLength);
//Create checkBoxes
createcheckbxs();
//Label Generated psw
labelGnPswd = new Label("Generated pswd");
add(labelGnPswd);
//Generated Password;
generatedPswd = new JTextField("****", 5);
generatedPswd.setEditable(false);
add(generatedPswd);
//Button
generetor = new JButton("Generate pswd!");
generetor.addActionListener(event);
add(generetor);
setSize(200, 400);
setVisible(true);
}
public String getpsdLength() {
return psdLength.getText();
}
public void setGeneratedPswd(String pswd) {
generatedPswd.setText(pswd);
}
private void createcheckbxs() {
upperCase_letters = new JCheckBox("Include uppercase");
add(upperCase_letters);
lowerCase_letters = new JCheckBox("Include lowercase");
add(lowerCase_letters);
numbers = new JCheckBox("Include numbers ");
add(numbers);
symbols = new JCheckBox("Include symbols ");
add(symbols);
}
}
My question is that When I clicked on the generator JButton I get an error message along the line, "Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException..." I debugged the actionPerformed() and I found out that the Window Window is null when actionPefromed() is called after clicking genetor Jbutton. Why is Window null? Int x is working fine and it is not null. Isn't var x and window the same varaible scope. The only way I could keep the value of Window not null was to make Window a static variable.I hope my problem is a little more clear. Thanks in advance
Your problem seems in this line in Window.java:
generetor.addActionListener(event);
Variable event is declared in Window.java:
EventHandler event = new EventHandler();
The problem is that on event start() is not called so its instance variable Window Window remains null. The EventHandler you instantiate in main() is not used. What you might do is delete your start method and put its contents in the constructor.

When using JPanels in java how can you display user input data in one class to another

I've been having some difficulty with this. Basically I want a user to enter a name in one Java class and have that data displayed in another. I try using gets but I keep getting 'null'. Also how would you do this if I wanted to get an int from my class (exampl) to display in my second class. Would it basically be the same?
Here's an example to show what I mean
1st class
import javax.swing.*;
import java.awt.*;
import java.awt.event.*;
public class Exampl extends JFrame implements ActionListener {
JLabel intro = new JLabel("Welcome What is your name?");
JTextField answer = new JTextField(10);
JButton button = new JButton("Enter");
final int WIDTH = 330;
final int HEIGHT = 330;
JLabel greeting = new JLabel("");
JButton next =new JButton("Next");
String name = answer.getText();
public String getName() {
return name;
}
public void setName(String name) {
this.name = name;
}
public Exampl() {
super("Welcome");
setSize(WIDTH, HEIGHT);
setLayout(new FlowLayout());
add(intro);
add(answer);
add(button);
add(greeting);
setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
button.addActionListener(this);
setVisible(true);
}
public void actionPerformed(ActionEvent event) {
Object source = event.getSource();
if (source == button) {
String greet = "Hello there" + name;
greeting.setText(greet);
add(next);
next.addActionListener(this);
if(source==next)
dispose();
new Hello();
}
}
public static void main(String[] args) {
Exampl frame= new Exampl();
frame.setVisible(true);
}
}
2nd class
import javax.swing.*;
import java.awt.*;
import java.awt.event.*;
public class Hello extends JFrame implements ActionListener {
String name;
JLabel hi = new JLabel("Nice to see you "+name);
JButton button = new JButton("Enter");
final int WIDTH = 330;
final int HEIGHT = 330;
JLabel greeting = new JLabel("");
JButton next =new JButton("Next");
public Hello() {
super("Welcome");
setSize(WIDTH, HEIGHT);
setLayout(new FlowLayout());
add(hi);
add(button);
add(greeting);
setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
button.addActionListener(this);
setVisible(true);
}
public void actionPerformed(ActionEvent event) {
Object source = event.getSource();
if (source == button) {
String greet = "example " + name;
greeting.setText(greet);
add(next);
}
}
public static void main(String[] args) {
Exampl frame= new Exampl();
frame.setVisible(true);
}
}
My recommendations for you:
Don't extend JFrame, instead extend JPanel. See: Extends Frame vs creating it inside the program and Why shouldn't you extend JFrame and other components
Don't use multiple JFrames. See The use of multiple JFrames, good / bad practice? (BAD) instead you might want to try using Card Layout or JDialogs
What do you mean by extend? do you mean "public class Hello extends Exampl"? I'm new to java so I don't know much.
From that comment on another answer, you might want to learn the basics first in console applications before going into a GUI application which adds more complexity to your programs and thus your learning.
I'm planning on doing a project where I add the inputted name and a random int into a file so I can store it. Or would it make more sense to add that code into the same class?
From that comment, you can create a public method which returns the value in a JTextField in the format you need, then call that method from the other class, for example:
public String getUserName() {
return userNameField.getText();
}
public int getDigit() {
try {
return Integer.parseInt(digitField.getText());
} catch (NumberFormatException nfe) {
nfe.printStackTrace();
}
return -1; //In case it's not a number, or return 0, or whatever you need
}
//------In another class------//
int digit = object1.getDigit(); //Where object1 is an instance of the other class where those methods are defined
Make a getter in first class and extends second one class then use super.getter();

I'm trying to make a button to count characters in a text field

I'm trying to use the JButton count to count the number of characters entered into the JTextField t. I'm new to Java and GUIs but here's my code:
import java.awt.*;
import java.awt.event.*;
import javax.swing.*;
public class GUI1 extends Frame implements ActionListener{
TextField t;
Button count;
int a;
Choice choice;
public GUI1(){
this.t = new TextField("", 30);
this.count = new Button("count");
this.count.addActionListener(this);
JTextField x = new JTextField();
x.setEditable(false);
this.setTitle("Character count");
this.setLayout(new FlowLayout());
this.add(x);
this.add(t);
this.add(count);
this.pack();
this.setVisible(true);
}
public void actionPerformed(ActionEvent e) {
if(e.getSource()== this.count)
t.setText(choice.getSelectedItem()+ " " +a);
}
I'm also trying to enter the value in another uneditable JTextField x. Any help is appreciated.
Add this to your code
count.addActionListener(new ActionListener() {
public void actionPerformed(ActionEvent e) {
a = t.getText().length();
}
});
OR
You can use lambda expression like this
count.addActionListener(e -> a = t.getText().length());
For More
http://docs.oracle.com/javase/7/docs/api/java/awt/event/ActionListener.html
You need to add a listener
TextField t = new TextField();
Button b = new Button("Count");
b.addActionListener(new ActionListener() {
public void actionPerformed(ActionEvent e) {
int count = t.getText().length();
}
});
You can read more about here
http://www.tutorialspoint.com/awt/awt_button.htm
First of all, I recommend you not to use AWT elements, since it brings tons of problems and it has little to no support, instead you can try using Swing components which are a replacement/fix for AWT. You can read more about here. You might also want to read AWT vs Swing (Pros and Cons).
Now going into your problem:
You should avoid extending from JFrame, I might recommend you to create a new JFrame object instead. Here's the reason to why. That being said, you can also remove all your this.t and other calls with this.
I'm glad you're using a Layout Manager!
And now to count the number of characters on your JTextField and set text to your other JTextField you should use this code:
count.addActionListener(new ActionListener() {
public void actionPerformed(ActionEvent e) {
int count = t.getText().length();
System.out.println(count);
x.setText(t.getText());
}
});
Also I fixed your code, I changed AWT elements to Swing ones and added number of cols to your second JTextField so it would appear.
So, here's a running example that I made from your code (And removed Choice choice line since you didn't posted that code):
import java.awt.*;
import java.awt.event.*;
import javax.swing.*;
public class GUI1 {
JTextField t;
JButton count;
int a;
JFrame frame;
public GUI1(){
frame = new JFrame();
t = new JTextField("", 15);
count = new JButton("count");
JTextField x = new JTextField("", 15);
x.setEditable(false);
count.addActionListener(new ActionListener() {
public void actionPerformed(ActionEvent e) {
int count = t.getText().length();
System.out.println(count);
x.setText(t.getText());
}
});
frame.setTitle("Character count");
frame.setLayout(new FlowLayout());
frame.add(x);
frame.add(t);
frame.add(count);
frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
frame.pack();
frame.setVisible(true);
}
public static void main (String args[]) {
new GUI1();
}
}
When you click on a button you should
String str = t.getText(); // get string from jtextfield
To save the text from the texfield. Then you can use something like:
a = str..length(); // get length of string
x.setText(str + " " + a); //set it to the field
To set it to the JTextField.

Editable JComboBox gives error: <identifier> expected and illegal start of type

I am trying to create and Editable JComboBox to allow the user to type the name of the song to purchase. However when I set tunes.setEditable(true); I get an error... any help will be appreciated!
import javax.swing.*;
import java.awt.*;
import java.awt.event.*;
import javax.swing.JTextArea;
public class JTunes2 extends JFrame implements ItemListener
{
int songNum,songPrice;
int[] songAmount = {2,5,8,1,4,7,12,10,11,3,6,9};
String result;
JComboBox tunes = new JComboBox();
// set as editable
tunes.setEditable(true);
JLabel labelTunes = new JLabel("Song List");
JLabel outputs = new JLabel();
FlowLayout layout = new FlowLayout();
public JTunes2()
{
super("Song Selector");
setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
JPanel panel = new JPanel();
panel.setLayout(layout);
// add song names to combo box and register an item listener.
tunes.addItem("Song1");
tunes.addItem("Song2");
tunes.addItem("Song3");
tunes.addItem("Song4");
tunes.addItem("Song5");
tunes.addItem("Song6");
tunes.addItem("Song7");
tunes.addItem("Song8");
tunes.addItem("Song9");
tunes.addItem("Song10");
tunes.addItem("Song11");
tunes.addItem("Song12");
tunes.addItemListener(this);
panel.add(labelTunes);
panel.add(tunes);
panel.add(outputs);
//add panel to the frame
setContentPane(panel);
}
public void itemStateChanged(ItemEvent e)
{
//create source object
Object source = e.getSource();
//check the type size
if(source == tunes)
{
songNum = tunes.getSelectedIndex();
songPrice = songAmount[songNum];
result = "Total Price $" + songPrice;
//Display result
outputs.setText(result);
}
}
public static void main(String[] args)
{
// create class object
JTunes frame = new JTunes();
frame.setSize(250, 180);
frame.setVisible(true);
}
}
Thank you!
Actually, Java requires that you setup JComponents in the constructor. In order for your code to work, you need to call on setEditable(true) in the constructor, which means that you just need to move tunes.setEditable(true); to the constructor.
Tip: always allocate memory for JComponents in the constructor (you want to draw the components as soon as you create the Jframe). You can have a reference to the JComboBox at the class level.
Here is another version of your code:
import javax.swing.*;
import java.awt.*;
import java.awt.event.*;
import javax.swing.JTextArea;
public class JTunes2 extends JFrame implements ItemListener
{
int songNum,songPrice;
int[] songAmount = {2,5,8,1,4,7,12,10,11,3,6,9};
String result;
JComboBox tunes;
JLabel labelTunes = new JLabel("Song List");
JLabel outputs = new JLabel();
FlowLayout layout = new FlowLayout();
public JTunes2()
{
super("Song Selector");
setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
JPanel panel = new JPanel();
panel.setLayout(layout);
tunes = new JComboBox();
// set as editable
tunes.setEditable(true);
// add song names to combo box and register an item listener.
tunes.addItem("Song1");
tunes.addItem("Song2");
tunes.addItem("Song3");
tunes.addItem("Song4");
tunes.addItem("Song5");
tunes.addItem("Song6");
tunes.addItem("Song7");
tunes.addItem("Song8");
tunes.addItem("Song9");
tunes.addItem("Song10");
tunes.addItem("Song11");
tunes.addItem("Song12");
tunes.addItemListener(this);
panel.add(labelTunes);
panel.add(tunes);
panel.add(outputs);
//add panel to the frame
setContentPane(panel);
}
public void itemStateChanged(ItemEvent e)
{
//create source object
Object source = e.getSource();
//check the type size
if(source == tunes)
{
songNum = tunes.getSelectedIndex();
songPrice = songAmount[songNum];
result = "Total Price $" + songPrice;
//Display result
outputs.setText(result);
}
}
public static void main(String[] args)
{
// create class object
JTunes2 frame = new JTunes2();
frame.setSize(250, 180);
frame.setVisible(true);
}
}
You added the tunes.setEditable(true) at the class level, instead of at the method level. No statements allowed at the class level!
Here's a fixed version: I renamed JTunes2 to JTunes to fix the compilation errors, and moved the setEditable to the constructor. Also I fixed the indentation - this makes it harder to make this mistake:
import javax.swing.*;
import java.awt.*;
import java.awt.event.*;
import javax.swing.JTextArea;
public class JTunes extends JFrame implements ItemListener
{
int songNum,songPrice;
int[] songAmount = {2,5,8,1,4,7,12,10,11,3,6,9};
String result;
JComboBox tunes = new JComboBox();
JLabel labelTunes = new JLabel("Song List");
JLabel outputs = new JLabel();
FlowLayout layout = new FlowLayout();
public JTunes()
{
super("Song Selector");
setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
JPanel panel = new JPanel();
panel.setLayout(layout);
tunes.setEditable(true);
// add song names to combo box and register an item listener.
tunes.addItem("Song1");
tunes.addItem("Song2");
tunes.addItem("Song3");
tunes.addItem("Song4");
tunes.addItem("Song5");
tunes.addItem("Song6");
tunes.addItem("Song7");
tunes.addItem("Song8");
tunes.addItem("Song9");
tunes.addItem("Song10");
tunes.addItem("Song11");
tunes.addItem("Song12");
tunes.addItemListener(this);
panel.add(labelTunes);
panel.add(tunes);
panel.add(outputs);
//add panel to the frame
setContentPane(panel);
}
public void itemStateChanged(ItemEvent e)
{
//create source object
Object source = e.getSource();
//check the type size
if(source == tunes)
{
songNum = tunes.getSelectedIndex();
songPrice = songAmount[songNum];
result = "Total Price $" + songPrice;
//Display result
outputs.setText(result);
}
}
public static void main(String[] args)
{
// create class object
JTunes frame = new JTunes();
frame.setSize(250, 180);
frame.setVisible(true);
}
}

Need help to fix ComboBox programming error

I'm a kinda newbie actually and just self-studying. I really want to learn how to use JComboBox properly. I have created a simple program but it took me forever to fix it.
import javax.swing.*;
import java.awt.*;
import java.awt.event.*;
public class SampleButtonKo {
JComboBox combo;
public void ComboBox1() {
String course[] = {
"PM1", "PM2", "PM3", "PM4"
};
JFrame frame = new JFrame("Mang Inasal Ordering System");
JPanel panel = new JPanel();
combo = new JComboBox(course);
combo.setBackground(Color.gray);
combo.setForeground(Color.red);
panel.add(combo);
frame.add(panel);
combo.addItemListener(new ItemListener() {
public void itemStateChanged(ItemEvent ie) {
String str = (String) combo.getSelectedItem();
System.out.print("You have chosen " + str);
}
});
frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
frame.setSize(400, 400);
frame.setVisible(true);
}
public static void main(String[] args) {
JComboBox = new JComboBox();
}
}
You forgot a name for the variable
Instead of
JComboBox = new JComboBox();
try
JComboBox j = new JComboBox();
^
But perhaps, as iTech suggests, you want to create an instance of your class.
new SampleButtonKo();
There are obvious few errors in your code, you need to have the constructor named exactly as your class with no return type. Second, in your main you should create an instance of your class not the JComboBox
public class SampleButtonKo{
JComboBox combo;
public SampleButtonKo(){
// Copy your code from "ComboBox1" here
}
public static void main(String[] args) {
new SampleButtonKo();
}
}

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