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How to resolve NullPointerException? [duplicate]

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I am trying to create a JPanel that asks whether the player wants to hit or stand in a Black Jack game.
public class decisionLayout extends JFrame
{
JPanel decisionPanel;
public JButton buttons[];
private final String names[] = { "YES", "NO" };
final static String ASK_FOR_DECISION = "Decide whether to hit or not.";
public decisionLayout()
{
super( "decisionPanel demo" );
JPanel panel = new JPanel();
JPanel TextFieldPanel = new JPanel();
final JTextField tf = new JTextField();
tf.setText( ASK_FOR_DECISION );
tf.setEditable( false );
TextFieldPanel.add( tf );
// create the decisions
JPanel decisionPlace = new JPanel();
for ( int count = 0; count < buttons.length; count++ )
{
buttons[ count ] = new JButton( names[ count ] );
buttons[ count ].addActionListener(
new ActionListener()
{
public void actionPerformed( ActionEvent e )
{
if( e.getSource() == buttons[ 0 ] )
{
tf.setText( "Yes. You want more cards." );
}
else
{
tf.setText( "No. You do not want more cards." );
}
}
}
);
}
JPanel decisionPanel = new JPanel();
decisionPanel.add( decisionPlace );
panel.add( TextFieldPanel, BorderLayout.PAGE_START );
panel.add( decisionPanel, BorderLayout.CENTER );
}
} `
I do not see why there is a compilation error at line:
for ( int count = 0; count < buttons.length; count++ )
,which is what eclipse keeps telling me. Does anybody have any ideas why this is happening?

public JButton buttons[];
You never initialize your buttons, so when you try to call buttons.length you get an NPE. You probably want to actually place some buttons in that array

You never initialize the buttons[]. buttons[] has no length:
for ( int count = 0; count < **buttons.length**; count++ )

Can someone help me work out the kinks in my program? [closed]

Closed. This question needs details or clarity. It is not currently accepting answers.
Want to improve this question? Add details and clarify the problem by editing this post.
Closed 8 years ago.
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I have to write a calculator program using the Gui components. I have written the program but I'm having issues with my CE button. It is giving me errors. I'm also getting and error in my for loop where I add the operand labels to the expression. I was just hoping to get some help in fixing these errors. Thank you! Please ignore the comments. They are there for my to keep track of what I have to do.
import java.awt.*;
import javax.swing.*;
import java.awt.event.*;
public class Calculator
{
JFrame window;
// stuff for top panel
JPanel topPanel;
JTextField expr,result;
JButton equals;
// stuff for bottom panel
JPanel bottomPanel,digitsPanel,opsPanel;
JButton[] digits,ops;
JButton clear, clearEntry;
Container content;
Listener listener;
String[] oplabels = { "+", "-", "/", "*" };
public Calculator()
{
listener = new Listener(); // our Listener class implements ActionListener
window= new JFrame("GUI Calc");
content=window.getContentPane();
content.setLayout( new GridLayout(2,1) );
topPanel=new JPanel();
topPanel.setLayout( new GridLayout(1,3) );
// TOP PANEL WORK
expr = new JTextField( );
equals = new JButton("=");
equals.addActionListener( listener );
result = new JTextField( );
topPanel.add( expr );
topPanel.add( equals );
topPanel.add( result );
// BOTTOM PANEL WORK
bottomPanel = new JPanel();
bottomPanel.setLayout( new GridLayout(1,2) );
digitsPanel = new JPanel();
digitsPanel.setLayout( new GridLayout(4,3) );
opsPanel = new JPanel();
opsPanel.setLayout( new GridLayout(4,1) );
digits = new JButton[12];
ops = new JButton[4];
for (int i=0 ; i<10 ; i++)
{
digits[i] = new JButton( i+"" );
digits[i].addActionListener(listener);
digitsPanel.add( digits[i] );
}
clear = new JButton( "C" );
clearEntry = new JButton( "CE" );
clear.addActionListener(listener);
clearEntry.addActionListener(listener);
digitsPanel.add( clear );
digitsPanel.add( clearEntry);
for (int i=0 ; i<4 ; i++)
{
ops[i] = new JButton( oplabels[i] ) ;
ops[i].addActionListener(listener);
opsPanel.add( ops[i] );
}
bottomPanel.add( digitsPanel );
bottomPanel.add( opsPanel );
content.add( topPanel);
content.add( bottomPanel);
window.setVisible(true);
}
class Listener implements ActionListener
{
public void actionPerformed(ActionEvent e)
{
Component clicked = (Component) e.getSource();
if ( clicked == equals )
{
result.setText( evaluate( expr.getText() ) );
return;
}
for ( int i=0 ; i<10 ; i++)
{
if ( clicked == digits[i] )
{
expr.setText( expr.getText() + i );
return;
}
}
if ( clicked == clear )
{
expr.setText("0");
return;
// do something
}
for (int i=0; i < 10; i++)
{
if (clicked == digits[i])
{
int lastValue = i;
return;
}
}
if ( clicked == clearEntry )
{
expr.setText(expr.getText() - lastValue);
return;
// do something
}
for ( int i=0 ; i<4 ; i++ )
{
if(clicked == oplabels[i])
{
expr.setText(expr.getText + oplabels[i]);
return;
}
// tack on that operator to the expr string
}
}
String evaluate( String exp )
{
return "NOT WRITTEN YET";
}
}
public static void main(String [] args)
{
new Calculator();
}
}

Problem #1
The compiler is complaining about expr.setText(expr.getText() - lastValue); because lastValue is undefined.
Having a close look at the code, the only place that lastValue is defined is within the for-loop before the if statement...but the next issues thereturn` statement following it's decleration...
for (int i = 0; i < 10; i++) {
if (clicked == digits[i]) {
int lastValue = i;
return;
}
}
if (clicked == clearEntry) {
expr.setText(expr.getText() - lastValue);
return;
// do something
}
This not only makes the lastValue useless, but it's unlikely that the if statement would ever be reached...
Problem #2
for (int i = 0; i < 4; i++) {
if (clicked == oplabels[i]) {
expr.setText(expr.getText + oplabels[i]);
return;
}
// tack on that operator to the expr string
}
clicked is defined as Component clicked = (Component) e.getSource(); and oplables are defined as String[] oplabels = {"+", "-", "/", "*"};. String and Component are not comparable (also, you want expr.getText() not expr.getText).
I think you want to do something more like...
if (clicked instanceof JButton) {
JButton btn = (JButton)clicked)
for (int i = 0; i < oplabels.length; i++) {
if (oplabels[i].equals(btn.getText()) {
//...
}
}
}

Line 128: clicked is a Component and oplabels[i] is a string - you can't compare them directly.
Line 130: Missing () on getText() call.

show warning if users enter letter instead number in java applet

I am writing an tip calculator app in java applet with GUI, my question is how I make sure the error message will pop up if users enter letter instead of number
it is my first time asking question, please be easy on me! Thanks!!!
import objectdraw.*;
import java.awt.*;
import java.awt.event.*;
import javax.swing.*;
// Typing in the text field and hitting return adds text to text area.
// Clicking on button erases the text area.
public class TextApplet extends Controller implements ActionListener
{
private static final int ROWS = 1; // rows in TextArea
private static final int COLS = 10; // cols in text field & area
private String amount;
private float number;
private JTextField inField, output; // Input field
private JButton clear, calc;
// button to clear output
public void begin()
{
Container contentPane = getContentPane();
JPanel topPanel = new JPanel(); // prepare text field & label
JLabel inLabel = new JLabel("Bill Cost: ");
inField = new JTextField(COLS);
inField.addActionListener(this);
JLabel topTitle = new JLabel("Tip Calculator", JLabel.CENTER);
JPanel combinePanel = new JPanel();
combinePanel.add ( inLabel );
combinePanel.add ( inField );
JPanel combinePanel1 = new JPanel();
combinePanel1.add ( topTitle );
topPanel.add ( combinePanel1 );
topPanel.add ( combinePanel );
topPanel.setLayout ( new GridLayout ( 3,1) );
contentPane.add(topPanel,BorderLayout.NORTH);
JPanel centerPanel = new JPanel(); // prepare text area & label
JLabel outLabel = new JLabel("Bill + Tip:");
output = new JTextField(COLS);
output.setEditable(false); // Prevent user from wrting in output
centerPanel.add(outLabel);
centerPanel.add(output);
contentPane.add(centerPanel,BorderLayout.CENTER);
JPanel bottomPanel = new JPanel();
// create button
clear = new JButton(" Clear ");
calc = new JButton("Calculate");
calc.addActionListener(this);
clear.addActionListener(this);
bottomPanel.add(calc);
bottomPanel.add(clear);
contentPane.add(bottomPanel,BorderLayout.SOUTH);
validate();
}
// add text to area if user hits return, else erase text area
public void actionPerformed(ActionEvent evt)
{
if (evt.getSource() == calc )
{
amount = inField.getText();
number = ( Float.parseFloat( amount ) );
number = (15*number/100);
output.setText ( Float.toString ( number ) + '$' );
}
else if (evt.getSource() == clear )
{
output.setText("$");
inField.setText("");
}
}
}

There are any number of ways you might achieve this, you could use
An InputVerifier
A JFormattedTextField
A JSpinner
Or a DocumentFilter and examples

Take a look at javax.swing.InputVerifier. That can be easily attached to a JTextField
JTextField inputField = new JTextField();
inputField.setInputVerifier(new NumericInputVerifier());
private class NumericInputVerifier extends InputVerifier
{
#Override
public boolean verify(JComponent input)
{
if (((JTextField) input).getText().matches("[0-9]+"))
{
return true;
}
else
{
JOptionPane.showMessageDialog(input, "Only numbers are allowed", "Warning", JOptionPane.WARNING_MESSAGE);
return false;
}
}
}
A complete example can be found here.
Edit Added an example of how to use InputVerifier to limit to numeric input. You'll want to double check the regex, but the basic idea is there...

Use a JFormattedTextField or a DocumentFilter. Then the user won't even be able to enter a non-numeric digit. See:
How to Use Formatted Text Fields
Implementing a Document Filter
For the document filter you will need to check each chraacter as it is entered to make sure it is a digit.
It is always better to do simple edits like that as the user types, rather than wait until you click on a button to do processing.

Hello Friend I will give a suggestion
please add validation when call actionPerformed method
public void actionPerformed(ActionEvent evt)
{
if (evt.getSource() == calc )
{
if(validate()){
amount = inField.getText();
number = ( Float.parseFloat( amount ) );
number = (15*number/100);
output.setText ( Float.toString ( number ) + '$' );
}
else{
// show message for inter valid number or any other
}
}
else if (evt.getSource() == clear )
{
output.setText("$");
inField.setText("");
}
}
boolean validate(){
try{
amount = inField.getText();
number = ( Float.parseFloat( amount ) );
return true;
}
catch(Exception e){
return false;
}
}

If you try to call Float.parseFloat on a String that cannot be converted to a float, it will throw a NumberFormatException. You need to catch this exception.
try {
number = ( Float.parseFloat( amount ) );
number = (15*number/100);
output.setText ( Float.toString ( number ) + '$' );
} catch(NumberFormatException e) {
//code to show error message here
}

Well considering, you'd have to turn the string into an integer to do the math, you could do this:
try {
int number = Ineger.parseInt(inField.getText());
} catch (NumberFormatException e) {
//SHOW WARNING
}

if (Label1.getText().matches("[0-9]+"))
// does Label1 containts numbers.
{
// do math
}
else
{
// warning
Lavbel1.setText("");
}

didn't understand action listner of radio buttons

when i click adjacent two radio button then a line drawn between these button i put conditions in my code that line only will draw when two horizontal or vertical radio button selected else it show a error message ........... it works for me but if i click randomly two radio button it will throw an exception which i cant understand please give some discription about answer because i am beginner too............ and what is this means it shows when i compile prog..........Note: C:\Users\MUHAMMAD Umair\Desktop\ComponentLinkerTest.java uses unchecked or unsafe operations.
Note: Recompile with -Xlint:unchecked for details.
import javax.swing.*;
import java.awt.*;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import java.util.HashMap;
import java.util.Map;
import java.util.*;
/**
* #see http://stackoverflow.com/a/12389479/909085
*/
public class ComponentLinkerTest extends JComponent
{
// private Map<JComponent, JComponent> linked;
Map<JComponent, java.util.List<JComponent> > linked;// = new HashMap<>();
int n=1;
public ComponentLinkerTest ()
{
super ();
linked = new HashMap ();
}
static JRadioButton[] button = new JRadioButton[25];
public void gui()
{
setupLookAndFeel ();
JFrame frame = new JFrame ();
linker = new ComponentLinkerTest ();
frame.setGlassPane ( linker );
linker.setVisible ( true );
JPanel content = new JPanel ();
content.setLayout ( new GridLayout ( 5, 5, 5, 5 ) );
content.setBorder ( BorderFactory.createEmptyBorder ( 5, 5, 5, 5 ) );
frame.add ( content );
int i;
for (i = 0; i < 25; i++ )
{
// final JButton button = new JButton ( "Button" + i );
button[i] = new JRadioButton();
// panel.add(fontButtons[i]);
button[i].addActionListener ( new ActionListener ()
{
public void actionPerformed ( ActionEvent e )
{
link ((JRadioButton) e.getSource() );
}
} );
content.add ( button [i]);
}
frame.setDefaultCloseOperation ( JFrame.EXIT_ON_CLOSE );
frame.pack ();
frame.setLocationRelativeTo ( null );
frame.setVisible ( true );
}
/*public void link ( JComponent c1, JComponent c2 )
{
linked.put ( c1, c2 );
repaint ();
}*/
public void link ( JComponent c1, JComponent c2 )
{
if(linked.containsKey(c1)){
linked.get(c1).add(c2);
}else{
java.util.List<JComponent> list = new LinkedList<>();
list.add(c2);
linked.put ( c1, list );
}
repaint ();
}
/* protected void paintComponent ( Graphics g )
{
Graphics2D g2d = ( Graphics2D ) g;
g2d.setRenderingHint ( RenderingHints.KEY_ANTIALIASING, RenderingHints.VALUE_ANTIALIAS_ON );
g2d.setPaint ( Color.BLACK );
for ( JComponent c1 : linked.keySet () )
{
Point p1 = getRectCenter ( getBoundsInWindow ( c1 ) );
Point p2 = getRectCenter ( getBoundsInWindow ( linked.get ( c1 ) ) );
/* Stroke stroke = new BasicStroke(8//,
/*BasicStroke.CAP_BUTT, BasicStroke.JOIN_BEVEL, 0,
new float[] { 12, 12 }, 0);
g2d.setStroke(stroke);
g2d.setColor(Color.RED);
g2d.drawLine ( p1.x, p1.y, p2.x, p2.y );
}
}*/
#Override
protected void paintComponent ( Graphics g )
{
super.paintComponent(g);
Graphics2D g2d = ( Graphics2D ) g;
g2d.setRenderingHint ( RenderingHints.KEY_ANTIALIASING, RenderingHints.VALUE_ANTIALIAS_ON );
g2d.setPaint ( Color.BLACK );
for ( JComponent c1 : linked.keySet () )
{
for(JComponent c2 : linked.get(c1)){
Point p1 = getRectCenter ( getBoundsInWindow ( c1 ) );
Point p2 = getRectCenter ( getBoundsInWindow ( c2 ) );
/* Stroke stroke = new BasicStroke(8//,
/*BasicStroke.CAP_BUTT, BasicStroke.JOIN_BEVEL, 0,
new float[] { 12, 12 }, 0);
g2d.setStroke(stroke);*/
if(n==1)
{
g2d.setColor(Color.RED);
n=2;
}
else
{
g2d.setColor(Color.BLUE);
n=1;
}
g2d.drawLine ( p1.x, p1.y, p2.x, p2.y );
}
}
}
private Point getRectCenter ( Rectangle rect )
{
return new Point ( rect.x + rect.width / 2, rect.y + rect.height / 2 );
}
private Rectangle getBoundsInWindow ( Component component )
{
return getRelativeBounds ( component, getRootPaneAncestor ( component ) );
}
private Rectangle getRelativeBounds ( Component component, Component relativeTo )
{
return new Rectangle ( getRelativeLocation ( component, relativeTo ),
component.getSize () );
}
private Point getRelativeLocation ( Component component, Component relativeTo )
{
Point los = component.getLocationOnScreen ();
Point rt = relativeTo.getLocationOnScreen ();
return new Point ( los.x - rt.x, los.y - rt.y );
}
private JRootPane getRootPaneAncestor ( Component c )
{
for ( Container p = c.getParent (); p != null; p = p.getParent () )
{
if ( p instanceof JRootPane )
{
return ( JRootPane ) p;
}
}
return null;
}
public boolean contains ( int x, int y )
{
return false;
}
private static ComponentLinkerTest linker;
public static void main ( String[] args )
{
ComponentLinkerTest ct = new ComponentLinkerTest();
ct.gui();
}
private static JRadioButton last = null;
private static void link ( JRadioButton buton )
{
int a=0;
int i;
if ( last == null )
{
last = buton;
System.out.println(last.getX());
}
else
{
for( i=0 ;i<25 ;i++)
{
if(buton == button[i])
{
if(button[i-1] == last || button[i+1]==last || button[i-5] == last || button[i+5]==last)
{
System.out.println("in cond");
linker.link ( last, buton );
buton.setSelected(false);
last.setSelected(false);
last = null;
}
else
{
System.out.println("out cond");
buton.setSelected(false);
last.setSelected(false);
last = null;
JOptionPane.showMessageDialog(null,"Wrong position clicked ");
}
break;
}
else
{
System.out.println("button not found");
}
}
}
}
private static void setupLookAndFeel ()
{
try
{
UIManager.setLookAndFeel ( UIManager.getSystemLookAndFeelClassName () );
}
catch ( ClassNotFoundException e )
{
e.printStackTrace ();
}
catch ( InstantiationException e )
{
e.printStackTrace ();
}
catch ( IllegalAccessException e )
{
e.printStackTrace ();
}
catch ( UnsupportedLookAndFeelException e )
{
e.printStackTrace ();
}
}
}

You do no range checking...
This is going to cause an exception if you the last button is the on the last line, or the second button is one the first row
if (button[i - 1] == last || button[i + 1] == last || button[i - 5] == last || button[i + 5] == last)
You need to modify your checks to ensure that they don't extend below (0) or beyond (button.length - 1)...
if ((i > 0 && button[i - 1] == last) ||
(i < (button.length - 1) && button[i + 1] == last) ||
(i > 5 && button[i - 5] == last) ||
(i < (button.length - 1) && button[i - 5] == last)) {
Updated with alternative
Okay, that if statement was doing my head...so I took a different tact...
Basically, instead, I found the index of each button and calculated the distance that each button was from each other, they could only be 1 or 5 indices apart...
int lastIndex = indexOf(last);
int currentIndex = indexOf(buton);
int diff = Math.max(lastIndex, currentIndex) - Math.min(lastIndex, currentIndex);
if (diff == 1 || diff == 5) {
System.out.println("in cond");
linker.link(last, buton);
buton.setSelected(false);
last.setSelected(false);
last = null;
} else {
System.out.println("out cond");
buton.setSelected(false);
last.setSelected(false);
last = null;
JOptionPane.showMessageDialog(null, "Wrong position clicked ");
}
And the indexOf method...
protected static int indexOf(JRadioButton btn) {
int index = -1;
for (int value = 0; value < button.length; value++) {
if (button[value].equals(btn)) {
index = value;
break;
}
}
return index;
}
As a side note, I'd also beware of over using static. This suggest a bad design

adding listeners to JButton array;

I have the following code which creates a JButton array on button click. Ope[] is publically declared in class. I have problem that I got null pointer. That is it doesn't print 2nd in loop ie. doesn't go in inner iteration.
Please tell me how to handle the listeners for array. Thanks in advance.
for( int i=0,y=30; i<counter;i++,y+=15 )
{
open[i]=new JButton( "Open" );
open[i].setBounds( 380, y, 70, 15 );
open[i].addActionListener( this );
panelDisplay.add (open[i] );
System.out.println(""+i);
}
The event handling in actionPerformed function is as follows:
for( int j=0; j<open.length; j++ )
{
System.out.println("1st in a loop"+j);
if( ae.getSource() != null )
{
if( ae.getSource() == open[j] )
{
System.out.println("2nd in a loop" +j);
int id;
String stringid;
System.out.println("open of"+j+"is clicked");
stringid = ""+table.getValueAt(j, 0);
id = Integer.parseInt(stringid);
fetchData(id);
//ae.getSource().equals(null);
}
}
}

JButton inherits "setName" Method from Component. So if you set a name on the Button at initialization
open[i]=new JButton( "Open" );
open[i].setBounds( 380, y, 70, 15 );
open[i].setName("Button"+i);
open[i].addActionListener( this );
panelDisplay.add (open[i] );
System.out.println(""+i);
you can find wich button was pressed in the eventhandling
int buttonNumber = Integer.parseInt(ae.getSource().getName().replace("Button",""))
//... do eventhandling for Button["buttonNumber"]