Java while loop to draw lines

Question

Hello everyone i am trying to do a exercise from a Java book and what i need to do is draw lines and within a textfield i have to say how far the distance between the lines have to be. All of this has to be in a loop so no hardcoded lines.

i have made everything like i thought it should be done but i get some weird error in my console that is keep coming. the error code is as followed:

Exception in thread "AWT-EventQueue-0" java.lang.NumberFormatException: For input string: "" at java.lang.NumberFormatException.forInputString(NumberFormatException.java:48) at java.lang.Integer.parseInt(Integer.java:470) at java.lang.Integer.parseInt(Integer.java:499) at h03.LinePanel.paintComponent(LinePanel.java:30) at javax.swing.JComponent.paint(JComponent.java:1037) at javax.swing.JComponent._paintImmediately(JComponent.java:5106) at javax.swing.JComponent.paintImmediately(JComponent.java:4890) at javax.swing.RepaintManager.paintDirtyRegions(RepaintManager.java:812) at javax.swing.RepaintManager.paintDirtyRegions(RepaintManager.java:714) at javax.swing.RepaintManager.prePaintDirtyRegions(RepaintManager.java:694) at javax.swing.RepaintManager.access$700(RepaintManager.java:41) at javax.swing.RepaintManager$ProcessingRunnable.run(RepaintManager.java:1672) at java.awt.event.InvocationEvent.dispatch(InvocationEvent.java:209) at java.awt.EventQueue.dispatchEventImpl(EventQueue.java:702) at java.awt.EventQueue.access$400(EventQueue.java:82) at java.awt.EventQueue$2.run(EventQueue.java:663) at java.awt.EventQueue$2.run(EventQueue.java:661) at java.security.AccessController.doPrivileged(Native Method) at java.security.AccessControlContext$1.doIntersectionPrivilege(AccessControlContext.java:87) at java.awt.EventQueue.dispatchEvent(EventQueue.java:672) at java.awt.EventDispatchThread.pumpOneEventForFilters(EventDispatchThread.java:296) at java.awt.EventDispatchThread.pumpEventsForFilter(EventDispatchThread.java:211) at java.awt.EventDispatchThread.pumpEventsForHierarchy(EventDispatchThread.java:201) at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:196) at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:188) at java.awt.EventDispatchThread.run(EventDispatchThread.java:122)

since i am new to Java i don't know what i did wrong and how to troubleshoot this error so i would appreciate any help! the code in my panel that i use for this is

DrawLines lines = new DrawLines();

public void paintComponent(Graphics g) {

    super.paintComponent(g);
    int positionY = getHeight() - Integer.parseInt(afstand.getText()); // absolut positioning

    int yPos = 0;

    while(yPos <= positionY) {

        lines.drawLines(g, 0, yPos, getWidth(), yPos);
        yPos = yPos + Integer.parseInt(afstand.getText());
    }

}

public void actionPerformed(ActionEvent e) {

    try {

        repaint();


    }
    catch(NumberFormatException err) {

        JOptionPane.showMessageDialog(null, "something went wrong! heeft u wel    een waarde opgegeven?");

    }

}

assylias · Answer 1 · 2012-09-23T17:31:52.943

2

The issue is exactly what it says: you call Integer.parseInt() on an empty String, in your example: afstand.getText(), and it does not like it.

java.lang.NumberFormatException: For input string: "" at
[...]
java.lang.Integer.parseInt(Integer.java:470) at java.lang.Integer.parseInt(Integer.java:499)

To avoid the exception you could catch the exception:

try {
    int input = Integer.parseInt(afstand.getText());
    //rest of your code
} catch (NumberFormatException e) {
    //let the use know that the entry is invalid
}

edited Sep 23 '12 at 17:31

answered Sep 23 '12 at 17:20

assylias

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I'd like to add the suggestion, that you check the input for non-Number chars before trying to parse it to Integer. – Fildor Sep 23 '12 at 17:27
But how come it already checks the textfield when i start debugging? i mean it's not in the constructor so it has to wait till i have put in a value or am i thinking the wrong way? – Reshad Sep 23 '12 at 17:33
@Fildor I've done that - although that check should probably done in the component itself, for example through a Document, rather than in the paintComponent method. – assylias Sep 23 '12 at 17:33
@Reshad `paintComponent` is called as soon as your component is visible - and it means you have not had time to enter anything yet (unless you populate the field before it is shown). – assylias Sep 23 '12 at 17:35
@assylias I agree. This is a special case I guess. – Fildor Sep 23 '12 at 17:37
@assylias hmm and where do i have to put this try and catch block? and what do you mean with populate the field? – Reshad Sep 23 '12 at 17:51
@Reshad Quick and dirty: put the try/catch block in your paintComponent method. – assylias Sep 23 '12 at 17:56
@assylias i see in the console the error shown but after i put in a value it works kinda but this is a very dirty way is there not a better way to do this? – Reshad Sep 23 '12 at 18:03
@Reshad I think you should review your design and parse that information outside the paintComponent method. See for example: http://stackoverflow.com/questions/3412135/how-to-validate-if-text-entered-is-a-numeric-number Or you can use an InputVerifier: http://docs.oracle.com/javase/tutorial/uiswing/misc/focus.html#inputVerification – assylias Sep 23 '12 at 18:06
But how can i draw the lines in a while loop outside the paintComponent? – Reshad Sep 23 '12 at 18:20
@Reshad Read the input as in the examples I linked to, store the result in a field and access that field from paintComponent. – assylias Sep 23 '12 at 18:21
i changed it a bit and i get a different error now.. > Exception in thread "main" java.lang.NullPointerException at h03.LinePanel.(LinePanel.java:26) at h03.Frame.(Frame.java:16) at h03.Main.main(Main.java:10) the link where u can see my code https://github.com/reshadf/Java/blob/master/DeBasis/src/h03/LinePanel.java – Reshad Sep 23 '12 at 18:48

score 0 · Answer 2 · answered Sep 23 '12 at 17:25

0

getText().toString() to get the value of your Textfield. You can try System.out.println(afstand.getText) and with toString to see the difference.

Otherwise there is no integer value in your Textfield.

answered Sep 23 '12 at 17:25

JackTools.Net

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Java while loop to draw lines

2 Answers2