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 · Accepted Answer · 2012-09-23 17:31:52Z

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, 2012 at 17:31

answered Sep 23, 2012 at 17:20

assylias

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12 Comments

Fildor Over a year ago

I'd like to add the suggestion, that you check the input for non-Number chars before trying to parse it to Integer.

Reshad Over a year ago

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?

assylias Over a year ago

@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 Over a year ago

@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).

Fildor Over a year ago

@assylias I agree. This is a special case I guess.

|

JackTools.Net · Accepted Answer · 2012-09-23 17:25:21Z

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, 2012 at 17:25

JackTools.Net

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