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Thread: NullPointerException when canceling JOptionPane.showInputDialog

  1. #1
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    Question NullPointerException when canceling JOptionPane.showInputDialog

    Hello folks,

    Not sure what the problem is here. When I run this code it creates directories just fine. If I hit the "cancel" button, however, it creates a directory called "null". When I attempt to check the string for null, I get a NullPointerException. Any help is greatly appreciated.

    	public void makeTables() throws IOException{
    		String dirPath = "data/tendtables/";
    		String tname = (String)JOptionPane.showInputDialog(
    				"Enter the name of the new strain:");
     
    		// Canceled?
    		if (tname.equals(null))
    			return;
     
    		// Check if directory already exists
    		File tendTableDir = new File(dirPath + tname);
    		if (tendTableDir.exists()){
    			JOptionPane.showMessageDialog(null, "Strain already created!");
    		}else{
    				// Create the directory
    				tendTableDir.mkdir();


  2. #2
    Super Moderator Norm's Avatar
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    Default Re: NullPointerException when canceling JOptionPane.showInputDialog

    If tname is null, then there is no object with an equals() method to dereference. Hence a NPE
    The way to test if an object reference such as tname is null it to use ==
    For example: if (tname == null) ...

  3. The Following 2 Users Say Thank You to Norm For This Useful Post:

    Json (July 2nd, 2010), Shaddam (July 1st, 2010)

  4. #3
    Member Charlie's Avatar
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    Default Re: NullPointerException when canceling JOptionPane.showInputDialog

    Or to try/catch them NPE's. Try/catching is good practice, youll need to be adept at it later anyways . Not to mention the fact that teachers love error-handling.

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