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Thread: Error: java.util.ConcurrentModificationException

  1. #1
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    Default Error: java.util.ConcurrentModificationException

    Yesterday I hoped I had finished program 1, but it appears to have a fault in it.
    And that is that it won't take empty cells.
    (It is a program that read in an excel file to a MultiHashMap, uses Apache POI)
    So know I have made this:

    	public void OpenReadFileTwo() {
    		// Open JFileChooser. 
    		chooser2.showOpenDialog(null); 
    		// Get the selected file. 
    		java.io.File file2 = chooser2.getSelectedFile();
    		// Create a FileInputStream. 
    		FileInputStream fileInput2 = null; 
    		//Get the name of the selected file. 
    		fileName2= file2.getName();
    		// Create a FileInputStream that will be used to read excel files. 
    		try {
    			fileInput2 = new FileInputStream(file2);
    			// Create an excel workbook. 
    			XSSFWorkbook workbook = new XSSFWorkbook(fileInput2); 
    			// Get the first sheet. 
    			XSSFSheet fileSheet2 = workbook.getSheetAt(0);
    			// Create Iterator. 
    			Iterator<Row> rows2 = fileSheet2.rowIterator();
     
    			while (rows2.hasNext()) {
    				//XSSF = .xlsx files, HSSF = .xls files. 
    				XSSFRow row = (XSSFRow) rows2.next();
    				// Get the empty cells in between as blank. 
    				//row.getCell(blank, Row.CREATE_NULL_AS_BLANK);
    				//System.out.println("Blank: " + row.getCell(blank, Row.CREATE_NULL_AS_BLANK));
     
    				// Iterate the row. 
    				Iterator<Cell> cells2 = row.cellIterator();
    				// Count every time +1. 
    				counterFile2 = counterFile2 +1; 
    				//Get only the first column in the excel file, column A. 
    				columnNumbers2 = "A" +counterFile2; 
    				CellReference cellReference2 = new CellReference(columnNumbers2);
    				Cell cellDataFirstColumn2 = row.getCell(cellReference2.getCol()); 
    				// Change cellDataFirstColumn2 to a String format. 
    				cellDataFirstColumn2String = "" + cellDataFirstColumn2; 
    				// Store the first column data in an arrayList. 
    				ArrayListFileData2.add(cellDataFirstColumn2String);
     
    				while (cells2.hasNext()) {
    					//
    					XSSFCell cell2 = (XSSFCell) cells2.next();
    					//
    					row.getCell(blank, Row.CREATE_NULL_AS_BLANK);
    					//System.out.println("Blank?: " + row.getCell(blank, Row.CREATE_NULL_AS_BLANK));
    					String BlankOrNot = ("" + row.getCell(blank, Row.CREATE_NULL_AS_BLANK));; 
    					//
    					if (BlankOrNot.length() != 0) {
    						System.out.println("Fish");
    						//continue;
    						Object cell2Object = "" + cell2;
    						//
    						mapFile2.put(cellDataFirstColumn2String, cell2Object);
    						System.out.println("cell2Object: " + cell2Object);
    					}
    					else {
    						System.out.println("Salmon");
    						//
    						Object blankeCell = " "; 
    						mapFile2.put(cellDataFirstColumn2String, blankeCell);
    						System.out.println("cell2Object: " + blankeCell);
    					}
     
    					blank = blank +1; 					
    					System.out.println(blank);
     
     
    				}
    				blank = 0; 
    			} counterFile2++; 
    		} catch (IOException e2) {
    			JOptionPane.showMessageDialog(null, "No file 2 selected, or the file is not a .xlsx file!");
    		} finally {
    			// Show the user the name of the selected file that was loaded in.
    			label2.setText(" File selected: " + fileName2); 
    			label2.setForeground(Color.BLUE);
    		}
    	}

    And it looks like it is working, because it now grabs the empty cell and prints " " on my screen.
    But after an empty cell have been found he runs to the catch.
    And I looked up this error, and it says something is in use that I am not allowed to change.
    But I don't see what is in use....

    Also it only runs to the catch when an empty cell is found.
    So if goes well, but else not.

    And I don't understand why. -_-


  2. #2
    Super Moderator Norm's Avatar
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    Default Re: Error: java.util.ConcurrentModificationException

    this error, and it says something is in use that I am not allowed to change.
    Can you post the full contents of the error message here?
    If you don't understand my answer, don't ignore it, ask a question.

  3. #3
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    Default Re: Error: java.util.ConcurrentModificationException

    Sure.
    This is the error code I am having:

    Exception in thread "AWT-EventQueue-0" java.util.ConcurrentModificationException
    	at java.util.TreeMap$PrivateEntryIterator.nextEntry(Unknown Source)
    	at java.util.TreeMap$ValueIterator.next(Unknown Source)
    	at SecondMatcherGridLayout.OpenReadFileTwo(SecondMatcherGridLayout.java:359)
    	at SecondMatcherGridLayout.actionPerformed(SecondMatcherGridLayout.java:198)
    	at javax.swing.AbstractButton.fireActionPerformed(Unknown Source)
    	at javax.swing.AbstractButton$Handler.actionPerformed(Unknown Source)
    	at javax.swing.DefaultButtonModel.fireActionPerformed(Unknown Source)
    	at javax.swing.DefaultButtonModel.setPressed(Unknown Source)
    	at javax.swing.plaf.basic.BasicButtonListener.mouseReleased(Unknown Source)
    	at java.awt.Component.processMouseEvent(Unknown Source)
    	at javax.swing.JComponent.processMouseEvent(Unknown Source)
    	at java.awt.Component.processEvent(Unknown Source)
    	at java.awt.Container.processEvent(Unknown Source)
    	at java.awt.Component.dispatchEventImpl(Unknown Source)
    	at java.awt.Container.dispatchEventImpl(Unknown Source)
    	at java.awt.Component.dispatchEvent(Unknown Source)
    	at java.awt.LightweightDispatcher.retargetMouseEvent(Unknown Source)
    	at java.awt.LightweightDispatcher.processMouseEvent(Unknown Source)
    	at java.awt.LightweightDispatcher.dispatchEvent(Unknown Source)
    	at java.awt.Container.dispatchEventImpl(Unknown Source)
    	at java.awt.Window.dispatchEventImpl(Unknown Source)
    	at java.awt.Component.dispatchEvent(Unknown Source)
    	at java.awt.EventQueue.dispatchEventImpl(Unknown Source)
    	at java.awt.EventQueue.access$000(Unknown Source)
    	at java.awt.EventQueue$3.run(Unknown Source)
    	at java.awt.EventQueue$3.run(Unknown Source)
    	at java.security.AccessController.doPrivileged(Native Method)
    	at java.security.ProtectionDomain$1.doIntersectionPrivilege(Unknown Source)
    	at java.security.ProtectionDomain$1.doIntersectionPrivilege(Unknown Source)
    	at java.awt.EventQueue$4.run(Unknown Source)
    	at java.awt.EventQueue$4.run(Unknown Source)
    	at java.security.AccessController.doPrivileged(Native Method)
    	at java.security.ProtectionDomain$1.doIntersectionPrivilege(Unknown Source)
    	at java.awt.EventQueue.dispatchEvent(Unknown Source)
    	at java.awt.EventDispatchThread.pumpOneEventForFilters(Unknown Source)
    	at java.awt.EventDispatchThread.pumpEventsForFilter(Unknown Source)
    	at java.awt.EventDispatchThread.pumpEventsForHierarchy(Unknown Source)
    	at java.awt.EventDispatchThread.pumpEvents(Unknown Source)
    	at java.awt.EventDispatchThread.pumpEvents(Unknown Source)
    	at java.awt.EventDispatchThread.run(Unknown Source)

    Line 359 is:
    XSSFCell cell2 = (XSSFCell) cells2.next();

    What is does now is:
    - It looks if there are next cells.
    - And write the information from those cells away to a MultiHashMap.
    - If there is a cell with nothing in between put a " " in the MultiHashMap.
    - And after that add the next cell to MultiHashmap.

    It all goes into the MultiHashMap, and if I let it print it does it the right way.
    But after that it throws this error. (After there is an empty cell found.)
    Last edited by Purple01; September 28th, 2012 at 07:16 AM.

  4. #4
    Super Moderator Norm's Avatar
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    Default Re: Error: java.util.ConcurrentModificationException

    Where does the code make any changes to the collection while it is looking at it with an iterator?
    If you don't understand my answer, don't ignore it, ask a question.

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    Default Re: Error: java.util.ConcurrentModificationException

    That is a bit of the problem.
    It only runs this subclass.
    So it is not used somewhere else yet.

  6. #6
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    Default Re: Error: java.util.ConcurrentModificationException

    Where is the collection modified while iterating over it? A reason for the error from the API doc:
    If a single thread issues a sequence of method invocations that violates the contract of an object, the object may throw this exception
    If you don't understand my answer, don't ignore it, ask a question.

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    Default Re: Error: java.util.ConcurrentModificationException

    I think it is this one:
    (When I remove this one I don't get the error.)

    String BlankOrNot = ("" + row.getCell(blank, Row.CREATE_NULL_AS_BLANK));
    But that is some code I need to find empty cells in between.

    I found that code here:
    java - How to get an Excel Blank Cell Value in Apache POI? - Stack Overflow

    And I think that was the right way to implement it in my code.
    Did I maybe made a mistake here?

  8. #8
    Super Moderator Norm's Avatar
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    Default Re: Error: java.util.ConcurrentModificationException

    It's strange that a method named getCell makes changes to the object.
    What is the definition for that method?
    If you don't understand my answer, don't ignore it, ask a question.

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    Default Re: Error: java.util.ConcurrentModificationException

    There is not so much to be found on this function.
    It only should make sure that empty cells are not skipped over.
    And nothing else.

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    Default Re: Error: java.util.ConcurrentModificationException

    Maybe it helps if I show some output this makes?

    This is my excel file:
    Sample_4 Cy3 Cat Old
    Sample_7 <empty cell here> Human Old

    This is what is hapening when it runs:
    {Sample_4=[Sample_4, Cy3, Cat]}

    {Sample_4=[Sample_4, Cy3, Cat, Old]}

    {Sample_4=[Sample_4, Cy3, Cat, Old] Sample_7=[Sample_7]}

    {Sample_4=[Sample_4, Cy3, Cat, Old] Sample_7=[Sample_7,  , Human]}

    And then this exception shows up:
    Exception in thread "AWT-EventQueue-0" java.util.ConcurrentModificationException

    ###


    I so hope I can fix this problem today, before the weekend starts. :p
    Last edited by Purple01; September 28th, 2012 at 03:18 AM.

  11. #11
    Super Moderator jps's Avatar
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    Default Re: Error: java.util.ConcurrentModificationException

    Quote Originally Posted by Purple01 View Post
    Exception in thread "AWT-EventQueue-0" java.util.ConcurrentModificationException
    I would suggest you utilize your favorite search engine on java.util.ConcurrentModificationException so you understand what is going on.
    Then if you have questions please post your code with the question. Just a little fyi, it is much easier to follow if you past more code that shows what you are trying to do rather than a bunch of snippets and comments about them.

    Good luck with the exception, once you understand the exception you may be able to fix the cause of the problem.

  12. #12
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    Default Re: Error: java.util.ConcurrentModificationException

    I solved it.
    I have put that line of code a few lines above and I made the line Return something.
    Now it does work.

    Thanks everyone.

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