I am relativly new to java and honestly to programing. I am working on

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Asked: June 15, 20262026-06-15T14:23:26+00:00 2026-06-15T14:23:26+00:00

I am relativly new to java and honestly to programing. I am working on

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I am relativly new to java and honestly to programing. I am working on a little tool. One of the functionalites is a pseudo-ftp-client showing the content on ftp hosts, also allowing to switch between the different folders. For the file/folder Listing i am using a TableView. Now I wanted to tune the visual stuff and ad some graphics to the cell depending on its content if it is a file or a folder. But when I use the following code in my CellFactory, it sets the Graphic not only on the cell fullfilling the statement, further when scrolling the TableView up and down, the graphic appears on different cells randomly and after a few seconds it crashes.

    public BorderPane addFTPBorderPane(){

    ftpBorderpane = new BorderPane();
    ftpBorderpane.setPadding(new Insets(0));
    GridPane grid = new GridPane();
    grid.setPadding(new Insets(10));
    grid.setHgap(20);
    grid.setId("simplebluegradient");
    Button getFTPButton = new Button("Verbinden");
    grid.add(getFTPButton, 1, 0);
    ftpBorderpane.setTop(grid);

    VBox vbox = new VBox();
    final TableView<MyFTPFile> ftpTable = new TableView<MyFTPFile>();

    Callback<TableColumn<MyFTPFile,String>, TableCell<MyFTPFile,String>> cellFactory =
            new Callback<TableColumn<MyFTPFile,String>, TableCell<MyFTPFile,String>>() {
                public TableCell<MyFTPFile,String> call(TableColumn<MyFTPFile,String> p) {
                    cell = new TableCell<MyFTPFile, String>() {
                        @Override
                        public void updateItem(String item, boolean empty) {
                            super.updateItem(item, empty);
                            setText(item);
                            if(item.equals("test")){
                                Image folderIcon = new Image(imagesPath+"folder.png");
                                setGraphic(new ImageView(folderIcon));
                            }

                        }

                    };

                    cell.addEventFilter(MouseEvent.MOUSE_CLICKED, new EventHandler<MouseEvent>() {
                        @Override
                        public void handle(MouseEvent event) {
                            if (event.getClickCount() > 1) {
                                cell = (TableCell<MyFTPFile,String>) event.getSource();
                                System.out.println(cell.getTableRow().getItem());
                                try {
                                    MyFTPClient.getInstance().changeFTPdir(cell.getTableRow().getItem());
                                } catch (IOException e) {
                                    // TODO Auto-generated catch block
                                    e.printStackTrace();
                                }
                                ftpObservablelist = MyFTPClient.getInstance().getFtpObservableList(); 
                                ftpTable.setItems(ftpObservablelist);

                            }
                        }
                    });

                    return cell;
                }
            };


    ftpTable.setEditable(true);

    TableColumn<MyFTPFile,String> nameColumn = new TableColumn<MyFTPFile,String>("Dateiname");
    nameColumn.setMinWidth(200);
    nameColumn.setCellFactory(cellFactory);
    nameColumn.setCellValueFactory(new PropertyValueFactory<MyFTPFile,String>("name"));

    TableColumn<MyFTPFile,String> sizeColumn = new TableColumn<MyFTPFile,String>("Grösse");
    sizeColumn.setCellValueFactory(new PropertyValueFactory<MyFTPFile,String>("size"));

    TableColumn<MyFTPFile,String> dateColumn = new TableColumn<MyFTPFile,String>("Änderungsdatum");
    dateColumn.setMinWidth(140);
    dateColumn.setCellValueFactory(new PropertyValueFactory<MyFTPFile,String>("date"));

    TableColumn<MyFTPFile,String> rightsColumn = new TableColumn<MyFTPFile,String>("Berechtigungen");
    rightsColumn.setCellValueFactory(new PropertyValueFactory<MyFTPFile,String>("rights"));

    TableColumn<MyFTPFile,String> userColumn = new TableColumn<MyFTPFile,String>("User");
    userColumn.setCellValueFactory(new PropertyValueFactory<MyFTPFile,String>("owner"));

    TableColumn<MyFTPFile,String> groupColumn = new TableColumn<MyFTPFile,String>("Group");
    groupColumn.setCellValueFactory(new PropertyValueFactory<MyFTPFile,String>("group"));

    ftpTable.getColumns().addAll(nameColumn,sizeColumn, dateColumn, rightsColumn, userColumn, groupColumn);

    vbox.getChildren().add(ftpTable);
    ftpBorderpane.setCenter(vbox);

    getFTPButton.setOnAction(new EventHandler<ActionEvent>() {
        public void handle(ActionEvent e){
            try {
                MyFTPClient.getInstance().doConnection(kunde);
                MyFTPClient.getInstance().reqFTPdir();


            } catch (Exception e1) {
                // TODO Auto-generated catch block
                e1.printStackTrace();
            }
            ftpObservablelist = MyFTPClient.getInstance().getFtpObservableList(); 
            ftpTable.setItems(ftpObservablelist);


        }   
    });



    return ftpBorderpane;       

}

The following Error appears when it crashes:

java.lang.NullPointerException
at ch.trusted.java.hcst.gui.GuiTest$19$1.updateItem(GuiTest.java:972)
at ch.trusted.java.hcst.gui.GuiTest$19$1.updateItem(GuiTest.java:1)
at javafx.scene.control.TableCell.updateItem(TableCell.java:539)
at javafx.scene.control.TableCell.indexChanged(TableCell.java:105)
at javafx.scene.control.TableCell$1.invalidated(TableCell.java:93)
at com.sun.javafx.binding.ExpressionHelper$SingleInvalidation.fireValueChangedEvent(ExpressionHelper.java:155)
at com.sun.javafx.binding.ExpressionHelper.fireValueChangedEvent(ExpressionHelper.java:100)
at javafx.beans.property.ReadOnlyIntegerWrapper$ReadOnlyPropertyImpl.fireValueChangedEvent(ReadOnlyIntegerWrapper.java:195)
at javafx.beans.property.ReadOnlyIntegerWrapper.fireValueChangedEvent(ReadOnlyIntegerWrapper.java:161)
at javafx.beans.property.IntegerPropertyBase.markInvalid(IntegerPropertyBase.java:130)
at javafx.beans.property.IntegerPropertyBase.set(IntegerPropertyBase.java:163)
at javafx.scene.control.IndexedCell.updateIndex(IndexedCell.java:112)
at com.sun.javafx.scene.control.skin.TableRowSkin.updateCells(TableRowSkin.java:251)
at com.sun.javafx.scene.control.skin.TableRowSkin.doUpdateCheck(TableRowSkin.java:151)
at com.sun.javafx.scene.control.skin.TableRowSkin.layoutChildren(TableRowSkin.java:157)
at javafx.scene.Parent.layout(Parent.java:1018)
at com.sun.javafx.scene.control.skin.TableRowSkin.handleControlPropertyChanged(TableRowSkin.java:110)
at com.sun.javafx.scene.control.skin.SkinBase$3.changed(SkinBase.java:282)
at javafx.beans.value.WeakChangeListener.changed(WeakChangeListener.java:107)
at com.sun.javafx.binding.ExpressionHelper$SingleChange.fireValueChangedEvent(ExpressionHelper.java:196)
at com.sun.javafx.binding.ExpressionHelper.fireValueChangedEvent(ExpressionHelper.java:100)
at javafx.beans.property.ObjectPropertyBase.fireValueChangedEvent(ObjectPropertyBase.java:123)
at javafx.beans.property.ObjectPropertyBase.markInvalid(ObjectPropertyBase.java:130)
at javafx.beans.property.ObjectPropertyBase.set(ObjectPropertyBase.java:163)
at javafx.scene.control.Cell.setItem(Cell.java:333)
at javafx.scene.control.Cell.updateItem(Cell.java:557)
at javafx.scene.control.TableRow.updateItem(TableRow.java:246)
at javafx.scene.control.TableRow.indexChanged(TableRow.java:92)
at javafx.scene.control.TableRow$1.invalidated(TableRow.java:85)
at com.sun.javafx.binding.ExpressionHelper$SingleInvalidation.fireValueChangedEvent(ExpressionHelper.java:155)
at com.sun.javafx.binding.ExpressionHelper.fireValueChangedEvent(ExpressionHelper.java:100)
at javafx.beans.property.ReadOnlyIntegerWrapper$ReadOnlyPropertyImpl.fireValueChangedEvent(ReadOnlyIntegerWrapper.java:195)
at javafx.beans.property.ReadOnlyIntegerWrapper.fireValueChangedEvent(ReadOnlyIntegerWrapper.java:161)
at javafx.beans.property.IntegerPropertyBase.markInvalid(IntegerPropertyBase.java:130)
at javafx.beans.property.IntegerPropertyBase.set(IntegerPropertyBase.java:163)
at javafx.scene.control.IndexedCell.updateIndex(IndexedCell.java:112)
at com.sun.javafx.scene.control.skin.VirtualFlow.setCellIndex(VirtualFlow.java:1596)
at com.sun.javafx.scene.control.skin.VirtualFlow.addLeadingCells(VirtualFlow.java:1049)
at com.sun.javafx.scene.control.skin.VirtualFlow.layoutChildren(VirtualFlow.java:1005)
at javafx.scene.Parent.layout(Parent.java:1018)
at javafx.scene.Scene.layoutDirtyRoots(Scene.java:513)
at javafx.scene.Scene.doLayoutPass(Scene.java:484)
at javafx.scene.Scene.access$3900(Scene.java:169)
at javafx.scene.Scene$ScenePulseListener.pulse(Scene.java:2199)
at com.sun.javafx.tk.Toolkit.firePulse(Toolkit.java:363)
at com.sun.javafx.tk.quantum.QuantumToolkit.pulse(QuantumToolkit.java:460)
at com.sun.javafx.tk.quantum.QuantumToolkit$9.run(QuantumToolkit.java:329)
at com.sun.glass.ui.InvokeLaterDispatcher$Future.run(InvokeLaterDispatcher.java:76)

If I just remove the if statement including the setGraphic out it works and no crashes appear.

Any Idea what the problem is here?

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1 Answer

Editorial Team · Answer 1 · 2026-06-15T14:23:27+00:00

Item can be null (for instance in an empty cell).

Write:

if (item != null && item.equals("test"))

Or another way to do the test because the constant string will never be null nor equal null:

if ("test".equals(item))

There may be other errors in your code, but that one seemed obvious on a quick review.

You might also be interested in the custom cell factory for row based processing in this question. It is very well written.

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