如何修复 NullPointerException 并将数据放入 NatTable

【问题标题】：How do I fix NullPointerException and putting data into NatTable如何修复 NullPointerException 并将数据放入 NatTable
【发布时间】：2018-05-05 12:39:26
【问题描述】：

当我尝试使用这段代码时：

students.add(new Student("John","1", 3));
students.add(new Student("Chris","2", 2));
NatTable table = createTable(shell, students);
table.refresh();

出现NullPointerException，可能是因为Student类的getter有问题或者生成表的时候出现了问题。出现此问题时的消息之一是 ReflectiveColumnPropertyAccessor.getDataValue。我还需要修复 NatTable 中的 ColumnGroup 以正确显示许多考试。

它给了我（我的代码中的第 48 行是学生构造函数）：

WARNING: java.lang.NullPointerException
java.lang.RuntimeException: java.lang.NullPointerException
    at net.sourceforge.nattable.data.ReflectiveColumnPropertyAccessor.getDataValue(ReflectiveColumnPropertyAccessor.java:48)
at net.sourceforge.nattable.data.ListDataProvider.getDataValue(ListDataProvider.java:36)
at net.sourceforge.nattable.layer.DataLayer.getDataValueByPosition(DataLayer.java:289)
at net.sourceforge.nattable.layer.AbstractLayerTransform.getDataValueByPosition(AbstractLayerTransform.java:319)
at net.sourceforge.nattable.layer.AbstractLayerTransform.getDataValueByPosition(AbstractLayerTransform.java:319)
at net.sourceforge.nattable.layer.AbstractLayerTransform.getDataValueByPosition(AbstractLayerTransform.java:319)
at net.sourceforge.nattable.layer.AbstractLayerTransform.getDataValueByPosition(AbstractLayerTransform.java:319)
at net.sourceforge.nattable.layer.AbstractLayerTransform.getDataValueByPosition(AbstractLayerTransform.java:319)
at net.sourceforge.nattable.layer.AbstractLayerTransform.getDataValueByPosition(AbstractLayerTransform.java:319)
at net.sourceforge.nattable.layer.AbstractLayerTransform.getDataValueByPosition(AbstractLayerTransform.java:319)
at net.sourceforge.nattable.layer.CompositeLayer.getDataValueByPosition(CompositeLayer.java:547)
at net.sourceforge.nattable.layer.cell.LayerCell.getDataValue(LayerCell.java:166)
at net.sourceforge.nattable.layer.cell.CellDisplayConversionUtils.convertDataType(CellDisplayConversionUtils.java:10)
at net.sourceforge.nattable.painter.cell.AbstractTextPainter.convertDataType(AbstractTextPainter.java:104)
at net.sourceforge.nattable.painter.cell.TextPainter.paintCell(TextPainter.java:96)
at net.sourceforge.nattable.painter.cell.CellPainterWrapper.paintCell(CellPainterWrapper.java:51)
at net.sourceforge.nattable.painter.cell.decorator.LineBorderDecorator.paintCell(LineBorderDecorator.java:64)
at net.sourceforge.nattable.painter.layer.CellLayerPainter.paintCell(CellLayerPainter.java:60)
at net.sourceforge.nattable.selection.SelectionLayerPainter.paintCell(SelectionLayerPainter.java:143)
at net.sourceforge.nattable.painter.layer.CellLayerPainter.paintLayer(CellLayerPainter.java:32)
at net.sourceforge.nattable.painter.layer.GridLineCellLayerPainter.paintLayer(GridLineCellLayerPainter.java:31)
at net.sourceforge.nattable.selection.SelectionLayerPainter.paintLayer(SelectionLayerPainter.java:34)
at net.sourceforge.nattable.layer.CompositeLayer$CompositeLayerPainter.paintLayer(CompositeLayer.java:898)
at net.sourceforge.nattable.painter.layer.NatLayerPainter.paintLayer(NatLayerPainter.java:26)
at net.sourceforge.nattable.NatTable.paintNatTable(NatTable.java:313)
at net.sourceforge.nattable.NatTable.paintControl(NatTable.java:309)
at org.eclipse.swt.widgets.TypedListener.handleEvent(Unknown Source)
at org.eclipse.swt.widgets.EventTable.sendEvent(Unknown Source)
at org.eclipse.swt.widgets.Display.sendEvent(Unknown Source)
at org.eclipse.swt.widgets.Widget.sendEvent(Unknown Source)
at org.eclipse.swt.widgets.Widget.sendEvent(Unknown Source)
at org.eclipse.swt.widgets.Widget.sendEvent(Unknown Source)
at org.eclipse.swt.widgets.Control.gtk_draw(Unknown Source)
at org.eclipse.swt.widgets.Composite.gtk_draw(Unknown Source)
at org.eclipse.swt.widgets.Canvas.gtk_draw(Unknown Source)
at org.eclipse.swt.widgets.Widget.windowProc(Unknown Source)
at org.eclipse.swt.widgets.Control.windowProc(Unknown Source)
at org.eclipse.swt.widgets.Display.windowProc(Unknown Source)
at org.eclipse.swt.internal.gtk.OS._gtk_main_do_event(Native Method)
at org.eclipse.swt.internal.gtk.OS.gtk_main_do_event(Unknown Source)
at org.eclipse.swt.widgets.Display.eventProc(Unknown Source)
at org.eclipse.swt.internal.gtk.OS._g_main_context_iteration(Native Method)
at org.eclipse.swt.internal.gtk.OS.g_main_context_iteration(Unknown Source)
at org.eclipse.swt.widgets.Display.readAndDispatch(Unknown Source)
at student.StudentMain.main(StudentMain.java:585)

我最初的考试和学生课程。

public static class Student {
    private String name;
    private String groupNumber;
    private Exam[] exams;
    public Student() { }
    public Student(String name, String groupNumber, int numbOfExams) {
        this.name = name;
        this.groupNumber = groupNumber;
        this.exams = new Exam[numbOfExams];
        for(int i=0;i<numbOfExams;i++) {
            exams[i]= new Exam();
            this.exams[i].setName("");
            this.exams[i].setMark(0);
        }           
    }
    public String getName() {
        return name;
    }
    public String getGroupNumber() {
        return groupNumber;
    }
    public void setName(String name) {
        this.name = name;
    }
    public void setGroupNumber(String groupNumber) {
        this.groupNumber = groupNumber;
    }
    public void setExam(Exam[] exams) {
        this.exams = exams;
    }
    public Exam[] getExam() {
        return exams;
    }       
}
public static class Exam {
    private String name;
    private int mark;
    public Exam() { }
    public Exam(String name, int mark) {
        this.name = name;
        this.mark = mark;
    }
    public String getName() {
        return name;
    }
    public int getMark() {
        return mark;
    }    
    public void setName(String name) {
        this.name = name;
    }
    public void setMark(int mark) {
        this.mark = mark;
    }
}

我对 NatTable 的实现

public static NatTable createTable(final Shell shell, ArrayList<Student> students) {
    final ColumnGroupModel columnGroupModel = new ColumnGroupModel();
    ColumnHeaderLayer columnHeaderLayer;

    String[] propertyNames = { "name", "groupNumber", "examName", "examMark" };
    Map<String, String> propertyToLabelMap = new HashMap<String, String>();
    propertyToLabelMap.put("name", "Full Name");
    propertyToLabelMap.put("groupNumber", "Group");
    propertyToLabelMap.put("examName", "Name");
    propertyToLabelMap.put("examMark", "Mark");
    DefaultBodyDataProvider<Student> bodyDataProvider = new DefaultBodyDataProvider<Student>(students, propertyNames);
    ColumnGroupBodyLayerStack bodyLayer = new ColumnGroupBodyLayerStack(new DataLayer(bodyDataProvider), columnGroupModel);

    DefaultColumnHeaderDataProvider defaultColumnHeaderDataProvider = new DefaultColumnHeaderDataProvider(propertyNames, propertyToLabelMap);
    DefaultColumnHeaderDataLayer columnHeaderDataLayer = new DefaultColumnHeaderDataLayer(defaultColumnHeaderDataProvider);
    columnHeaderLayer = new ColumnHeaderLayer(columnHeaderDataLayer, bodyLayer, bodyLayer.getSelectionLayer());
    ColumnGroupHeaderLayer columnGroupHeaderLayer = new ColumnGroupHeaderLayer(columnHeaderLayer, bodyLayer.getSelectionLayer(), columnGroupModel);

    columnGroupHeaderLayer.addColumnsIndexesToGroup("Exams", 2, 3);
    columnGroupHeaderLayer.setGroupUnbreakable(2);

    final DefaultRowHeaderDataProvider rowHeaderDataProvider = new DefaultRowHeaderDataProvider(bodyDataProvider);
    DefaultRowHeaderDataLayer rowHeaderDataLayer = new DefaultRowHeaderDataLayer(rowHeaderDataProvider);
    ILayer rowHeaderLayer = new RowHeaderLayer(rowHeaderDataLayer, bodyLayer, bodyLayer.getSelectionLayer());

    final DefaultCornerDataProvider cornerDataProvider = new DefaultCornerDataProvider(defaultColumnHeaderDataProvider, rowHeaderDataProvider);
    DataLayer cornerDataLayer = new DataLayer(cornerDataProvider);
    ILayer cornerLayer = new CornerLayer(cornerDataLayer, rowHeaderLayer, columnGroupHeaderLayer);

    GridLayer gridLayer = new GridLayer(
            bodyLayer,
            columnGroupHeaderLayer,
            rowHeaderLayer,
            cornerLayer);

    NatTable table = new NatTable(shell, gridLayer, true);
    return table;
}

【问题讨论】：

  • 你在哪一行得到 NPE？
  • 没有堆栈跟踪，没有行号，......谁能帮忙？
  • 第一个代码 sn-p 没有说明 shell 实例。您从哪里获得它或如何创建它？
  • 鉴于您调用this.exams[i].setMark(); 并且方法签名setMark(int mark) 需要一个int 值，您的代码如何编译？
  • 首先，您使用的是旧版本的 NatTable。 SourceForge 版本已被弃用数年，因为我们大约在六年前迁移到 Eclipse。您不会得到对该版本的太多支持，并且可能会遇到许多问题。我不明白为什么人们仍然使用那个。其次，您有嵌套对象（Exam 在 Student 下面）并且您希望可以通过反射神奇地检查它？实现自定义数据提供程序或使用具有正确属性定义的 ExtendedReflectiveDataProvider。查看我们的入门教程可能会有所帮助。

标签： java nullpointerexception swt nattable


【解决方案1】：

你的问题是你有嵌套的对象，你想通过反射来访问它们。那是行不通的！

如果您的 Student 只参加一次考试，您需要将 propertyNames 更改为：

String[] propertyNames = { "name", "groupNumber", "exam.name", "exam.mark" };

以及数据提供者的定义如下：

IDataProvider bodyDataProvider =
            new ListDataProvider<>(
                    students,
                    new ExtendedReflectiveColumnPropertyAccessor<Student>(propertyNames));

但是您的 Student 类有一个 Exam 对象数组。你想如何在表格中可视化这样的树结构？ NatTable 能够做到这一点，但是你需要更多的东西，比如TreeLayer 等。而这反过来又不适用于你的数据结构。

我建议首先考虑您想要可视化的内容，然后检查哪个控件适合您的需求（是表格还是树？），然后检查一些肯定存在的教程，例如 NatTable Getting Started Tutorial 或JFace Table tutorial 或 JFace Tree tutorial

【讨论】：

    猜你喜欢
    • 2017-12-15
    • 2020-04-23
    • 2020-11-26
    • 1970-01-01
    • 1970-01-01
    • 1970-01-01
    • 1970-01-01
    • 1970-01-01
    相关资源
    最近更新 更多
    热门标签
    Java Python linux javascript Mysql C# Docker 算法 前端 SpringBoot Redis Vue spring 设计模式 .net core .net kubernetes c++ 数据库 数据结构 大数据 js 机器学习 微服务 Android Go 程序员 面试 JVM ASP.net core 云原生 人工智能 后端 PHP git CSS golang k8s Nginx Django mybatis 深度学习 多线程 React 架构 devops 爬虫 云计算 Spring Boot LeetCode