java - 在文本字段上调用setPropertyDataSource时，自定义转换器将引发java.lang.ClassCastException

我正在尝试为TextField实现自定义转换器，以将其转换为自定义数据类型（CellElement）。我正在按照Book of Vaadin中的教程进行操作。
当我在TextField上调用setPropertyDataSource()时，调试器将抛出java.lang.ClassCastException: java.lang.String cannot be cast to com.example.demo.CellElement

    board = new Board();//this initializes all the cells

    // connect the tile to the display
    for( int col = 0; col < 9; col++ )
        for( int row = 0; row < 9; row++ )
        {
            TextField cell_text = new TextField();
            cell_text.setConverter(new StringToCellElementConverter());
            Property temp_ce = board.getCellElement(col, row);
            //error on next line
            cell_text.setPropertyDataSource(temp_ce);

            grid.addComponent( cell_text, col, row );
        }

我认为在实现stringToCellElementConverter类时犯了一个错误，但是我无法确定是什么。我遵循了调试器步骤，并在AbstractField：内的这段代码中引发错误：

@Override
public void setPropertyDataSource(Property newDataSource) {

    ...

    // Gets the value from source. This requires that a valid converter has
    // been set.
    try {
        if (dataSource != null) {
            //next line throws error!
            T fieldValue = convertFromModel(getDataSourceValue());
            setInternalValue(fieldValue);
        }
        setModified(false);
        if (getCurrentBufferedSourceException() != null) {
            setCurrentBufferedSourceException(null);
        }
    } catch (final Throwable e) {
        setCurrentBufferedSourceException(new Buffered.SourceException(
                this, e));
        setModified(true);
        throw getCurrentBufferedSourceException();
    }

这是自定义Converter的代码：

public class StringToCellElementConverter implements Converter<String, CellElement> {

    private static final long serialVersionUID = -3138155389689170574L;

    @Override
    public CellElement convertToModel(String value,
            Class<? extends CellElement> targetType, Locale locale)
            throws com.vaadin.data.util.converter.Converter.ConversionException {
        return new CellElement(Integer.parseInt(value), true);
    }

    @Override
    public String convertToPresentation(CellElement value,
            Class<? extends String> targetType, Locale locale)
            throws com.vaadin.data.util.converter.Converter.ConversionException {
        String ret_val = value.getValue();
        return ret_val;
    }

    @Override
    public Class<CellElement> getModelType() {
        return CellElement.class;
    }

    @Override
    public Class<String> getPresentationType() {
        return String.class;
    }

}

CellElement的代码。我可能在此类中遇到一些配置问题。它最初不是为此目的而设计的：

public class CellElement implements Property
{

    private static final long serialVersionUID = -6205569838194226408L;
    private Integer value;
    private boolean readOnly;
    private boolean selected;

    public CellElement( Integer value, boolean readOnly )
    {
        this.value = value;
        this.readOnly = readOnly;
        this.selected = false;
    }

    public boolean isSelected()
    {
        return selected;
    }

    public void setSelected( boolean selected )
    {
        this.selected = selected;
    }

    public Integer getIntegerValue()
    {
        return value;
    }

    public int getIntValue()
    {
        return value.intValue();
    }

    @Override
    public String getValue()
    {
        return value.toString();
    }

    @Override
    public void setValue(Object newValue)
                throws ReadOnlyException, ConversionException {
            if (readOnly)
                throw new ReadOnlyException();

            // Already the same type as the internal representation
            if (newValue instanceof Integer)
                value = (Integer) newValue;

            // Conversion from a string is required
            else if (newValue instanceof String)
                try {
                    value = Integer.parseInt((String) newValue);
                } catch (NumberFormatException e) {
                    throw new ConversionException();
                }
            else
                 // Don't know how to convert any other types
                throw new ConversionException();
        }

    @Override
    public Class getType() {
        return this.getClass();
    }

    @Override
    public boolean isReadOnly() {
        return this.readOnly;
    }

    @Override
    public void setReadOnly(boolean newStatus) {
        this.readOnly = newStatus;
    }
}

这是堆栈跟踪：：

com.vaadin.data.Buffered$SourceException
    at com.vaadin.ui.AbstractField.setPropertyDataSource(AbstractField.java:637)
    at com.example.demo.Sudoku.init(Sudoku.java:134)
    at com.vaadin.ui.UI.doInit(UI.java:639)
    at com.vaadin.server.communication.UIInitHandler.getBrowserDetailsUI(UIInitHandler.java:222)
    at com.vaadin.server.communication.UIInitHandler.synchronizedHandleRequest(UIInitHandler.java:74)
    at com.vaadin.server.SynchronizedRequestHandler.handleRequest(SynchronizedRequestHandler.java:41)
    at com.vaadin.server.VaadinService.handleRequest(VaadinService.java:1402)
    at com.vaadin.server.VaadinServlet.service(VaadinServlet.java:305)
    at javax.servlet.http.HttpServlet.service(HttpServlet.java:727)
    at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:303)
    at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:208)
    at org.apache.tomcat.websocket.server.WsFilter.doFilter(WsFilter.java:52)
    at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:241)
    at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:208)
    at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:220)
    at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:122)
    at org.apache.catalina.authenticator.AuthenticatorBase.invoke(AuthenticatorBase.java:501)
    at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:171)
    at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:103)
    at org.apache.catalina.valves.AccessLogValve.invoke(AccessLogValve.java:950)
    at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:116)
    at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:408)
    at org.apache.coyote.http11.AbstractHttp11Processor.process(AbstractHttp11Processor.java:1070)
    at org.apache.coyote.AbstractProtocol$AbstractConnectionHandler.process(AbstractProtocol.java:611)
    at org.apache.tomcat.util.net.JIoEndpoint$SocketProcessor.run(JIoEndpoint.java:316)
    at java.util.concurrent.ThreadPoolExecutor.runWorker(Unknown Source)
    at java.util.concurrent.ThreadPoolExecutor$Worker.run(Unknown Source)
    at org.apache.tomcat.util.threads.TaskThread$WrappingRunnable.run(TaskThread.java:61)
    at java.lang.Thread.run(Unknown Source)
Caused by: java.lang.ClassCastException: java.lang.String cannot be cast to com.example.demo.CellElement
    at com.example.demo.StringToCellElementConverter.convertToPresentation(StringToCellElementConverter.java:1)
    at com.vaadin.data.util.converter.ConverterUtil.convertFromModel(ConverterUtil.java:94)
    at com.vaadin.ui.AbstractField.convertFromModel(AbstractField.java:708)
    at com.vaadin.ui.AbstractField.convertFromModel(AbstractField.java:693)
    at com.vaadin.ui.AbstractField.setPropertyDataSource(AbstractField.java:629)
    ... 28 more

Nov 21, 2014 4:01:31 PM org.apache.catalina.core.StandardWrapperValve invoke
SEVERE: Servlet.service() for servlet [com.example.demo.Sudoku$Servlet] in context with path [/VaadinUltimate] threw exception [com.vaadin.server.ServiceException: com.vaadin.data.Buffered$SourceException] with root cause
java.lang.ClassCastException: java.lang.String cannot be cast to com.example.demo.CellElement
    at com.example.demo.StringToCellElementConverter.convertToPresentation(StringToCellElementConverter.java:1)
    at com.vaadin.data.util.converter.ConverterUtil.convertFromModel(ConverterUtil.java:94)
    at com.vaadin.ui.AbstractField.convertFromModel(AbstractField.java:708)
    at com.vaadin.ui.AbstractField.convertFromModel(AbstractField.java:693)
    at com.vaadin.ui.AbstractField.setPropertyDataSource(AbstractField.java:629)
    at com.example.demo.Sudoku.init(Sudoku.java:134)
    at com.vaadin.ui.UI.doInit(UI.java:639)
    at com.vaadin.server.communication.UIInitHandler.getBrowserDetailsUI(UIInitHandler.java:222)
    at com.vaadin.server.communication.UIInitHandler.synchronizedHandleRequest(UIInitHandler.java:74)
    at com.vaadin.server.SynchronizedRequestHandler.handleRequest(SynchronizedRequestHandler.java:41)
    at com.vaadin.server.VaadinService.handleRequest(VaadinService.java:1402)
    at com.vaadin.server.VaadinServlet.service(VaadinServlet.java:305)
    at javax.servlet.http.HttpServlet.service(HttpServlet.java:727)
    at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:303)
    at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:208)
    at org.apache.tomcat.websocket.server.WsFilter.doFilter(WsFilter.java:52)
    at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:241)
    at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:208)
    at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:220)
    at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:122)
    at org.apache.catalina.authenticator.AuthenticatorBase.invoke(AuthenticatorBase.java:501)
    at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:171)
    at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:103)
    at org.apache.catalina.valves.AccessLogValve.invoke(AccessLogValve.java:950)
    at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:116)
    at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:408)
    at org.apache.coyote.http11.AbstractHttp11Processor.process(AbstractHttp11Processor.java:1070)
    at org.apache.coyote.AbstractProtocol$AbstractConnectionHandler.process(AbstractProtocol.java:611)
    at org.apache.tomcat.util.net.JIoEndpoint$SocketProcessor.run(JIoEndpoint.java:316)
    at java.util.concurrent.ThreadPoolExecutor.runWorker(Unknown Source)
    at java.util.concurrent.ThreadPoolExecutor$Worker.run(Unknown Source)
    at org.apache.tomcat.util.threads.TaskThread$WrappingRunnable.run(TaskThread.java:61)
    at java.lang.Thread.run(Unknown Source)

最佳答案

一切似乎还好。由于您没有提供堆栈跟踪，所以我最好的选择是转换器中的异常处理。

 @Override
public CellElement convertToModel(String value,
        Class<? extends CellElement> targetType, Locale locale)
        throws com.vaadin.data.util.converter.Converter.ConversionException {
    return new CellElement(Integer.parseInt(value), true);
}

@Override
public String convertToPresentation(CellElement value,
        Class<? extends String> targetType, Locale locale)
        throws com.vaadin.data.util.converter.Converter.ConversionException {
    String ret_val = value.getValue();
    return ret_val;
}

如果Integer.parseInt或value.getValue失败，这两种方法都将引发异常。
尝试根据您的企业登录信息添加尝试/捕获流程

try {
    return new CellElement(Integer.parseInt(value), true);
} catch (Exception e) {
    //value is null or NaN
    return new CellElement(0, true);
}

对于第一个或

if (value == null) {
    return "";
} else {
    return value.getValue();
}

第二。

问候