java - 如何防止列表字段自动刷新后显示重复数据

Question

我使用下面的代码每分钟刷新一个列表字段，其中包含从 xml Web 服务返回的数据。但第一次刷新后，数据会重复。如何防止重复？

public final class MyScreen extends MainScreen implements ListFieldCallback
{
    ResponseHandler handler = new ResponseHandler();
    ListField listUsers = new ListField(handler.getItem().size());
    Manager mainManager = getMainManager();

    protected void onUiEngineAttached(boolean attached) {
        if (attached) {
            Timer timer = new Timer();
            // schedule the web service task to run every minute
            timer.schedule(new WebServiceTask(), 0, 3000);
        }
    }

    public MyScreen() {
        setTitle("Uikro");
        listUsers.setEmptyString("No Users found", 0);
        listUsers.setCallback(this);
        add(listUsers);
        handler.getItem().removeAllElements();
    }

    private class WebServiceTask extends TimerTask {
        public void run() {
            //Fetch the xml from the web service
            String wsReturnString = GlobalV.Fetch_Webservice("myDs");
            //Parse returned xml
            SAXParserImpl saxparser = new SAXParserImpl();
            ByteArrayInputStream stream = new ByteArrayInputStream(wsReturnString.getBytes());
            try {
                saxparser.parse( stream, handler );
            }
            catch ( Exception e ) {
                response.setText( "Unable to parse response.");
            }

            // now, update the UI back on the UI thread:
            UiApplication.getUiApplication().invokeLater(new Runnable() {
                public void run() {
                    //Return vector sze from the handler class
                    int currentIndex = listUsers.getSelectedIndex();
                    int scrollPosition = getMainManager().getVerticalScroll();
                    //Return vector sze from the handler class
                    listUsers.setSize(handler.getItem().size());
                    listUsers.setSelectedIndex(currentIndex);
                    getMainManager().setVerticalScroll(scrollPosition);
                }
            });
        }
    }

    public void drawListRow(ListField list, Graphics g, int index, int y, int w) {
        String personToDraw = (String) this.get(list, index);
        int drawColor = Color.RED;
        g.setColor(drawColor);
        g.drawText(personToDraw, 0, y, 0, w);
    }

    // get the selected index from the correct Vector
    public Object get(ListField list, int index) {
        return handler.getItem().elementAt(index);
    }

    public void insert(String toInsert, int index) {
        handler.getItem().insertElementAt(toInsert, index);
    }

    public int getPreferredWidth(ListField list) {
        return Display.getWidth();
    }

    public int indexOfList(ListField listField, String prefix, int start) {
        return handler.getItem().indexOf(prefix, start);
    }
}

public class ResponseHandler extends DefaultHandler
{
    boolean username=false;
    Vector _vectorUser = new Vector();

    public void startElement(String uri, String localName, String qName, Attributes attributes){
        if (qName.equalsIgnoreCase("U_Username")){
            username = true;
        }
    }

    public void characters(char[] ch, int start, int length) {
        if (username){
            _vectorUser.addElement(new String( ch, start, length ));
            username=false;
        }
    }

    Vector getItem(){
        return _vectorUser;
    }
}

Answer 1

所以，我认为问题出在 ResponseHandler 类中。该类使用成员变量_vectorUser来存储数据。该 vector 是在创建 ResponseHandler 时创建的，并在调用 characters(char[],int,int) 方法时添加元素。

但是，_vectorUser从未真正被清除(它在使用之前已被清除，但这并不能阻止此问题)。

因此，您应该在每次向网络服务请求新数据时清除 vector :

private class WebServiceTask extends TimerTask {
    public void run() {
        //Fetch the xml from the web service
        String wsReturnString = GlobalV.Fetch_Webservice("myDs");
        //Parse returned xml
        SAXParserImpl saxparser = new SAXParserImpl();
        ByteArrayInputStream stream = new ByteArrayInputStream(wsReturnString.getBytes());
        try {
            // clear out the vector first
            handler.getItem().removeAllElements();
            saxparser.parse( stream, handler );
        }
        catch ( Exception e ) {
            response.setText( "Unable to parse response.");
        }

或者，只需创建一个新的ResponseHandler，它也将创建一个新的 vector :

private class WebServiceTask extends TimerTask {
    public void run() {
        //Fetch the xml from the web service
        String wsReturnString = GlobalV.Fetch_Webservice("myDs");
        //Parse returned xml
        SAXParserImpl saxparser = new SAXParserImpl();
        ByteArrayInputStream stream = new ByteArrayInputStream(wsReturnString.getBytes());
        try {
            // create a new handler
            handler = new ResponseHandler();
            saxparser.parse( stream, handler );
        }
        catch ( Exception e ) {
            response.setText( "Unable to parse response.");
        }