在Head First Android开发中无法从rss feed nasa读取

Question

我是Android的新手。我尝试从头部优先Android开发教程中的rss feed获取数据。但是，当我运行应用程序时出现错误。 这是我遇到的错误

05-12 09:48:15.833: E/AndroidRuntime(2135): FATAL EXCEPTION: main
05-12 09:48:15.833: E/AndroidRuntime(2135): java.lang.RuntimeException: Unable to start activity ComponentInfo{com.example.nasadailyimage/com.example.nasadailyimage.NasaDailyImage}: java.lang.NullPointerException
05-12 09:48:15.833: E/AndroidRuntime(2135):     at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:1647)
05-12 09:48:15.833: E/AndroidRuntime(2135):     at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:1663)
05-12 09:48:15.833: E/AndroidRuntime(2135):     at android.app.ActivityThread.access$1500(ActivityThread.java:117)
05-12 09:48:15.833: E/AndroidRuntime(2135):     at android.app.ActivityThread$H.handleMessage(ActivityThread.java:931)
05-12 09:48:15.833: E/AndroidRuntime(2135):     at android.os.Handler.dispatchMessage(Handler.java:99)
05-12 09:48:15.833: E/AndroidRuntime(2135):     at android.os.Looper.loop(Looper.java:130)
05-12 09:48:15.833: E/AndroidRuntime(2135):     at android.app.ActivityThread.main(ActivityThread.java:3683)
05-12 09:48:15.833: E/AndroidRuntime(2135):     at java.lang.reflect.Method.invokeNative(Native Method)
05-12 09:48:15.833: E/AndroidRuntime(2135):     at java.lang.reflect.Method.invoke(Method.java:507)
05-12 09:48:15.833: E/AndroidRuntime(2135):     at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:839)
05-12 09:48:15.833: E/AndroidRuntime(2135):     at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:597)
05-12 09:48:15.833: E/AndroidRuntime(2135):     at dalvik.system.NativeStart.main(Native Method)
05-12 09:48:15.833: E/AndroidRuntime(2135): Caused by: java.lang.NullPointerException
05-12 09:48:15.833: E/AndroidRuntime(2135):     at com.example.nasadailyimage.NasaDailyImage.resetDisplay(NasaDailyImage.java:74)
05-12 09:48:15.833: E/AndroidRuntime(2135):     at com.example.nasadailyimage.NasaDailyImage.onCreate(NasaDailyImage.java:32)
05-12 09:48:15.833: E/AndroidRuntime(2135):     at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1047)
05-12 09:48:15.833: E/AndroidRuntime(2135):     at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:1611)
05-12 09:48:15.833: E/AndroidRuntime(2135):     ... 11 more

这是我的IotdHandler.java

package com.example.nasadailyimage;

import java.io.IOException;
import java.io.InputStream;
import java.net.HttpURLConnection;
import java.net.URL;
import java.util.jar.Attributes;

import javax.xml.parsers.SAXParser;
import javax.xml.parsers.SAXParserFactory;

import org.xml.sax.InputSource;
import org.xml.sax.SAXException;
import org.xml.sax.XMLReader;
import org.xml.sax.helpers.DefaultHandler;

import android.graphics.Bitmap;
import android.graphics.BitmapFactory;

public class IotdHandler extends DefaultHandler {
private String url = "http://www.nasa.gov/rss/dyn/image_of_the_day.rss";
//private String url = "http://www.nasa.gov/rss/image_of_the_day.rss";
//"http://www.nasa.gov/rss/dyn/image_of_the_day.rss"
private boolean inUrl = false;
private boolean inTitle = false;
private boolean inDescription = false;
private boolean inItem = false;
private boolean inDate = false;
private Bitmap image = null;
private String title = null;
private StringBuffer description = new StringBuffer();
private String date = null;

    public void processFeed() {
        try {
            SAXParserFactory factory = SAXParserFactory.newInstance();
            SAXParser parser = factory.newSAXParser();
            XMLReader reader = parser.getXMLReader();
            reader.setContentHandler(this);
            InputStream inputStream = new URL(url).openStream();
            reader.parse(new InputSource(inputStream));
        } catch (Exception e) {}
    }

    private Bitmap getBitmap(String url) {
        try {
            HttpURLConnection connection = (HttpURLConnection)new URL(url).openConnection();
            connection.setDoInput(true);
            connection.connect();
            InputStream input = connection.getInputStream();
            Bitmap bitmap = BitmapFactory.decodeStream(input);
            input.close();
            return bitmap;
        } catch (IOException ioe) { return null; }
    }

    public void startElement(String uri, String localName, String qName, Attributes attributes) throws SAXException {
        if (localName.equals("url")) { inUrl = true; }
        else { inUrl = false; }

        if (localName.startsWith("item")) { inItem = true; }
        else 
        if (inItem) {
            if (localName.equals("title")) { inTitle = true; }
            else { inTitle = false; }

            if (localName.equals("description")) { inDescription = true; }
            else { inDescription = false; }

            if (localName.equals("pubDate")) { inDate = true; }
            else { inDate = false; }
        }
    }

    public void characters(char ch[], int start, int length) {
        String chars = new String(ch).substring(start, start + length);

        if (inUrl && url == null) { image = getBitmap(chars); }
        if (inTitle && title == null) { title = chars; }
        if (inDescription) { description.append(chars); }
            if (inDate && date == null) { date = chars; }
    }   

    public Bitmap getImage() { return image; }
    /*public String getImage() { return image.toString(); }*/
    public String getTitle() { return title; }
    public StringBuffer getDescription() { return description; }
    public String getDate() { return date; }
}

我在Activity类

中添加了一些代码

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_nasa_daily_image);

    if (savedInstanceState == null) {
        getSupportFragmentManager().beginTransaction()
                .add(R.id.container, new PlaceholderFragment())
                .commit();
    }

    IotdHandler handler = new IotdHandler();
    handler.processFeed();
    resetDisplay(handler.getTitle(), handler.getDate(), handler.getImage(), handler.getDescription());
}

private void resetDisplay(String title, String date, Bitmap imageUrl, StringBuffer description) {
    TextView titleView = (TextView) findViewById(R.id.imageTitle);
    titleView.setText(title);

    TextView dateView = (TextView) findViewById(R.id.imageDate);
    dateView.setText(date);

    ImageView imageView = (ImageView) findViewById(R.id.imageDisplay);
    imageView.setImageBitmap(imageUrl);

    TextView descriptionView = (TextView) findViewById(R.id.imageDescription);    
    descriptionView.setText(description);
}

如何解决此错误？