关于 android xmlSerializer 的问题

Question

package com.pro.XMLHandle;import java.io.StringWriter;import org.xmlpull.v1.XmlPullParserFactory;
import org.xmlpull.v1.XmlSerializer;import android.util.Log;import com.pro.entity.PhotoInfo;public class XMLInfoAdder { private StringWriter stringWriter;
private XmlPullParserFactory factory;
private XmlSerializer xmlSerializer; public String addPhotoInfo(PhotoInfo photoInfo) { try {
stringWriter = new StringWriter();
factory = XmlPullParserFactory.newInstance();
xmlSerializer = factory.newSerializer();
xmlSerializer.setOutput(stringWriter);
String[] tags = new String[] { "photo", "street", "building",
"direction", "describe" };
Log.i("myInfo", photoInfo.getPhoto() + photoInfo.getStreet()
+ photoInfo.getBuilding() + photoInfo.getDirection()
+ photoInfo.getDescribe());
String[] texts = new String[] { photoInfo.getPhoto(),
photoInfo.getStreet(), photoInfo.getBuilding(),
photoInfo.getDirection(), photoInfo.getDescribe() };
xmlSerializer.startTag(null, "photoInfo");
for (int i = 0; i < tags.length; i++) {
Log.i("myInfo", i + "");
xmlSerializer.startTag(null, tags[i]);
xmlSerializer.text(texts[i]);
xmlSerializer.endTag(null, tags[i]);
}
xmlSerializer.endTag(null, "photoInfo"); } catch (Exception e) {
e.printStackTrace();
}
System.out.println(stringWriter.toString());
return stringWriter.toString();
} public String addXMLHead() {
try {
stringWriter = new StringWriter();
factory = XmlPullParserFactory.newInstance();
xmlSerializer = factory.newSerializer();
xmlSerializer.setOutput(stringWriter); xmlSerializer.startDocument("utf-8", true);
xmlSerializer
.startTag("http://picture_world.pro.com", "photoInfos"); } catch (Exception e) {
e.printStackTrace();
}
return stringWriter.toString();
} public String addXMLTail() {
try {
stringWriter = new StringWriter();
factory = XmlPullParserFactory.newInstance();
xmlSerializer = factory.newSerializer();
xmlSerializer.setOutput(stringWriter); xmlSerializer.endTag(null, "photoInfos");
xmlSerializer.endDocument(); } catch (Exception e) {
e.printStackTrace();
}
return stringWriter.toString();
}}
为什么返回值为空呢 ？哪里错了吗？该得到的参数都不是空的