我在这里和其他地方研究过这个错误，但原因似乎很多样。 我是新手，并将一个原始的天气应用程序放在一起练习。输入一个城市和州，然后转到Weatherunderground，提取XML Feed，解析它并显示一些数据点。它是线程化的，工作正常。今天我添加了一个Intent来打开一个新的第二个屏幕并在那里显示数据。崩溃。不知道为什么。这是错误日志。我在日志中搜索了一些条款也没有成功。
08-14 14:03:27.314: E/AndroidRuntime(950): FATAL EXCEPTION: main
08-14 14:03:27.314: E/AndroidRuntime(950): java.lang.RuntimeException: Unable to instantiate activity ComponentInfo{com.glenngilchrist.weatherapp/com.glenngilchrist.weatherapp.WeatherMainActivity}: java.lang.NullPointerException
08-14 14:03:27.314: E/AndroidRuntime(950): at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2137)
08-14 14:03:27.314: E/AndroidRuntime(950): at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2261)
08-14 14:03:27.314: E/AndroidRuntime(950): at android.app.ActivityThread.access$600(ActivityThread.java:141)
08-14 14:03:27.314: E/AndroidRuntime(950): at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1256)
08-14 14:03:27.314: E/AndroidRuntime(950): at android.os.Handler.dispatchMessage(Handler.java:99)
08-14 14:03:27.314: E/AndroidRuntime(950): at android.os.Looper.loop(Looper.java:137)
08-14 14:03:27.314: E/AndroidRuntime(950): at android.app.ActivityThread.main(ActivityThread.java:5103)
08-14 14:03:27.314: E/AndroidRuntime(950): at java.lang.reflect.Method.invokeNative(Native Method)
08-14 14:03:27.314: E/AndroidRuntime(950): at java.lang.reflect.Method.invoke(Method.java:525)
08-14 14:03:27.314: E/AndroidRuntime(950): at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:737)
08-14 14:03:27.314: E/AndroidRuntime(950): at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:553)
08-14 14:03:27.314: E/AndroidRuntime(950): at dalvik.system.NativeStart.main(Native Method)
08-14 14:03:27.314: E/AndroidRuntime(950): Caused by: java.lang.NullPointerException
08-14 14:03:27.314: E/AndroidRuntime(950): at android.content.ContextWrapper.getPackageName(ContextWrapper.java:135)
08-14 14:03:27.314: E/AndroidRuntime(950): at android.content.ComponentName.<init>(ComponentName.java:75)
08-14 14:03:27.314: E/AndroidRuntime(950): at android.content.Intent.<init>(Intent.java:3662)
08-14 14:03:27.314: E/AndroidRuntime(950): at com.glenngilchrist.weatherapp.WeatherMainActivity.<init>(WeatherMainActivity.java:31)
08-14 14:03:27.314: E/AndroidRuntime(950): at java.lang.Class.newInstanceImpl(Native Method)
08-14 14:03:27.314: E/AndroidRuntime(950): at java.lang.Class.newInstance(Class.java:1130)
08-14 14:03:27.314: E/AndroidRuntime(950): at android.app.Instrumentation.newActivity(Instrumentation.java:1061)
08-14 14:03:27.314: E/AndroidRuntime(950): at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2128)
08-14 14:03:27.314: E/AndroidRuntime(950): ... 11 more
这是一些代码：
public class WeatherMainActivity extends Activity implements OnClickListener {
String WunderURL = "http://api.wunderground.com/api/1d4c3256ea6ea9fa/conditions/q/";
TextView tv; // text box for retrieved data
EditText city, state; // data entered by user
Intent i = new Intent(this, WeatherDetailDisplay.class);
//StrictMode.ThreadPolicy policy = new StrictMode.ThreadPolicy.Builder().permitAll().build();
Handler handler = new Handler() {
@Override
public void handleMessage(Message msg) {
tv = (TextView)findViewById(R.id.results);
Bundle bundle = msg.getData();
String string = bundle.getString("myKey");
if (string.equals("Valid")) {
// go to detail screen
String strcity = bundle.getString("myCity");
String strstate = bundle.getString("myState");
String strtempF = bundle.getString("myTempF");
i.putExtra("myCity", strcity);
i.putExtra("myState", strstate);
i.putExtra("myTempF", strtempF);
startActivity(i);
} else {
tv.setText(string);
}
}
};
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_weather_main);
//Button b = (Button)findViewById(R.id.button1);
tv = (TextView)findViewById(R.id.results);
city = (EditText)findViewById(R.id.city);
state = (EditText)findViewById(R.id.state);
city.setText("Burlington");
state.setText("VT");
//StrictMode.setThreadPolicy(policy);
}
@Override
public boolean onCreateOptionsMenu(Menu menu) {
// Inflate the menu; this adds items to the action bar if it is present.
getMenuInflater().inflate(R.menu.weather_main, menu);
return true;
}
@Override
public void onClick(View view) {
// run this is a different thread
Runnable runnable = new Runnable() {
public void run() {
// build a URl and go out to the web for data
String c = city.getText().toString();
String s = state.getText().toString();
StringBuilder url = new StringBuilder(WunderURL);
url.append(s+"/"+c+".xml");
String fullUrl = url.toString();
Message msg = handler.obtainMessage();
Bundle bundle = new Bundle();
try{
URL weatherURL = new URL(fullUrl);
// set up an xmlreader to parse the data
SAXParserFactory spf = SAXParserFactory.newInstance();
SAXParser sp = spf.newSAXParser();
XMLReader xreader = sp.getXMLReader();
// tell the reader to user our handler class
XMLHandler xh = new XMLHandler();
xreader.setContentHandler(xh);
// now open the stream
xreader.parse(new InputSource(weatherURL.openStream()));
//String text = xh.GetInfoString();
String city = xh.getCity();
String state = xh.getState();
String tempF = xh.getTemp();
if (city.contains("null")) {
//tv.setText("Invalid Location");
bundle.putString("myKey", "Invalid Location");
bundle.putString("myCity", "");
bundle.putString("myState", "");
bundle.putString("myTempF", "");
msg.setData(bundle);
handler.sendMessage(msg);
} else {
//tv.setText(text);
bundle.putString("myKey", "Valid");
bundle.putString("myCity", city);
bundle.putString("myState", state);
bundle.putString("myTempF", tempF);
msg.setData(bundle);
handler.sendMessage(msg);
}
} catch (Exception e) {
//tv.setText("error");
bundle.putString("myKey", "error");
bundle.putString("myCity", "");
bundle.putString("myState", "");
bundle.putString("myTempF", "");
msg.setData(bundle);
handler.sendMessage(msg);
}
}
};
Thread mythread = new Thread(runnable);
mythread.start();
}
}
答案 0 :(得分：0)
您需要确保在AndroidManifest.xml文件中定义了新活动。最低限度，它会是这样的：
<activity android:name=".TheNewActivityNameHere"/>
请发布主要天气活动的代码以获取更多详细信息。
答案 1 :(得分：0)
WeatherMainActivity.java:31
您正在为Android找不到的活动创建一个Intent。
这是我看到您的错误日志时的第一印象。
您可能想看看 ContextWrapper.getPackageName（ContextWrapper.java:135）
中导致NullPE的原因这很可能是像MiStr所说的配置错误。我同意你的看法，比NPE更清楚的错误信息： - ）
Caused by: java.lang.NullPointerException
08-14 14:03:27.314: E/AndroidRuntime(950): at android.content.ContextWrapper.getPackageName(ContextWrapper.java:135)
08-14 14:03:27.314: E/AndroidRuntime(950): at android.content.ComponentName.<init>(ComponentName.java:75)
08-14 14:03:27.314: E/AndroidRuntime(950): at android.content.Intent.<init>(Intent.java:3662)
08-14 14:03:27.314: E/AndroidRuntime(950): at com.glenngilchrist.weatherapp.WeatherMainActivity.<init>(WeatherMainActivity.java:31)