大家好！
我正在努力纠正我在开发Android应用时遇到的这个错误。我试图在模拟器中运行，但它崩溃并给出nullpointerexception错误。然后，我创建了一个新项目，并从头开始将这些文件复制到一个新项目中，仍然是同样的错误。然后，我将应用程序的一部分变成了一个新项目，但它仍然给出了同样的错误。 该错误与启动我正在使用的活动有关。 以下是给定的必要文件，我希望有人可以指导我找到正确的方向并告诉我在做什么错误。 非常感谢！
DISPLAYPOINTS.JAVA
package com.example.safedrive;
import java.io.BufferedReader;
import java.io.InputStream;
import java.io.InputStreamReader;
import java.util.ArrayList;
import org.apache.http.HttpEntity;
import org.apache.http.HttpResponse;
import org.apache.http.NameValuePair;
import org.apache.http.client.HttpClient;
import org.apache.http.client.entity.UrlEncodedFormEntity;
import org.apache.http.client.methods.HttpPost;
import org.apache.http.impl.client.DefaultHttpClient;
import org.apache.http.message.BasicNameValuePair;
import org.json.JSONObject;
import android.text.Html;
import android.app.Activity;
import android.os.AsyncTask;
import android.os.Bundle;
import android.util.Log;
import android.view.Menu;
import android.view.MenuItem;
import android.view.View;
import android.widget.Button;
import android.widget.TextView;
import android.widget.Toast;
public class DisplayPoints extends Activity {
String name = "Ammaar";
TextView display_points;
Button btn_start = (Button) findViewById(R.id.btnStart); //Start Button Variable
String displayPoints;
InputStream is=null;
String result=null;
String line=null;
int points;
int code;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.display_points);
TextView lblName = (TextView) findViewById(R.id.lblName);
display_points = (TextView) findViewById(R.id.lblCurrPoints); //POINTS TO DISPLAY IN
lblName.setText(Html.fromHtml("Welcome <b>" + name + "! </b>"));
new Insert().execute();
btn_start.setOnClickListener(new View.OnClickListener() {
public void onClick(View arg0) {
//MainScreen.IsStarted=true;
//SpeedPoints start = new SpeedPoints();
//Intent i = new Intent(getApplicationContext(), SpeedPoints.class);
//startActivity(i);
Toast.makeText(getApplicationContext(), "The app has started monitoring your drive...",
Toast.LENGTH_LONG).show();
//start.startAnimation();
}
});
}
class Insert extends AsyncTask<String, Void, String> {
// Do the long-running work in here
protected String doInBackground(String... args) {
ArrayList<NameValuePair> nameValuePairs = new ArrayList<NameValuePair>(1);
nameValuePairs.add(new BasicNameValuePair("fname", name));
try
{
HttpClient httpclient = new DefaultHttpClient();
HttpPost httppost = new HttpPost("http://ammar123.net84.net/capstone/displayPoints.php");
httppost.setEntity(new UrlEncodedFormEntity(nameValuePairs));
HttpResponse response = httpclient.execute(httppost);
HttpEntity entity = response.getEntity();
is = entity.getContent();
Log.e("pass 1", "connection success ");
}
catch(Exception e)
{
Log.e("Fail 1", e.toString());
Toast.makeText(getApplicationContext(), "Invalid IP Address",
Toast.LENGTH_LONG).show();
}
try
{
BufferedReader reader = new BufferedReader
(new InputStreamReader(is,"iso-8859-1"),8);
StringBuilder sb = new StringBuilder();
while ((line = reader.readLine()) != null)
{
sb.append(line + "\n");
}
is.close();
result = sb.toString();
Log.e("pass 2", "connection success ");
}
catch(Exception e)
{
Log.e("Fail 2", e.toString());
}
try
{
JSONObject json_data = new JSONObject(result);
code=(json_data.getInt("code"));
points=(json_data.getInt("pointss"));
displayPoints = Integer.toString(points);
}
catch(Exception e)
{
Log.e("Fail 3", e.toString());
}
return displayPoints;
}//End of doInBackground method
protected void onPostExecute(String displaypoints) {
display_points.setText(displayPoints);
Toast.makeText(getBaseContext(), "Points Retrieved!",
Toast.LENGTH_SHORT).show();
}//End of onPostExecute
}
@Override
public boolean onCreateOptionsMenu(Menu menu) {
// Inflate the menu; this adds items to the action bar if it is present.
getMenuInflater().inflate(R.menu.display_points, menu);
return true;
}
@Override
public boolean onOptionsItemSelected(MenuItem item) {
// Handle action bar item clicks here. The action bar will
// automatically handle clicks on the Home/Up button, so long
// as you specify a parent activity in AndroidManifest.xml.
int id = item.getItemId();
if (id == R.id.action_settings) {
return true;
}
return super.onOptionsItemSelected(item);
}
}
预告文件
<?xml version="1.0" encoding="utf-8"?>
<manifest xmlns:android="http://schemas.android.com/apk/res/android"
package="com.example.safedrive"
android:versionCode="1"
android:versionName="1.0" >
<uses-sdk
android:minSdkVersion="8"
android:targetSdkVersion="21" />
<uses-permission android:name="android.permission.INTERNET"/>
<application
android:allowBackup="true"
android:icon="@drawable/ic_launcher"
android:label="@string/app_name"
android:theme="@style/AppTheme" >
<activity
android:name="com.example.safedrive.DisplayPoints"
android:label="@string/app_name" >
<intent-filter>
<action android:name="android.intent.action.MAIN" />
<category android:name="android.intent.category.LAUNCHER" />
</intent-filter>
</activity>
</application>
</manifest>
LOGCAT错误：
06-09 18:52:55.467: ERROR/ResourceType(87): Style contains key with bad entry: 0x01010479
06-09 18:52:56.567: ERROR/AndroidRuntime(771): FATAL EXCEPTION: main
06-09 18:52:56.567: ERROR/AndroidRuntime(771): java.lang.RuntimeException: Unable to instantiate activity ComponentInfo{com.example.safedrive/com.example.safedrive.DisplayPoints}: java.lang.NullPointerException
06-09 18:52:56.567: ERROR/AndroidRuntime(771): at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:1879)
06-09 18:52:56.567: ERROR/AndroidRuntime(771): at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:1980)
06-09 18:52:56.567: ERROR/AndroidRuntime(771): at android.app.ActivityThread.access$600(ActivityThread.java:122)
06-09 18:52:56.567: ERROR/AndroidRuntime(771): at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1146)
06-09 18:52:56.567: ERROR/AndroidRuntime(771): at android.os.Handler.dispatchMessage(Handler.java:99)
06-09 18:52:56.567: ERROR/AndroidRuntime(771): at android.os.Looper.loop(Looper.java:137)
06-09 18:52:56.567: ERROR/AndroidRuntime(771): at android.app.ActivityThread.main(ActivityThread.java:4340)
06-09 18:52:56.567: ERROR/AndroidRuntime(771): at java.lang.reflect.Method.invokeNative(Native Method)
06-09 18:52:56.567: ERROR/AndroidRuntime(771): at java.lang.reflect.Method.invoke(Method.java:511)
06-09 18:52:56.567: ERROR/AndroidRuntime(771): at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:784)
06-09 18:52:56.567: ERROR/AndroidRuntime(771): at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:551)
06-09 18:52:56.567: ERROR/AndroidRuntime(771): at dalvik.system.NativeStart.main(Native Method)
06-09 18:52:56.567: ERROR/AndroidRuntime(771): Caused by: java.lang.NullPointerException
06-09 18:52:56.567: ERROR/AndroidRuntime(771): at android.app.Activity.findViewById(Activity.java:1794)
06-09 18:52:56.567: ERROR/AndroidRuntime(771): at com.example.safedrive.DisplayPoints.<init>(DisplayPoints.java:34)
06-09 18:52:56.567: ERROR/AndroidRuntime(771): at java.lang.Class.newInstanceImpl(Native Method)
06-09 18:52:56.567: ERROR/AndroidRuntime(771): at java.lang.Class.newInstance(Class.java:1319)
06-09 18:52:56.567: ERROR/AndroidRuntime(771): at android.app.Instrumentation.newActivity(Instrumentation.java:1023)
06-09 18:52:56.567: ERROR/AndroidRuntime(771): at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:1870)
06-09 18:52:56.567: ERROR/AndroidRuntime(771): ... 11 more
答案 0 :(得分：2)
把这一行
btn_start = (Button) findViewById(R.id.btnStart); //Start Button Variable
后
setContentView(R.layout.display_points);
findViewById仅在定义布局后才可用。