尝试从Facebook Graph API接收数据，但我似乎得到了NullPointerException。有人可以帮忙吗？这是我的活动文件，包含xml和logCat输出。
package com.example.fbtest;
//省略所有导入语句。
public class TestActivity extends Activity implements OnClickListener {
String APP_ID;
Facebook fb;
private SharedPreferences sp;
TextView tv;
ImageView login;
Button b;
@Override
public void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_test);
APP_ID = getString(R.string.APP_ID);
fb = new Facebook(APP_ID);
sp = getPreferences(MODE_PRIVATE);
String access_token = sp.getString("access_token", null);
long expires = sp.getLong("access_expires", 0);
if (access_token != null) {
fb.setAccessToken(access_token);
}
if (expires != 0) {
fb.setAccessExpires(expires);
}
login = (ImageView) findViewById(R.id.login);
login.setOnClickListener(this);
b=(Button)findViewById(R.id.button1);
b.setOnClickListener(this);
}
@Override
public boolean onCreateOptionsMenu(Menu menu) {
getMenuInflater().inflate(R.menu.activity_test, menu);
return true;
}
private void post() {
if (fb.isSessionValid()) {
login.setImageResource(R.drawable.logout_button);
JSONObject obj;
try {
obj = Util.parseJson(fb.request("ronakshah"));
String id = obj.optString("id");
String name = obj.optString("name");
tv.setText("Welcome, " + name + id);
} catch (MalformedURLException e) {
e.printStackTrace();
} catch (JSONException e) {
e.printStackTrace();
} catch (FacebookError fe) {
} catch (IOException e) {
e.printStackTrace();
}
}
}
@Override
public void onClick(View v) {
// TODO Auto-generated method stub
switch (v.getId()) {
case R.id.login:
if (fb.isSessionValid()) {
// button close our session - log out of facebook
try {
fb.logout(getApplicationContext());
} catch (MalformedURLException e) {
// TODO Auto-generated catch block
e.printStackTrace();
} catch (IOException e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
} else {
// login to facebook
fb.authorize(TestActivity.this, new String[] { "email",
"read_stream" }, new DialogListener() {
public void onFacebookError(FacebookError e) {
// TODO Auto-generated method stub
Toast.makeText(TestActivity.this, "fbError",
Toast.LENGTH_SHORT).show();
}
public void onError(DialogError e) {
// TODO Auto-generated method stub
Toast.makeText(TestActivity.this, "onError",
Toast.LENGTH_SHORT).show();
}
public void onComplete(Bundle values) {
// TODO Auto-generated method stub
Editor editor = sp.edit();
editor.putString("access_token", fb.getAccessToken());
editor.putLong("access_expires", fb.getAccessExpires());
editor.commit();
}
public void onCancel() {
// TODO Auto-generated method stub
Toast.makeText(TestActivity.this, "onCancel",
Toast.LENGTH_SHORT).show();
}
});
}
case R.id.button1:
post();
}
}
}
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:tools="http://schemas.android.com/tools"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:orientation="vertical"
>
<TextView
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:padding="@dimen/padding_medium"
android:text="@string/hello_world"
tools:context=".TestActivity" />
<ImageView
android:id="@+id/login"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:src="@drawable/login_button"
android:text="Button" />
<Button
android:id="@+id/button1"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:text="Button" />
LogCat：
07-18 14:10:42.636: I/dalvikvm(385): Jit: resizing JitTable from 512 to 1024
07-18 14:10:47.216: D/dalvikvm(385): GC_CONCURRENT freed 555K, 52% free 2998K/6151K, external 908K/1038K, paused 9ms+8ms
07-18 14:13:39.126: D/dalvikvm(385): GC_CONCURRENT freed 492K, 52% free 2988K/6151K, external 918K/1038K, paused 9ms+9ms
07-18 14:13:39.235: D/webviewglue(385): nativeDestroy view: 0x270d38
07-18 14:13:40.525: D/dalvikvm(385): GC_CONCURRENT freed 554K, 54% free 2867K/6151K, external 726K/1038K, paused 14ms+10ms
07-18 14:13:41.395: D/AndroidRuntime(385): Shutting down VM
07-18 14:13:41.395: W/dalvikvm(385): threadid=1: thread exiting with uncaught exception (group=0x40015560)
07-18 14:13:41.416: E/AndroidRuntime(385): FATAL EXCEPTION: main
07-18 14:13:41.416: E/AndroidRuntime(385): java.lang.NullPointerException
07-18 14:13:41.416: E/AndroidRuntime(385): at com.example.fbtest.TestActivity.post(TestActivity.java:73)
07-18 14:13:41.416: E/AndroidRuntime(385): at com.example.fbtest.TestActivity.onClick(TestActivity.java:136)
07-18 14:13:41.416: E/AndroidRuntime(385): at android.view.View.performClick(View.java:2485)
07-18 14:13:41.416: E/AndroidRuntime(385): at android.view.View$PerformClick.run(View.java:9080)
07-18 14:13:41.416: E/AndroidRuntime(385): at android.os.Handler.handleCallback(Handler.java:587)
07-18 14:13:41.416: E/AndroidRuntime(385): at android.os.Handler.dispatchMessage(Handler.java:92)
07-18 14:13:41.416: E/AndroidRuntime(385): at android.os.Looper.loop(Looper.java:123)
07-18 14:13:41.416: E/AndroidRuntime(385): at android.app.ActivityThread.main(ActivityThread.java:3683)
07-18 14:13:41.416: E/AndroidRuntime(385): at java.lang.reflect.Method.invokeNative(Native Method)
07-18 14:13:41.416: E/AndroidRuntime(385): at java.lang.reflect.Method.invoke(Method.java:507)
07-18 14:13:41.416: E/AndroidRuntime(385): at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:839)
07-18 14:13:41.416: E/AndroidRuntime(385): at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:597)
07-18 14:13:41.416: E/AndroidRuntime(385): at dalvik.system.NativeStart.main(Native Method)
提前致谢！
答案 0 :(得分：1)
您永远不会为tv变量分配任何内容。
假设您要在XML文件中引用TextView，则必须为其指定一个ID：
android:id="@+id/welcomeText"
然后将其分配给您的tv变量：
tv = (TextView) findViewById(R.id.welcomeText);