Can not find a View with findViewById()

问题

I cannot find a TextView by calling findViewById(), even though the ID does exist.

OtherActivity:

public class OtherActivity extends Activity { 
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        TextView textView = (TextView)findViewById(R.ID.txt02);
        super.onCreate(savedInstanceState);//line 5
        setContentView(R.layout.other_activity);
        //textView.setText(ACCESSIBILITY_SERVICE);
        Button button = (Button)findViewById(R.ID.otherActivity_Btn);
        button.setText(R.string.otherActivityBtn);
        button.setOnClickListener(new GoBackBtnListener(this));
    }

    class GoBackBtnListener implements OnClickListener {
        OtherActivity other = null;
        public GoBackBtnListener(OtherActivity p_other) {
            other = p_other;
        }
        public void onClick(View v) {
            Intent intent = new Intent();
            intent.setClass(other, Activity01Activity.class);
            other.startActivity(intent);
        }
    }
}

I added a breakpoint at line 5, and started the project in debug mode. When it stopped at the break point, I moved the mouse to variable textView, and it was null.

The other_activity layout:

<?xml version=\"1.0\" encoding=\"utf-8\"?>
<LinearLayout
  xmlns:android=\"http://schemas.android.com/apk/res/android\"
  android:orientation=\"vertical\"
  android:layout_width=\"fill_parent\"
  android:layout_height=\"fill_parent\">

<TextView 
    android:id=\"@+ID/txt02\"
    android:layout_width=\"fill_parent\"
    android:layout_height=\"wrap_content\"
/>

<Button 
    android:id=\"@+ID/otherActivity_Btn\"
    android:layout_width=\"fill_parent\"
    android:layout_height=\"wrap_content\"
/>
</LinearLayout>

AndroidManifest.xml:

<?xml version=\"1.0\" encoding=\"utf-8\"?>
<manifest xmlns:android=\"http://schemas.android.com/apk/res/android\"
    package=\"com.hp\" android:versionCode=\"1\" android:versionName=\"1.0\">
    <uses-sdk android:minSdkVersion=\"4\" />

    <application android:icon=\"@drawable/icon\" android:label=\"@string/app_name\">
        <activity android:name=\".Activity01Activity\" android:label=\"@string/app_name\">
            <intent-filter>
                <action android:name=\"android.intent.action.MAIN\" />
                <category android:name=\"android.intent.category.LAUNCHER\" />
            </intent-filter>
        </activity>
        <activity android:name=\".OtherActivity\" android:label=\"@string/other\"></activity>
    </application>
</manifest>

The generated R.java class:

public final class R {
    public static final class ID {
        public static final int Activity01_btn=0x7f050001;
        public static final int otherActivity_Btn=0x7f050003;
        public static final int txt01=0x7f050000;
        public static final int txt02=0x7f050002;
    }
    public static final class attr {
    }
    public static final class drawable {
        public static final int icon=0x7f020000;
    }
    public static final class layout {
        public static final int main=0x7f030000;
        public static final int other_activity=0x7f030001;
    }
    public static final class string {
        public static final int app_name=0x7f040001;
        public static final int hello=0x7f040000;
        public static final int other=0x7f040002;
        public static final int otherActivityBtn=0x7f040003;
    }
}

I don\'t think the TextView should not be found, but why is the variable textView null?

回答1:

You have to call setContentView(...) BEFORE you attempt to find any of the UI components. You're trying to find the TextView before you've called it.

Change your code to this...

super.onCreate(savedInstanceState);
setContentView(R.layout.other_activity);
TextView textView = (TextView)findViewById(R.id.txt02);

回答2:

change your code like ::

  TextView textView = (TextView)findViewById(R.id.txt02);

There is no capital letter like "ID"

Main :

 <TextView android:id="@+id/txt02" android:layout_width="fill_parent"
    android:layout_height="wrap_content" />

回答3:

I actually just had this problem it was caused by now having build automatically checked. Just either build your project manually or click build automatically and that should solve your problem!

回答4:

You have to make sure that your TextView element is already generated on the creation of your activity.

Most likely, if it's a Menu element, you will have to call the findViewById method on the onCreateOptionsMenu() callback and not on the Oncreate() callback.

来源：https://stackoverflow.com/questions/6761366/can-not-find-a-view-with-findviewbyid