Can't retrieve view with findViewById

Question

I'm very new to Android. I did read many Tutorials, and also the Google example about building first app.

I was able to correctly run the app only once! The problem is as written in the title of this question. I checked every question and answer provided here on the forum, but I can't figure what's going on.

The problem is simple. I started a new project with Eclipse and added a id to the TextView. When I try to get the view I receive a null pointer.

This is the XML

<RelativeLayout 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:paddingBottom="@dimen/activity_vertical_margin"
  android:paddingLeft="@dimen/activity_horizontal_margin"
  android:paddingRight="@dimen/activity_horizontal_margin"
  android:paddingTop="@dimen/activity_vertical_margin"
  tools:context="com.cguida1978it.prova.MainActivity$PlaceholderFragment" >

  <TextView
      android:id="@+id/text_view"
      android:layout_width="wrap_content"
      android:layout_height="wrap_content"
      android:text="@string/hello_world" />
</RelativeLayout> 

And this is the MainActivity.java

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main );

    TextView t = (TextView)findViewById( R.id.text_view );

    if (savedInstanceState == null) {
        getFragmentManager().beginTransaction()
                .add(R.id.container, new PlaceholderFragment()).commit();
    }
}

I checked also in the R.java and the text_view is in the id class

I try to post the error log:

05-03 16:04:22.729: E/AndroidRuntime(2123): FATAL EXCEPTION: main
05-03 16:04:22.729: E/AndroidRuntime(2123): Process: com.cguida1978it.prova, PID:2123
05-03 16:04:22.729: E/AndroidRuntime(2123): java.lang.RuntimeException: Unable to start activity ComponentInfo{com.cguida1978it.prova/com.cguida1978it.prova.MainActivity}: java.lang.NullPointerException
05-03 16:04:22.729: E/AndroidRuntime(2123):     at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2195)
05-03 16:04:22.729: E/AndroidRuntime(2123):     at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2245)
05-03 16:04:22.729: E/AndroidRuntime(2123):     at android.app.ActivityThread.access$800(ActivityThread.java:135)
05-03 16:04:22.729: E/AndroidRuntime(2123):     at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1196)
05-03 16:04:22.729: E/AndroidRuntime(2123):     at android.os.Handler.dispatchMessage(Handler.java:102)
05-03 16:04:22.729: E/AndroidRuntime(2123):     at android.os.Looper.loop(Looper.java:136)
05-03 16:04:22.729: E/AndroidRuntime(2123):     at android.app.ActivityThread.main(ActivityThread.java:5017)
05-03 16:04:22.729: E/AndroidRuntime(2123):     at java.lang.reflect.Method.invokeNative(Native Method)
05-03 16:04:22.729: E/AndroidRuntime(2123):     at java.lang.reflect.Method.invoke(Method.java:515)
05-03 16:04:22.729: E/AndroidRuntime(2123):     at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:779)
05-03 16:04:22.729: E/AndroidRuntime(2123):     at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:595)
05-03 16:04:22.729: E/AndroidRuntime(2123):     at dalvik.system.NativeStart.main(Native Method)
05-03 16:04:22.729: E/AndroidRuntime(2123): Caused by: java.lang.NullPointerException
05-03 16:04:22.729: E/AndroidRuntime(2123):     at com.cguida1978it.prova.MainActivity.onCreate(MainActivity.java:23)
05-03 16:04:22.729: E/AndroidRuntime(2123):     at android.app.Activity.performCreate(Activity.java:5231)
05-03 16:04:22.729: E/AndroidRuntime(2123):     at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1087)
05-03 16:04:22.729: E/AndroidRuntime(2123):     at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2159)
05-03 16:04:22.729: E/AndroidRuntime(2123):     ... 11 more
05-03 16:04:24.989: I/Process(2123): Sending signal. PID: 2123 SIG: 9

Can anyone of you explain me what could be the problem?

Is it an Eclipse issue/bug? May I reset Eclipse to default settings? When I started studying Android it worked a few times... why?? I'm a little bit confused...

Thank you very much!

EDIT TheFRedFox here are my imports

import android.app.Activity;
import android.widget.*;
import android.app.ActionBar;
import android.app.Fragment;
import android.os.Bundle;
import android.view.LayoutInflater;
import android.view.Menu;
import android.view.MenuItem;
import android.view.View;
import android.view.ViewGroup;
import android.os.Build;
import android.view.View.OnClickListener;

EDIT 2 This is a piace of the R.java

public static final class id {
    public static final int action_settings=0x7f080002;
    public static final int container=0x7f080000;
    public static final int text_view=0x7f080001;
}
public static final class layout {
    public static final int activity_main=0x7f030000;
    public static final int fragment_main=0x7f030001;
}

And this is the AndroidManifest.java

<?xml version="1.0" encoding="utf-8"?>
<manifest xmlns:android="http://schemas.android.com/apk/res/android"
    package="com.cguida1978it.prova"
    android:versionCode="1"
    android:versionName="1.0" >

    <uses-sdk
        android:minSdkVersion="14"
        android:targetSdkVersion="19" />

    <application
        android:allowBackup="true"
        android:icon="@drawable/ic_launcher"
        android:label="@string/app_name"
        android:theme="@style/AppTheme" >
        <activity
            android:name="com.cguida1978it.prova.MainActivity"
            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>

I also tried to retrieve the fragment in the onCreate() method using the getFragmentManager().findFragmentById() but also the fragment results in a null pointer!

I also tried to delete the content of R.java as written in one of your answer here in the forum to force Eclipse to regenerate it, but the problem persists!

I really can't figure what is going wrong here. This is a near blank-porject, created from scratch! The only edits are the addition of the TextView id and the lines to retrieve it in the Java code!

Oh... Maybe I did not mention (or it is not clear), I started many new projects from scratch and not modified, but the id and relative codes. Each new project has this issue!

Do you think is an Eclipse issue or ADT/AVD (virtual device) issue? Is there any other IDE I could use?

Really I'm a visually-impaired developer so, even in Eclipse I do not use the graphics designer. I always edit XML files, so I was wondering if it is easy to compile from command line and debug. I read about that in the "Getting started" Google guida. Before starting to read more deep I ask you if you would suggest it!

EDIT 3 The laalto answer seems to fix the problem. Inside the MainActivity class I found a PlaceHolder class that extends Fragment class and moving the TextView stuff inside the onCreateView() using rootView.findViewById() I can set the text property of the TextView.

Why Eclipse does not create a separate class for the fragment fragment_main? Instead it creates a generic class inside the MainActivity.java?

Do you think I should create a separate class for fragments? or use the activity class as suggested in the answer 2?

In the Google's sample they extends the MainActivity class using the activity bar that is a subclass of Fragment, but, really, I was not able to retrieve resources inside the simple onCreate() of the MainActivity class...

This is causing me a little confusion...

Thank you for all of your answer and for your further explanation... I really appreciate your effort...

Answer 1

Your text_view is in the fragment layout and not in the activity layout. It won't be in the activity view hierarchy yet in onCreate() and findViewById() won't find it, returning null. The NPE in your stacktrace is caused by trying to invoke a method on this null.

Move the findViewById() and related code to the fragment's onCreateView(), calling findViewById() on the rootView inflated there.

Answer 2

I can't comment yet, so I have to post an answer. ^^

Can you show us your imports. Sometimes Eclipse adds accidentally not your R class but the android's one. In this android.R class your id surely is not referenced. So you'll have to import your R class (when this is the problem).

EDIT:

Thanks for your imports. I have to correct myself. Your R class doesn't need to be imported as it is automatically chosen. One silly question: It is the right layout you are setting as the content view, right? When it is the right layout, I unfortunately don't know more yet. But I would give you two links to read to regenerate the R.class and/or fix some problem using adt version 22, which cause into some strange errors:

Regenerate R.class: https://stackoverflow.com/a/12195532/2625488

Problems with adt version 22: Eclipse error: R cannot be resolved to a variable