textView setText() NullPointerException

I have an almost completely Vanilla App. All I'm trying to do is change the text of a textView, but it gives me a NullPointerException. I have no XML configuration, no added methods, nothing special. I checked my class, and it's pointing to the correct XML, and the XML does have a textView element in it with the correct id.

I have also tried creating the textView instance outside of onCreate() on the class level, and the app just fails to start no matter what. I'm sure I'm overlooking simple, but I can't find the answer. Thanks for the help. EDIT: I realize that I need to move this code so that it accesses fragment resources, but when I do, it gives me an error:

Cannot make a static reference to the non-static method findViewById(int) from the type Activity

Where should I put my code if ALL of my views are in my fragment, and not in my Activity XML? or should I just have the activity load the fragment to begin with? package com.example.testapp;

import android.support.v7.app.ActionBarActivity;
import android.support.v7.app.ActionBar;
import android.support.v4.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.widget.TextView;
import android.os.Build;

public class MainActivity extends ActionBarActivity {

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

        TextView tv = (TextView) findViewById(R.id.textView1);
        tv.setText("Teststring");

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


    }



    @Override
    public boolean onCreateOptionsMenu(Menu menu) {


        // Inflate the menu; this adds items to the action bar if it is present.
        getMenuInflater().inflate(R.menu.main, 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);
    }


    public static class PlaceholderFragment extends Fragment {

        public PlaceholderFragment() {
        }

        @Override
        public View onCreateView(LayoutInflater inflater, ViewGroup container,
                Bundle savedInstanceState) {
            View rootView = inflater.inflate(R.layout.fragment_main, container,
                    false);
            return rootView;
        }

    }


}

ERROR LOG:

06-14 17:02:27.405: E/AndroidRuntime(1420): FATAL EXCEPTION: main
06-14 17:02:27.405: E/AndroidRuntime(1420): Process: com.example.testapp, PID: 1420
06-14 17:02:27.405: E/AndroidRuntime(1420): java.lang.RuntimeException: Unable to start activity ComponentInfo{com.example.testapp/com.example.testapp.MainActivity}: java.lang.NullPointerException
06-14 17:02:27.405: E/AndroidRuntime(1420):     at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2195)
06-14 17:02:27.405: E/AndroidRuntime(1420):     at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2245)
06-14 17:02:27.405: E/AndroidRuntime(1420):     at android.app.ActivityThread.access$800(ActivityThread.java:135)
06-14 17:02:27.405: E/AndroidRuntime(1420):     at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1196)
06-14 17:02:27.405: E/AndroidRuntime(1420):     at android.os.Handler.dispatchMessage(Handler.java:102)
06-14 17:02:27.405: E/AndroidRuntime(1420):     at android.os.Looper.loop(Looper.java:136)
06-14 17:02:27.405: E/AndroidRuntime(1420):     at android.app.ActivityThread.main(ActivityThread.java:5017)
06-14 17:02:27.405: E/AndroidRuntime(1420):     at java.lang.reflect.Method.invokeNative(Native Method)
06-14 17:02:27.405: E/AndroidRuntime(1420):     at java.lang.reflect.Method.invoke(Method.java:515)
06-14 17:02:27.405: E/AndroidRuntime(1420):     at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:779)
06-14 17:02:27.405: E/AndroidRuntime(1420):     at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:595)
06-14 17:02:27.405: E/AndroidRuntime(1420):     at dalvik.system.NativeStart.main(Native Method)
06-14 17:02:27.405: E/AndroidRuntime(1420): Caused by: java.lang.NullPointerException
06-14 17:02:27.405: E/AndroidRuntime(1420):     at com.example.testapp.MainActivity.onCreate(MainActivity.java:23)
06-14 17:02:27.405: E/AndroidRuntime(1420):     at android.app.Activity.performCreate(Activity.java:5231)
06-14 17:02:27.405: E/AndroidRuntime(1420):     at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1087)
06-14 17:02:27.405: E/AndroidRuntime(1420):     at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2159)
06-14 17:02:27.405: E/AndroidRuntime(1420):     ... 11 more

XML fragment_main

<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.example.testapp.MainActivity$PlaceholderFragment" >

    <TextView
        android:id="@+id/textView1"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="@string/hello_world" />

</RelativeLayout>

回答1:

Can you post xml? It's likely that the id your java is assuming "R.id.textView1" is wrong. Maybe R.id.textview1?

Well if your text view lives inside the fragment just do this:

@Override
        public View onCreateView(LayoutInflater inflater, ViewGroup container,
                Bundle savedInstanceState) {
            View rootView = inflater.inflate(R.layout.fragment_main, container,
                    false);
            TextView tv = (TextView) rootView.findViewById(R.id.textView1);
        tv.setText("Teststring");
            return rootView;
        }

回答2:

You are giving setContentView() the wrong layout if your XML is actually declared as fragment_main. That is why the controls are currently null.

// The layout file is not correct.
setContentView(R.layout.activity_second);

回答3:

The TextView textView1 you are trying to refer is in the Fragment's inflated layout. The content view you have set in the Activity is activity_main.xml, which probably doesn't have the textView1 component.

The solution is to override the Fragment's onViewCreated() method and set the text there

@Override
public void onViewCreated (View view, Bundle savedInstanceState)
{
    TextView tv = (TextView) view.findViewById(R.id.textView1);
    tv.setText("My text");
    /* Other code here */
}

Remember to call view.findViewById(R.id.textView1) and not just findViewById(R.id.textView1) because in the latter, you are trying to access the enclosing class non-static method in a static inner class. By calling view.findViewById(R.id.textView1) you are calling a method of a local variable in the static inner class.