can't access to TextView

问题

I have MainActivite. And I try to set text to TextView tvUserName. Until today everything was Ok, but now I can't access to this variable by findViewById. Value of this variable is null.

Maybe it's a wrong code, and it's stopped to work correctly after system libs update.

I didn't do anything special, and don't understand what to do.

Caused by: java.lang.NullPointerException

when I try to set text to this variable

private static TextView tvUserName;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        currentLocale = getResources().getConfiguration().locale;
        LocaleHelper.onCreate(this, currentLocale + "");
        setContentView(R.layout.activity_main);

        context = this;

        fragmentManager = getSupportFragmentManager();

        NavigationView navigationView = (NavigationView) findViewById(R.id.nav_view);
        navigationView.setNavigationItemSelectedListener(MainActivity.this);
        navigationView.setEnabled(false);

        DrawerLayout mDrawer = (DrawerLayout) findViewById(R.id.drawer_layout);
        mDrawer.setDrawerLockMode(DrawerLayout.LOCK_MODE_LOCKED_CLOSED);

        tvUserName = (TextView) findViewById(R.id.navUserName);

        //?????
    }

--------- activity_main.xml ------------------

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

    <android.support.v4.widget.DrawerLayout xmlns:android="http://schemas.android.com/apk/res/android"
        xmlns:app="http://schemas.android.com/apk/res-auto"
        xmlns:tools="http://schemas.android.com/tools" android:id="@+id/drawer_layout"
        android:layout_width="match_parent" android:layout_height="match_parent"
        android:fitsSystemWindows="true" tools:openDrawer="start">

                <include layout="@layout/app_bar_main" android:layout_width="match_parent"
                    android:layout_height="match_parent" />

            <android.support.design.widget.NavigationView android:id="@+id/nav_view"
                android:layout_width="wrap_content" android:layout_height="match_parent"
                android:layout_gravity="start" android:fitsSystemWindows="true"
                app:headerLayout="@layout/nav_header_main" app:menu="@menu/activity_main_drawer"/>

        </android.support.v4.widget.DrawerLayout>

</LinearLayout>

------- nav_header_main -----------------

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="match_parent" android:layout_height="wrap_content"
    android:paddingLeft="16dp"
    android:background="@drawable/side_nav_bar"
    android:theme="@style/ThemeOverlay.AppCompat.Dark" android:orientation="vertical"
    android:gravity="bottom">

    <TextView android:layout_width="match_parent" android:layout_height="wrap_content"
        android:paddingTop="@dimen/nav_header_vertical_spacing" android:text="Phone4Pay"
        android:textAppearance="@style/TextAppearance.AppCompat.Body1" />
    <LinearLayout
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        android:paddingTop="2dp"
        android:paddingBottom="15dp" >
        <TextView android:layout_width="wrap_content" android:layout_height="wrap_content"
            android:id="@+id/navUserName"/>

    </LinearLayout>
</LinearLayout>

回答1:

    navigationView = (NavigationView)findViewById(R.id.nav_header_main);
    View headerLayout = navigationView.getHeaderView(0);
    tvUserName = (TextView) headerLayout.findViewById(R.id.navUserName);

回答2:

TextView tv = (TextView)navigationView.findViewById(id);

Cause TextView is in the headerLayout, and the headerLayout is the root layout of navigationView.

This answer is wrong.

get more here

回答3:

The main reason was in changing version of support lib

from

compile 'com.android.support:design:23.0.1'

to

compile 'com.android.support:design:23.1.1'

But at the old version (23.0.1) this code work ok :

tvUserName = (TextView) findViewById(R.id.navUserName);

and this code don't work in common "can't recognise method getHeaderView"

View headerLayout = navigationView.getHeaderView(0);

Be careful when you change support library version, developers can change everything without any warnings. No back support.

来源：https://stackoverflow.com/questions/35800395/cant-access-to-textview