Question

我通过简单地点击android studio中的“新导航抽屉活动”创建了导航抽屉，我的主要活动工作得很好，但我想重写它，所以我可以在我的所有活动中使用导航抽屉。

看起来像是：

public class MainActivity extends AppCompatActivity
    implements NavigationView.OnNavigationItemSelectedListener {

protected DrawerLayout drawer;

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);
    Toolbar toolbar = (Toolbar) findViewById(R.id.toolbar);
    setSupportActionBar(toolbar);



    DrawerLayout drawer = (DrawerLayout) findViewById(R.id.drawer_layout);
    ActionBarDrawerToggle toggle = new ActionBarDrawerToggle(
            this, drawer, toolbar, R.string.navigation_drawer_open, R.string.navigation_drawer_close);
    drawer.setDrawerListener(toggle);
    toggle.syncState();

    NavigationView navigationView = (NavigationView) findViewById(R.id.nav_view);
    navigationView.setNavigationItemSelectedListener(this);
}

@Override
public void onBackPressed() {
    DrawerLayout drawer = (DrawerLayout) findViewById(R.id.drawer_layout);
    if (drawer.isDrawerOpen(GravityCompat.START)) {
        drawer.closeDrawer(GravityCompat.START);
    } else {
        super.onBackPressed();
    }
}

@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();

    //noinspection SimplifiableIfStatement
    if (id == R.id.action_settings) {
        return true;
    }

    return super.onOptionsItemSelected(item);
}

@SuppressWarnings("StatementWithEmptyBody")
@Override
public boolean onNavigationItemSelected(MenuItem item) {
    // Handle navigation view item clicks here.
    int id = item.getItemId();

    if (id == R.id.nav_main) {
        // Handle the camera action
    } else if (id == R.id.nav_subscribed) {
        startActivity(new Intent(this, ManageActivity.class));

    } else if (id == R.id.nav_searchForNew) {
        startActivity(new Intent(this, SearchActivity.class));

    }

    DrawerLayout drawer = (DrawerLayout) findViewById(R.id.drawer_layout);
    drawer.closeDrawer(GravityCompat.START);
    return true;
}

我的ManageActivity：

public class ManageActivity extends MainActivity {
    ListView list;
    ArrayAdapter<String> adapter ;
    List<String> showTitles = new ArrayList<String>();



@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);

    LayoutInflater inflater = (LayoutInflater) this
            .getSystemService(Context.LAYOUT_INFLATER_SERVICE);
    View contentView = inflater.inflate(R.layout.activity_manage, null, false);
    drawer.addView(contentView, 0);

当我尝试从导航栏访问ManageActivity时，我得到java.lang.RuntimeException: Unable to start activity ComponentInfo{com.example.user.test/com.example.user.test.ManageActivity}: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.support.v4.widget.DrawerLayout.addView(android.view.View, int)' on a null object reference

styles.xml

<!-- Base application theme. -->
<style name="AppTheme" parent="Theme.AppCompat.Light.NoActionBar">
    <!-- Customize your theme here. -->
    <item name="colorPrimary">@color/colorPrimary</item>
    <item name="colorPrimaryDark">@color/colorPrimaryDark</item>
    <item name="colorAccent">@color/colorAccent</item>
</style>

<style name="AppTheme.NoActionBar">
    <item name="windowActionBar">false</item>
    <item name="windowNoTitle">true</item>

</style>

<style name="AppTheme.AppBarOverlay" parent="ThemeOverlay.AppCompat.Dark.ActionBar" />

<style name="AppTheme.PopupOverlay" parent="ThemeOverlay.AppCompat.Light" />

activity_main.xml中

<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" />

app_bar_main.xml

<android.support.design.widget.AppBarLayout
    android:layout_width="match_parent"
    android:layout_height="wrap_content"
    android:theme="@style/AppTheme.AppBarOverlay">

    <android.support.v7.widget.Toolbar
        android:id="@+id/toolbar"
        android:layout_width="match_parent"
        android:layout_height="?attr/actionBarSize"
        android:background="?attr/colorPrimary"
        app:popupTheme="@style/AppTheme.PopupOverlay"
        app:theme="@style/ThemeOverlay.AppCompat.ActionBar"/>

</android.support.design.widget.AppBarLayout>

<include layout="@layout/content_main" />

寻找解决方案并尝试将主题更改为其他用户提到的主题，但它不起作用。有什么想法吗？

Answer 1

在MainActivity.onCreate（）中，您将$mysqli = new mysqli("localhost", "root", "", "praktikumweb"); if ($mysqli->connect_errno) { printf("Connect failed: %s\n", $mysqli->connect_error); exit(); }设置为局部变量与成员变量drawer。在ManageActivity中无法访问this.drawer局部变量，这就是您获得NullPointerException的原因。

void null对象引用上的android.support.v4.widget.DrawerLayout.addView（android.view.View，int）'

1 个答案: