我遇到了这个NullPointerException。我搜索了类似的帖子并尝试了所有这些帖子，但都没有用。好像我错过了什么，但我找不到它是什么。请帮忙。
这是Debug输出：
E/AndroidRuntime: FATAL EXCEPTION: main
Process: project_name, PID: 4101
java.lang.RuntimeException: Unable to start activity ComponentInfo{project_name/project_name.activity.HomeActivity}: java.lang.NullPointerException: Attempt to invoke virtual method 'android.view.View android.support.design.widget.NavigationView.getHeaderView(int)' on a null object reference
at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2416)
at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2476)
at android.app.ActivityThread.-wrap11(ActivityThread.java)
at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1344)
at android.os.Handler.dispatchMessage(Handler.java:102)
at android.os.Looper.loop(Looper.java:148)
at android.app.ActivityThread.main(ActivityThread.java:5417)
at java.lang.reflect.Method.invoke(Native Method)
at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:726)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:616)
Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'android.view.View android.support.design.widget.NavigationView.getHeaderView(int)' on a null object reference
at project_name.activity.HomeActivity.onCreate(HomeActivity.java:93)
at android.app.Activity.performCreate(Activity.java:6237)
at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1107)
at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2369)
at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2476)
at android.app.ActivityThread.-wrap11(ActivityThread.java)
at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1344)
at android.os.Handler.dispatchMessage(Handler.java:102)
at android.os.Looper.loop(Looper.java:148)
at android.app.ActivityThread.main(ActivityThread.java:5417)
at java.lang.reflect.Method.invoke(Native Method)
at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:726)
com.android.internal.os.ZygoteInit.main(ZygoteInit.java:616)
我的活动：
package project_name.activity;
import android.content.Intent;
import android.os.Bundle;
import android.os.Handler;
import android.support.design.widget.FloatingActionButton;
import android.support.design.widget.NavigationView;
import android.support.design.widget.Snackbar;
import android.support.v4.app.Fragment;
import android.support.v4.app.FragmentTransaction;
import android.support.v4.view.GravityCompat;
import android.support.v4.widget.DrawerLayout;
import android.support.v7.app.ActionBarDrawerToggle;
import android.support.v7.app.AppCompatActivity;
import android.support.v7.widget.Toolbar;
import android.view.LayoutInflater;
import android.view.Menu;
import android.view.MenuItem;
import android.view.View;
import android.widget.ImageView;
import android.widget.TextView;
import android.widget.Toast;
import com.bumptech.glide.Glide;
import com.bumptech.glide.load.engine.DiskCacheStrategy;
import project_name.activity.R;
import project_name.activity.fragment.DashboardFragment;
import project_name.activity.fragment.PhotosFragment;
import project_name.activity.other.CircleTransform;
public class HomeActivity extends AppCompatActivity {
private NavigationView navigationView;
private DrawerLayout drawer;
private View navHeader;
private ImageView imgNavHeaderBg, imgProfile;
private TextView txtName, txtWebsite;
private Toolbar toolbar;
private FloatingActionButton fab;
// urls to load navigation header background image
// and profile image
private static final String urlNavHeaderBg = "http://api.androidhive.info/images/nav-menu-header-bg.jpg";
private static final String urlProfileImg = "https://lh3.googleusercontent.com/eCtE_G34M9ygdkmOpYvCag1vBARCmZwnVS6rS5t4JLzJ6QgQSBquM0nuTsCpLhYbKljoyS-txg";
// index to identify current nav menu item
public static int navItemIndex = 0;
// tags used to attach the fragments
private static final String TAG_HOME = "home";
private static final String TAG_PHOTOS = "photos";
private static final String TAG_MOVIES = "movies";
private static final String TAG_NOTIFICATIONS = "notifications";
private static final String TAG_SETTINGS = "settings";
public static String CURRENT_TAG = TAG_HOME;
// toolbar titles respected to selected nav menu item
private String[] activityTitles;
// flag to load home fragment when user presses back key
private boolean shouldLoadHomeFragOnBackPress = true;
private Handler mHandler;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
toolbar = (Toolbar) findViewById(R.id.toolbar);
setSupportActionBar(toolbar);
mHandler = new Handler();
drawer = (DrawerLayout) findViewById(R.id.drawer_layout);
navigationView = (NavigationView) findViewById(R.id.nav_view);
fab = (FloatingActionButton) findViewById(R.id.fab);
// Workarounds with Null Pointer Exception
// View headerLayout =
// navigationView.inflateHeaderView(R.layout.nav_header_home);
// navigationView = (NavigationView) headerLayout.findViewById(R.id.nav_view);
// navHeader = navigationView.inflateHeaderView(R.layout.nav_header_home);
// navHeader = LayoutInflater.from(this).inflate(R.layout.nav_header_home, null);
// navigationView.addHeaderView(navHeader);
// TextView text = (TextView) navHeader.findViewById(R.id.textView);
// text.setText("HELLO");
// navHeader = navigationView.inflateHeaderView(R.layout.nav_header_home);
// Navigation view header
navHeader = navigationView.getHeaderView(0);
txtName = (TextView) navHeader.findViewById(R.id.name);
txtWebsite = (TextView) navHeader.findViewById(R.id.website);
imgNavHeaderBg = (ImageView) navHeader.findViewById(R.id.img_header_bg);
imgProfile = (ImageView) navHeader.findViewById(R.id.img_profile);
// load toolbar titles from string resources
activityTitles = getResources().getStringArray(R.array.nav_item_activity_titles);
fab.setOnClickListener(new View.OnClickListener() {
@Override
public void onClick(View view) {
Snackbar.make(view, "Replace with your own action", Snackbar.LENGTH_LONG)
.setAction("Action", null).show();
}
});
// load nav menu header data
loadNavHeader();
// initializing navigation menu
setUpNavigationView();
if (savedInstanceState == null) {
navItemIndex = 0;
CURRENT_TAG = TAG_HOME;
loadHomeFragment();
}
}
/***
* Load navigation menu header information
* like background image, profile image
* name, website, notifications action view (dot)
*/
private void loadNavHeader() {
// name, website
txtName.setText("My Name");
txtWebsite.setText("project_name");
// loading header background image
Glide.with(this).load(urlNavHeaderBg)
.crossFade()
.diskCacheStrategy(DiskCacheStrategy.ALL)
.into(imgNavHeaderBg);
// Loading profile image
Glide.with(this).load(urlProfileImg)
.crossFade()
.thumbnail(0.5f)
.bitmapTransform(new CircleTransform(this))
.diskCacheStrategy(DiskCacheStrategy.ALL)
.into(imgProfile);
// showing dot next to notifications label
navigationView.getMenu().getItem(3).setActionView(R.layout.menu_dot);
}
/***
* Returns respected fragment that user
* selected from navigation menu
*/
private void loadHomeFragment() {
// selecting appropriate nav menu item
selectNavMenu();
// set toolbar title
setToolbarTitle();
// if user select the current navigation menu again, don't do anything
// just close the navigation drawer
if (getSupportFragmentManager().findFragmentByTag(CURRENT_TAG) != null) {
drawer.closeDrawers();
// show or hide the fab button
toggleFab();
return;
}
// Sometimes, when fragment has huge data, screen seems hanging
// when switching between navigation menus
// So using runnable, the fragment is loaded with cross fade effect
// This effect can be seen in GMail app
Runnable mPendingRunnable = new Runnable() {
@Override
public void run() {
// update the main content by replacing fragments
Fragment fragment = getHomeFragment();
FragmentTransaction fragmentTransaction = getSupportFragmentManager().beginTransaction();
fragmentTransaction.setCustomAnimations(android.R.anim.fade_in,
android.R.anim.fade_out);
fragmentTransaction.replace(R.id.frame, fragment, CURRENT_TAG);
fragmentTransaction.commitAllowingStateLoss();
}
};
// If mPendingRunnable is not null, then add to the message queue
if (mPendingRunnable != null) {
mHandler.post(mPendingRunnable);
}
// show or hide the fab button
toggleFab();
//Closing drawer on item click
drawer.closeDrawers();
// refresh toolbar menu
invalidateOptionsMenu();
}
private Fragment getHomeFragment() {
switch (navItemIndex) {
case 0:
// home
DashboardFragment homeFragment = new DashboardFragment();
return homeFragment;
case 1:
// photos
PhotosFragment photosFragment = new PhotosFragment();
return photosFragment;
case 2:
// movies fragment
DashboardFragment moviesFragment = new DashboardFragment();
return moviesFragment;
case 3:
// notifications fragment
DashboardFragment notificationsFragment = new DashboardFragment();
return notificationsFragment;
case 4:
// settings fragment
DashboardFragment settingsFragment = new DashboardFragment();
return settingsFragment;
default:
return new DashboardFragment();
}
}
private void setToolbarTitle() {
getSupportActionBar().setTitle(activityTitles[navItemIndex]);
}
private void selectNavMenu() {
navigationView.getMenu().getItem(navItemIndex).setChecked(true);
}
private void setUpNavigationView() {
//Setting Navigation View Item Selected Listener to handle the item click of the navigation menu
navigationView.setNavigationItemSelectedListener(new NavigationView.OnNavigationItemSelectedListener() {
// This method will trigger on item Click of navigation menu
@Override
public boolean onNavigationItemSelected(MenuItem menuItem) {
//Check to see which item was being clicked and perform appropriate action
switch (menuItem.getItemId()) {
//Replacing the main content with ContentFragment Which is our Inbox View;
case R.id.nav_home:
navItemIndex = 0;
CURRENT_TAG = TAG_HOME;
break;
case R.id.nav_photos:
navItemIndex = 1;
CURRENT_TAG = TAG_PHOTOS;
break;
case R.id.nav_movies:
navItemIndex = 2;
CURRENT_TAG = TAG_MOVIES;
break;
case R.id.nav_notifications:
navItemIndex = 3;
CURRENT_TAG = TAG_NOTIFICATIONS;
break;
case R.id.nav_settings:
navItemIndex = 4;
CURRENT_TAG = TAG_SETTINGS;
break;
case R.id.nav_about_us:
// launch new intent instead of loading fragment
startActivity(new Intent(HomeActivity.this, MainActivity.class));
drawer.closeDrawers();
return true;
case R.id.nav_privacy_policy:
// launch new intent instead of loading fragment
startActivity(new Intent(HomeActivity.this, MainActivity.class));
drawer.closeDrawers();
return true;
default:
navItemIndex = 0;
}
//Checking if the item is in checked state or not, if not make it in checked state
if (menuItem.isChecked()) {
menuItem.setChecked(false);
} else {
menuItem.setChecked(true);
}
menuItem.setChecked(true);
loadHomeFragment();
return true;
}
});
ActionBarDrawerToggle actionBarDrawerToggle = new ActionBarDrawerToggle(this, drawer, toolbar, R.string.openDrawer, R.string.closeDrawer) {
@Override
public void onDrawerClosed(View drawerView) {
// Code here will be triggered once the drawer closes as we dont want anything to happen so we leave this blank
super.onDrawerClosed(drawerView);
}
@Override
public void onDrawerOpened(View drawerView) {
// Code here will be triggered once the drawer open as we dont want anything to happen so we leave this blank
super.onDrawerOpened(drawerView);
}
};
//Setting the actionbarToggle to drawer layout
drawer.setDrawerListener(actionBarDrawerToggle);
//calling sync state is necessary or else your hamburger icon wont show up
actionBarDrawerToggle.syncState();
}
@Override
public void onBackPressed() {
if (drawer.isDrawerOpen(GravityCompat.START)) {
drawer.closeDrawers();
return;
}
// This code loads home fragment when back key is pressed
// when user is in other fragment than home
if (shouldLoadHomeFragOnBackPress) {
// checking if user is on other navigation menu
// rather than home
if (navItemIndex != 0) {
navItemIndex = 0;
CURRENT_TAG = TAG_HOME;
loadHomeFragment();
return;
}
}
super.onBackPressed();
}
@Override
public boolean onCreateOptionsMenu(Menu menu) {
// Inflate the menu; this adds items to the action bar if it is present.
// show menu only when home fragment is selected
if (navItemIndex == 0) {
getMenuInflater().inflate(R.menu.home, menu);
}
// when fragment is notifications, load the menu created for notifications
if (navItemIndex == 3) {
getMenuInflater().inflate(R.menu.notifications, 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_logout) {
Toast.makeText(getApplicationContext(), "Logout user!", Toast.LENGTH_LONG).show();
return true;
}
// user is in notifications fragment
// and selected 'Mark all as Read'
if (id == R.id.action_mark_all_read) {
Toast.makeText(getApplicationContext(), "All notifications marked as read!", Toast.LENGTH_LONG).show();
}
// user is in notifications fragment
// and selected 'Clear All'
if (id == R.id.action_clear_notifications) {
Toast.makeText(getApplicationContext(), "Clear all notifications!", Toast.LENGTH_LONG).show();
}
return super.onOptionsItemSelected(item);
}
// show or hide the fab
private void toggleFab() {
if (navItemIndex == 0)
fab.show();
else
fab.hide();
}
}
我认为其他活动和碎片无关紧要。所以我会给你布局文件。这是抽屉：
<?xml version="1.0" encoding="utf-8"?>
<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_home"
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_home"
app:menu="@menu/activity_home_drawer" />
</android.support.v4.widget.DrawerLayout>
抽屉本身的标题：
<?xml version="1.0" encoding="utf-8"?>
<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:app="http://schemas.android.com/apk/res-auto"
android:id="@+id/view_container"
android:layout_width="match_parent"
android:layout_height="@dimen/nav_header_height"
android:gravity="bottom"
android:orientation="vertical"
android:theme="@style/ThemeOverlay.AppCompat.Dark">
<ImageView
android:id="@+id/img_header_bg"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:scaleType="fitXY"
android:src="@drawable/favicon" />
<LinearLayout
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_centerVertical="true"
android:orientation="vertical"
android:padding="@dimen/activity_horizontal_margin">
<ImageView
android:id="@+id/img_profile"
android:layout_width="@dimen/profile_width"
android:layout_height="@dimen/profile_height"
android:paddingTop="@dimen/nav_header_vertical_spacing"
app:srcCompat="@android:drawable/sym_def_app_icon" />
<TextView
android:id="@+id/name"
android:layout_width="match_parent"
android:layout_height="wrap_content"
android:paddingTop="@dimen/nav_header_vertical_spacing"
android:textAppearance="@style/TextAppearance.AppCompat.Body1" />
<TextView
android:id="@+id/website"
android:layout_width="wrap_content"
android:layout_height="wrap_content" />
</LinearLayout>
</RelativeLayout>
请帮忙，我现在真的迷路了。
答案 0 :(得分：0)
当您致电findViewById()时，您的Activity布局仅搜索 ，navigationView位于DrawerLayout。
这会导致navigationView为null并在空对象上调用getHeaderView(0)，会导致NullPointerException。
尝试切换到：
drawer = (DrawerLayout) findViewById(R.id.drawer_layout);
navigationView = (NavigationView) drawer.findViewById(R.id.nav_view);
答案 1 :(得分：0)
如果您已经尝试过，请忽略此答案。 我觉得你的代码没问题，只是尝试清理缓存并清理项目然后重新运行