Android對象在onCreate後返回null

Android新手在這裏。Android對象在onCreate後返回null

我想在LinearLayout上設置一個onClick事件。但我不斷收到錯誤 Attempt to invoke virtual method 'void android.widget.LinearLayout.setOnClickListener(android.view.View$OnClickListener)' on a null object reference

其中，如果我理解正確意味着該引用爲空。這對我來說沒有意義，因爲我打電話給它後onCreate

這是我的代碼，任何人有任何線索我做錯了什麼？

MainActivity - 與HERE!的地方是當碰撞正在發生

package ... 
import ... 

public class MainActivity extends AppCompatActivity 
     implements NavigationView.OnNavigationItemSelectedListener { 

    @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(); 

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

     setNavHeaderOnClickAction(); 

    } 


    @SuppressWarnings("StatementWithEmptyBody") 
    @Override 
    public boolean onNavigationItemSelected(MenuItem item) { 
     // ... 
    } 


    public void setNavHeaderOnClickAction(){ 
     // HERE! Here's the problemsome area 
     LinearLayout navHeaderUser = (LinearLayout) findViewById(R.id.nav_header_user); 
     navHeaderUser.setOnClickListener(new View.OnClickListener() { 
      @Override 
      public void onClick(View v) { 
      // Intent myIntent = new Intent(MainActivity.this, UserProfile.class); 
      // MainActivity.this.startActivity(myIntent); 
      // DrawerLayout drawer = (DrawerLayout) findViewById(R.id.drawer_layout); 
      // drawer.closeDrawer(GravityCompat.START); 
      } 
     }); 
    } 
}

activity_main.xml中

<?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_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>

app_bar_main.xml

<?xml version="1.0" encoding="utf-8"?> 
<android.support.design.widget.CoordinatorLayout 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:layout_width="match_parent" 
    android:layout_height="match_parent" 
    tools:context="com.example.trevorwood.biggles.MainActivity"> 



    <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" /> 

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

    <include android:id="@+id/main_content" layout="@layout/content_blank"/> 

    <android.support.design.widget.AppBarLayout 
     android:id="@+id/search_button" 
     android:layout_width="35dp" 
     android:layout_height="35dp" 
     android:layout_gravity="top|end" 
     android:layout_margin="10dp" 
     android:background="@android:drawable/ic_menu_search"> 


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

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

nav_header_main.xml包括（佈局）在activity_main.xml中

<?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" 
    android:layout_width="match_parent" 
    android:layout_height="120dp" 
    android:background="@drawable/side_nav_bar" 
    android:gravity="center" 
    android:orientation="vertical" 
    android:paddingBottom="@dimen/activity_vertical_margin" 
    android:paddingLeft="@dimen/activity_horizontal_margin" 
    android:paddingRight="@dimen/activity_horizontal_margin" 
    android:paddingTop="@dimen/activity_vertical_margin" 
    android:theme="@style/ThemeOverlay.AppCompat.Dark"> 

    <LinearLayout 
     android:id="@+id/nav_header_user" 
     android:layout_width="wrap_content" 
     android:layout_height="wrap_content" 
     android:layout_marginTop="10dp" 
     android:orientation="horizontal" 
     android:clickable="true"> 
     <ImageView 
      android:id="@+id/imageView" 
      android:layout_width="60dp" 
      android:layout_height="60dp" 
      android:paddingTop="@dimen/nav_header_vertical_spacing" 
      app:srcCompat="@android:drawable/sym_def_app_icon" 
      android:clickable="false"/> 
     <LinearLayout 
      android:layout_width="wrap_content" 
      android:layout_height="wrap_content" 
      android:orientation="vertical" 
      android:clickable="false"> 
      <TextView 
       android:id="@+id/textView" 
       android:layout_width="303dp" 
       android:layout_height="wrap_content" 
       android:paddingTop="@dimen/nav_header_vertical_spacing" 
       android:text="JohnSmith007" 
       android:textAppearance="@style/TextAppearance.AppCompat.Body1" 
       android:clickable="false"/> 
      <TextView 
       android:id="@+id/textView2" 
       android:layout_width="wrap_content" 
       android:layout_height="wrap_content" 
       android:text="1,500 points" 
       android:clickable="false"/> 

     </LinearLayout> 


    </LinearLayout> 
</LinearLayout>

activity_main_drawer.xml

<?xml version="1.0" encoding="utf-8"?> 
<menu xmlns:android="http://schemas.android.com/apk/res/android"> 

    <group 
     android:id="@+id/grp1" 
     android:checkableBehavior="single"> 
     <item 
      android:id="@+id/nav_home" 
      android:icon="@drawable/ic_menu_home" 
      android:title="Home" /> 
     <item 
      android:id="@+id/nav_library" 
      android:icon="@drawable/ic_menu_book" 
      android:title="Library" /> 
     <item 
      android:id="@+id/nav_create" 
      android:icon="@drawable/ic_menu_add" 
      android:title="Create" /> 
     <item 
      android:id="@+id/nav_user" 
      android:icon="@drawable/ic_menu_user" 
      android:title="My Account" /> 
     <item 
      android:id="@+id/nav_settings" 
      android:icon="@drawable/ic_menu_cog_wheel" 
      android:title="Settings" /> 

    </group> 

    <group 
     android:id="@+id/grp2" 
     android:checkableBehavior="single"> 
     <item 
      android:id="@+id/nav_about" 
      android:icon="@drawable/ic_menu_about" 
      android:title="About" /> 
     <item 
      android:id="@+id/nav_sign_out" 
      android:icon="@drawable/ic_menu_sign_out" 
      android:title="Sign Out" /> 
    </group> 

</menu>

content_blank.xml

<?xml version="1.0" encoding="utf-8"?> 
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android" 
    android:orientation="vertical" android:layout_width="match_parent" 
    android:layout_height="match_parent"> 

    <TextView 
     android:layout_width="wrap_content" 
     android:layout_height="wrap_content" 
     android:layout_marginTop="70dp" 
     android:text="content_blank"/> 

</LinearLayout>

來源

2017-09-27 Trevor Wood

添加你的main.xml –

你能發佈你的完整代碼嗎？ NavigationView.java，nav_header_main.xml。或者您可以嘗試使用'LinearLayout navHeaderUser =（LinearLayout）navigationView.findViewById（R.id.nav_header_user）;'在'setNavHeaderOnClickAction（）'方法中找到'LinearLayout''（請記住將'NavigationView'作爲參數傳遞**） –

@KingfisherPhuoc新增 –

NavigationView頭通常包括這樣：

<android.support.design.widget.NavigationView 
     android:id="@+id/nvView" 
     android:layout_width="wrap_content" 
     android:layout_height="match_parent" 
     android:layout_gravity="start" 
     android:background="@android:color/white" 
     app:headerLayout="@layout/main_drawer_header" 
     app:itemTextColor="@color/black" 
     app:menu="@menu/menu_main_drawer" 
     app:itemIconTint="@null"/>

由於支持庫版本23.1.0 NavigationView是使用RecyclerView並將標題添加爲RecyclerView的項目之一。

活動的findViewById不會找到標題及其內部。

要訪問它，你需要從NavigationView獲得頭，並呼籲findViewById相對於頭部的觀點：

NavigationView navigationView = (NavigationView) findViewById(R.id.nvView); 
View navHeaderview = navigationView.getHeaderView(0); 

LinearLayout navHeaderUser = (LinearLayout) navHeaderview.findViewById(R.id.nav_header_user); 
    navHeaderUser.setOnClickListener(new View.OnClickListener() { 
     @Override 
     public void onClick(View v) { 
     // Intent myIntent = new Intent(MainActivity.this, UserProfile.class); 
     // MainActivity.this.startActivity(myIntent); 
     // DrawerLayout drawer = (DrawerLayout) findViewById(R.id.drawer_layout); 
     // drawer.closeDrawer(GravityCompat.START); 
     } 
    });

希望我得到了它的權利，它會爲你工作！

來源

2017-09-27 02:49:42 Amaksoft

Android對象在onCreate後返回null

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