android.view.View.setVisibility(int) 使新活动崩溃[重复]

Question

所以最近我正在尝试实现一个 FAB，它将扩展到一个新的活动，但在按下 FAB 后它崩溃了。 此外，从我遵循实施步骤的这个问题开始：FloatingActionButton expand into a new activity

Logcat：

Process: com.example.jovie.canteen, PID: 6953
java.lang.NullPointerException: Attempt to invoke virtual method 'void android.view.View.setVisibility(int)' on a null object reference
    at com.example.jovie.canteen.Home$1$2.run(Home.java:105)
    at android.os.Handler.handleCallback(Handler.java:739)
    at android.os.Handler.dispatchMessage(Handler.java:95)
    at android.os.Looper.loop(Looper.java:148)
    at android.app.ActivityThread.main(ActivityThread.java:5417)
    at java.lang.reflect.Method.invoke(Native Method)
    at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:726)
    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:616)

Home.java

public class Home extends AppCompatActivity
    implements NavigationView.OnNavigationItemSelectedListener {
CoordinatorLayout homeLayout;

private GoogleApiClient client;
private Toolbar toolbar;
private TabLayout tabLayout;
private ViewPager viewPager;
private RevealLayout mRevealLayout;

View mViewToReveal;

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

    homeLayout = (CoordinatorLayout) findViewById(R.id.CoordinatorLayout);
    setSupportActionBar(toolbar);

    toolbar = (Toolbar) findViewById(R.id.toolbar);
    setSupportActionBar(toolbar);

    getSupportActionBar().setDisplayHomeAsUpEnabled(true);

    viewPager = (ViewPager) findViewById(R.id.viewpager);
    setupViewPager(viewPager);

    tabLayout = (TabLayout) findViewById(R.id.tabs);
    tabLayout.setupWithViewPager(viewPager);

    mRevealLayout = (RevealLayout) findViewById(R.id.reveal_layout);
    final View mRevealView=(View)
            findViewById(R.id.reveal_view);

     final FloatingActionButton mFab = (FloatingActionButton) findViewById(R.id.fab);
    mFab.setOnClickListener(new View.OnClickListener() {



        @Override
        public void onClick(View v) {
            mFab.setClickable(false); // Avoid naughty guys clicking FAB again and again...
            int[] location = new int[2];
            mFab.getLocationOnScreen(location);
            location[0] += mFab.getWidth() / 2;
            location[1] += mFab.getHeight() / 2;

            final Intent intent = new Intent(Home.this, SearchActivity.class);

            mRevealView.setVisibility(View.VISIBLE);
            mRevealLayout.setVisibility(View.VISIBLE);

            mRevealLayout.show(location[0], location[1]); // Expand from center of FAB. Actually, it just plays reveal animation.
            mFab.postDelayed(new Runnable() {
                @Override
                public void run() {
                    startActivity(intent);
                    /**
                     * Without using R.anim.hold, the screen will flash because of transition
                     * of Activities.
                     */
                    overridePendingTransition(0, R.anim.hold);
                }
            }, 600); // 600 is default duration of reveal animation in RevealLayout
            mFab.postDelayed(new Runnable() {
                @Override
                public void run() {
                    mFab.setClickable(true);
                    mRevealLayout.setVisibility(View.INVISIBLE);
                    mViewToReveal.setVisibility(View.INVISIBLE);
                }
            }, 960); // Or some numbers larger than 600.
        }
    });


    client = new GoogleApiClient.Builder(Home.this).addApi(AppIndex.API).build();

    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);



}

activity_home.xml

<com.example.jovie.canteen.RevealLayout
    android:id="@+id/reveal_layout"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:visibility="invisible">

    <View
        android:id="@+id/reveal_view"
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        android:visibility="invisible"/>

</com.example.jovie.canteen.RevealLayout>

<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:background="#ffffff" />

但是 android.view.View.setVisibility(int) 上的 null 指向 mViewToReveal.setVisibility(View.INVISIBLE); 并且不确定我需要在哪里寻找，请帮助。

Answer 1

这是因为你从未实例化mViewToReveal。

Answer 2

mViewToReveal 尚未初始化。

Answer 3

因为您没有将任何视图的引用传递给 mViewToReveal，所以它为 null 并抛出错误“尝试在空对象引用上调用虚拟方法 'void android.view.View.setVisibility(int)'”，需要在对其进行任何活动之前传递对 mViewToReveal 的引用

mViewToReveal =(View) findViewById(R.id.<id_of_ViewToReveal>);

Answer 4

因为你从未实例化mViewToReveal

将此添加到onCreate

mViewToReveal = findViewById(R.id.reveal_view);