我目前正致力于将项目放在一起，我开始收到此错误：
引起：java.lang.NullPointerException：尝试调用虚拟 null对象上的方法'void android.view.View.setVisibility（int）' 参考
我正在使用指南from this link，所以我不明白为什么它不起作用。
第一个activity.java
public class home extends AppCompatActivity {
private DrawerLayout drawer;
TextView fadetxt;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.index_layout);
FloatingActionButton fab = (FloatingActionButton) findViewById(R.id.fab);
fab.setOnClickListener(new View.OnClickListener() {
@Override
public void onClick(View view) {
presentActivity(view);
}
});
}
public void presentActivity(View view) {
ActivityOptionsCompat options = ActivityOptionsCompat.
makeSceneTransitionAnimation(this, view, "transition");
int revealX = (int) (view.getX() + view.getWidth() / 2);
int revealY = (int) (view.getY() + view.getHeight() / 2);
Intent intent = new Intent(this, MainActivity.class);
intent.putExtra(MainActivity.EXTRA_CIRCULAR_REVEAL_X, revealX);
intent.putExtra(MainActivity.EXTRA_CIRCULAR_REVEAL_Y, revealY);
ActivityCompat.startActivity(this, intent, options.toBundle());
}
}
第二个activity.java
public class MainActivity extends Activity implements View.OnClickListener {
public static final String EXTRA_CIRCULAR_REVEAL_X = "EXTRA_CIRCULAR_REVEAL_X";
public static final String EXTRA_CIRCULAR_REVEAL_Y = "EXTRA_CIRCULAR_REVEAL_Y";
View rootLayout;
private int revealX;
private int revealY;
// use a compound button so either checkbox or switch widgets work.
private CompoundButton autoFocus;
private CompoundButton useFlash;
private TextView statusMessage;
private TextView barcodeValue;
private static final int RC_BARCODE_CAPTURE = 9001;
private static final String TAG = "BarcodeMain";
public static final String DATA = "0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZ";
public static Random RANDOM = new Random();
public static String randomString(int len) {
StringBuilder sb = new StringBuilder(len);
for (int i = 0; i < len; i++) {
sb.append(DATA.charAt(RANDOM.nextInt(DATA.length())));
}
return sb.toString();
}
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
statusMessage = (TextView)findViewById(R.id.status_message);
barcodeValue = (TextView)findViewById(R.id.barcode_value);
autoFocus = (CompoundButton) findViewById(R.id.auto_focus);
useFlash = (CompoundButton) findViewById(R.id.use_flash);
autoFocus.setVisibility(View.INVISIBLE);
useFlash.setVisibility(View.INVISIBLE);
findViewById(R.id.read_barcode).setOnClickListener(this);
TextView alpha = (TextView) findViewById(R.id.auto_focus);
alpha.setText(randomString(6));
final Intent intent = getIntent();
rootLayout = findViewById(R.id.root_layout);
if (savedInstanceState == null && Build.VERSION.SDK_INT >= Build.VERSION_CODES.LOLLIPOP &&
intent.hasExtra(EXTRA_CIRCULAR_REVEAL_X) &&
intent.hasExtra(EXTRA_CIRCULAR_REVEAL_Y)) {
rootLayout.setVisibility(View.INVISIBLE);
revealX = intent.getIntExtra(EXTRA_CIRCULAR_REVEAL_X, 0);
revealY = intent.getIntExtra(EXTRA_CIRCULAR_REVEAL_Y, 0);
ViewTreeObserver viewTreeObserver = rootLayout.getViewTreeObserver();
if (viewTreeObserver.isAlive()) {
viewTreeObserver.addOnGlobalLayoutListener(new ViewTreeObserver.OnGlobalLayoutListener() {
@Override
public void onGlobalLayout() {
revealActivity(revealX, revealY);
rootLayout.getViewTreeObserver().removeOnGlobalLayoutListener(this);
}
});
}
} else {
rootLayout.setVisibility(View.VISIBLE);
}
我的浮动按钮
<?xml version="1.0" encoding="utf-8"?>
<!-- Use DrawerLayout as root container for activity -->
<android.support.v4.widget.DrawerLayout xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:app="http://schemas.android.com/apk/res-auto"
android:id="@+id/drawer_layout"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:fitsSystemWindows="true">
<!-- Layout to contain contents of main body of screen (drawer will slide over this) -->
<FrameLayout
android:id="@+id/fragment_container"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:background="#F7F7F7">
<android.support.v7.widget.Toolbar
android:id="@+id/toolbar"
android:layout_width="match_parent"
android:layout_height="?attr/actionBarSize"
android:background="#00FFFFFF"
android:theme="@style/ThemeOverlay.AppCompat.ActionBar"
android:elevation="4dp"/>
<RelativeLayout
android:layout_width="match_parent"
android:layout_height="match_parent"
android:layout_gravity="start"
>
<RelativeLayout
android:id="@+id/upper_section"
android:layout_width="match_parent"
android:layout_height="181dp"
android:background="#2699FB">
<TextView
android:id="@+id/navn"
android:layout_width="wrap_content"
android:layout_height="69dp"
android:layout_alignParentBottom="true"
android:layout_alignParentStart="true"
android:layout_marginBottom="44dp"
android:layout_marginStart="54dp"
android:text="OreoList"
android:textColor="#ffffff"
android:textSize="40sp" />
<TextView
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_alignParentBottom="true"
android:layout_alignStart="@+id/navn"
android:layout_marginBottom="37dp"
android:text="Family Milch"
android:textSize="20sp" />
</RelativeLayout>
<ListView
android:id="@+id/listview"
android:layout_width="match_parent"
android:layout_height="448dp"
android:layout_alignParentStart="true"
android:layout_below="@+id/textView3"
android:layout_marginStart="0dp" />
<TextView
android:id="@+id/textView3"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_alignParentStart="true"
android:layout_below="@+id/upper_section"
android:layout_marginBottom="10dp"
android:layout_marginStart="20dp"
android:layout_marginTop="10dp"
android:text="Indkøbsliste"
android:textColor="#707070"
android:textSize="20sp"
/>
<android.support.design.widget.FloatingActionButton
android:id="@+id/fab"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_alignBottom="@+id/textView"
android:layout_alignParentEnd="true"
android:layout_gravity="end|bottom"
android:layout_marginEnd="22dp"
app:backgroundTint="#114F84"
app:fabSize="mini"
android:src="@drawable/ic_add"/>
<TextView
android:id="@+id/textView"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_alignParentBottom="true"
android:layout_alignStart="@+id/textView3"
android:layout_marginBottom="34dp"
android:text="Completed"
android:textColor="#707070"
android:textSize="20sp" />
</RelativeLayout>
</FrameLayout>
<!-- Container for contents of drawer - use NavigationView to make configuration easier -->
<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"
/>
</android.support.v4.widget.DrawerLayout>