我正在尝试使用Android Studio制作完整的标题应用。我有MainActivity.java，navheader.xml，cardview.xml，activity_detail.xml。当我键入代码并构建代码时，没有发现错误。
当应用程序运行时，MainActivity.java中存在错误。
错误说：
java.lang.RuntimeException: Unable to start activity ComponentInfo{id.wdharmana.doahindu/id.wdharmana.doahindu.MainActivity}: java.lang.NullPointerException
和
at id.wdharmana.doahindu.MainActivity.onCreate(MainActivity.java:42)
这是我的MainActivity.java：
private CollapsingToolbarLayout ctb;
private int mutedColor;
private DrawerLayout drawerLayout;
private NavigationView navigationView;
private Snackbar snackbar;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
/* Top toolbar */
Toolbar toolbar = (Toolbar) findViewById(R.id.toolbar);
setSupportActionBar(toolbar);
/* Bottom toolbar. */
Toolbar bottomToolbar = (Toolbar) findViewById(R.id.bottom_toolbar);
bottomToolbar.inflateMenu(R.menu.menu_main);
bottomToolbar.setOnMenuItemClickListener(new Toolbar.OnMenuItemClickListener() {
@Override
public boolean onMenuItemClick(MenuItem item) {
String message = null;
if (item.getItemId() == R.id.action_copy){
message = "Copy clicked";
}else if (item.getItemId() == R.id.action_cut){
message = "Cut clicked";
}else if (item.getItemId() == R.id.action_record){
message = "Record clicked";
}
Toast.makeText(MainActivity.this, message, Toast.LENGTH_LONG).show();
return true;
}
});
RecyclerView recyclerView = (RecyclerView) findViewById(R.id.my_recycler_view);
recyclerView.setHasFixedSize(true);
recyclerView.setLayoutManager(new LinearLayoutManager(this));
// Add 8 cards
MyAdapter adapter = new MyAdapter(new String[8]);
recyclerView.setAdapter(adapter);
ctb = (CollapsingToolbarLayout) findViewById(R.id.collapsing_toolbar);
///* Define the image */
// ImageView image = (ImageView) findViewById(R.id.image);
///* Decode bitmap from the image */
// Bitmap bitmap = BitmapFactory.decodeResource(getResources(),
// R.drawable.full_image);
///* Generate palette from the image bitmap */
// Palette.from(bitmap).generate(new Palette.PaletteAsyncListener() {
// @Override
// public void onGenerated(Palette palette) {
// mutedColor = palette.getMutedColor
// (ContextCompat.getColor(MainActivity.this,
// R.color.primary));
// /* Set toolbar color from the palette */
// ctb.setContentScrimColor(mutedColor);
// }
// });
ctb.setContentScrimColor(ContextCompat.getColor(MainActivity.this, R.color.primary));
ctb.setTitle(getString(R.string.app_name));
navigationView = (NavigationView) findViewById(R.id.nav_view);
drawerLayout = (DrawerLayout) findViewById(R.id.drawer);
// On click of menu icon on toolbar
toolbar.setNavigationIcon(R.drawable.ic_menu_white_24dp);
toolbar.setNavigationOnClickListener(new View.OnClickListener() {
@Override
public void onClick(View v) {
drawerLayout.openDrawer(GravityCompat.START);
}
});
// On 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) {
// Set item in checked state
menuItem.setChecked(true);
//TODO: handle navigation
//Closing drawer on item click
drawerLayout.closeDrawers();
return true;
}
});
FloatingActionButton fab = (FloatingActionButton) findViewById(R.id.fab);
fab.setOnClickListener(new View.OnClickListener() {
@Override
public void onClick(View v) {
Snackbar.make(v, "Do something awesome?",
Snackbar.LENGTH_LONG).show();
}
});
}
/* Create RecylcerView Adapter. */
public static class MyAdapter extends RecyclerView.Adapter<MyAdapter.ViewHolder> {
private String[] mDataset;
public static class ViewHolder extends RecyclerView.ViewHolder {
public View view;
public TextView title;
public ViewHolder(View v) {
super(v);
view = v;
title = (TextView) v.findViewById(R.id.card_title);
}
}
public MyAdapter(String[] myDataset) {
mDataset = myDataset;
}
@Override
public MyAdapter.ViewHolder onCreateViewHolder(ViewGroup parent, int viewType) {
View cardview = LayoutInflater.from(parent.getContext())
.inflate(R.layout.cardview, parent, false);
return new ViewHolder(cardview);
}
@Override
public void onBindViewHolder(ViewHolder holder, int position) {
holder.title.setText("Card " + (position + 1));
}
@Override
public int getItemCount() {
return mDataset.length;
}
}
Activity_detail.xml
<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"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:fitsSystemWindows="true">
<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:tools="http://schemas.android.com/tools"
xmlns:app="http://schemas.android.com/apk/res-auto"
android:layout_width="match_parent"
android:layout_height="match_parent" tools:context=".MainActivity">
<android.support.design.widget.CoordinatorLayout
android:id="@+id/main_content"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:layout_alignParentTop="true"
android:layout_marginBottom="?attr/actionBarSize">
<android.support.design.widget.AppBarLayout
android:id="@+id/appbar"
android:layout_width="match_parent"
android:layout_height="192dp">
<android.support.design.widget.CollapsingToolbarLayout
android:id="@+id/collapsing_toolbar"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:theme="@style/ThemeOverlay.AppCompat.Dark"
app:layout_scrollFlags="scroll|exitUntilCollapsed">
<ImageView
android:id="@+id/image"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:background="@drawable/full_image"
android:fitsSystemWindows="true"
android:scaleType="centerCrop"
app:layout_collapseMode="parallax" />
<android.support.v7.widget.Toolbar
android:id="@+id/toolbar"
android:layout_width="match_parent"
android:layout_height="?attr/actionBarSize"
app:popupTheme="@style/ThemeOverlay.AppCompat.Light"
app:layout_collapseMode="pin" />
</android.support.design.widget.CollapsingToolbarLayout>
</android.support.design.widget.AppBarLayout>
<android.support.v7.widget.RecyclerView
android:id="@+id/my_recycler_view"
android:scrollbars="vertical"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:paddingTop="8dp"
android:paddingBottom="8dp"
android:clipToPadding="false"
app:layout_behavior="@string/appbar_scrolling_view_behavior"/>
<android.support.design.widget.FloatingActionButton
android:id="@+id/fab"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_marginBottom="16dp"
android:layout_marginRight="16dp"
android:layout_gravity="right|bottom"
android:tint="@android:color/white"
android:src="@drawable/ic_add"/>
</android.support.design.widget.CoordinatorLayout>
<android.support.v7.widget.Toolbar
android:id="@+id/bottom_toolbar"
android:layout_height="?attr/actionBarSize"
android:layout_width="match_parent"
android:theme="@style/Base.ThemeOverlay.AppCompat.Dark"
android:background="?attr/colorPrimaryDark"
android:layout_alignParentBottom="true"/>
</RelativeLayout>
<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:background="#ffffff"
android:clickable="true"
app:headerLayout="@layout/navheader"
app:menu="@menu/menu_navigation"
app:itemBackground="?attr/selectableItemBackground"/>
Card_View.xml
<android.support.v7.widget.CardView
xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:tools="http://schemas.android.com/tools"
xmlns:card_view="http://schemas.android.com/apk/res-auto"
android:id="@+id/card_view"
android:layout_gravity="center"
android:layout_width="match_parent"
android:layout_height="96dp"
android:layout_marginLeft="16dp"
android:layout_marginRight="16dp"
android:layout_marginTop="8dp"
android:layout_marginBottom="8dp"
card_view:contentPadding="8dp"
android:clickable="true"
android:foreground="?attr/selectableItemBackground">
<TextView
android:id="@+id/card_title"
android:layout_width="match_parent"
android:layout_height="wrap_content"
android:textAppearance="@style/TextAppearance.AppCompat.Title"
tools:text="Title text"/>
NavHeader.xml
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
android:layout_width="match_parent"
android:layout_height="222dp"
android:background="?attr/colorPrimaryDark"
android:padding="16dp"
android:orientation="vertical">
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