Android 4.1.1-RecyclerView上的findViewById在MainActivity中返回null

Question

我正在关注Android Fundamentals CodeLabs(https://developer.android.com/codelabs/android-training-create-recycler-view?index=..%2F..%2Fandroid-training#3(，考虑到我使用的是更高级版本的Android Studio(4.1.1(，并且在MainActivity onCreate方法中得到了一个空的recyclerView，即使该视图存在。我已经使用BasicActivity启动了这个项目。我该如何解决这个问题？

这里是我的XML for fragment_first.XML:

<?xml version="1.0" encoding="utf-8"?>
<androidx.constraintlayout.widget.ConstraintLayout 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=".FirstFragment">
<androidx.recyclerview.widget.RecyclerView
android:id="@+id/ourcards_view"
android:layout_width="match_parent"
android:layout_height="match_parent" />
<Button
android:id="@+id/button_first"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:text="@string/next"
app:layout_constraintBottom_toBottomOf="parent"
app:layout_constraintEnd_toEndOf="parent"
app:layout_constraintStart_toStartOf="parent"
/>
</androidx.constraintlayout.widget.ConstraintLayout>

这里是MainActivity.java文件内容：

package com.example.ourcards;
import android.os.Bundle;
import com.google.android.material.floatingactionbutton.FloatingActionButton;
import com.google.android.material.snackbar.Snackbar;
import androidx.appcompat.app.AppCompatActivity;
import androidx.appcompat.widget.Toolbar;
import androidx.recyclerview.widget.LinearLayoutManager;
import androidx.recyclerview.widget.RecyclerView;
import android.view.View;
import android.view.Menu;
import android.view.MenuItem;
import java.util.LinkedList;
public class MainActivity extends AppCompatActivity {
private final LinkedList<String> mCardList = new LinkedList<>();
private RecyclerView mRecyclerView;
private CardListAdapter mAdapter;

@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
Toolbar toolbar = findViewById(R.id.toolbar);
setSupportActionBar(toolbar);
FloatingActionButton fab = findViewById(R.id.fab);
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();
}
});
mCardList.addLast("Carta 1");
mCardList.addLast("Carta 2");
// Get a handle to the RecyclerView.
mRecyclerView = findViewById(R.id.ourcards_view);
// Create an adapter and supply the data to be displayed.
mAdapter = new CardListAdapter(this, mCardList);
// Connect the adapter with the RecyclerView.
mRecyclerView.setAdapter(mAdapter);
// Give the RecyclerView a default layout manager.
mRecyclerView.setLayoutManager(new LinearLayoutManager(this));
}
@Override
public boolean onCreateOptionsMenu(Menu menu) {
// Inflate the menu; this adds items to the action bar if it is present.
getMenuInflater().inflate(R.menu.menu_main, 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_settings) {
return true;
}
return super.onOptionsItemSelected(item);
}
}

编辑

这是我的activity_main.xml

<?xml version="1.0" encoding="utf-8"?>
<androidx.coordinatorlayout.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=".MainActivity">
<com.google.android.material.appbar.AppBarLayout
android:layout_width="match_parent"
android:layout_height="wrap_content"
android:theme="@style/Theme.OurCards.AppBarOverlay">
<androidx.appcompat.widget.Toolbar
android:id="@+id/toolbar"
android:layout_width="match_parent"
android:layout_height="?attr/actionBarSize"
android:background="?attr/colorPrimary"
app:popupTheme="@style/Theme.OurCards.PopupOverlay" />
</com.google.android.material.appbar.AppBarLayout>
<include layout="@layout/content_main" />
<com.google.android.material.floatingactionbutton.FloatingActionButton
android:id="@+id/fab"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_gravity="bottom|end"
android:layout_margin="@dimen/fab_margin"
app:srcCompat="@drawable/ic_add_for_fab" />
</androidx.coordinatorlayout.widget.CoordinatorLayout>

此处为content_main.xml

<?xml version="1.0" encoding="utf-8"?>
<androidx.constraintlayout.widget.ConstraintLayout 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="match_parent"
app:layout_behavior="@string/appbar_scrolling_view_behavior">
<fragment
android:id="@+id/nav_host_fragment"
android:name="androidx.navigation.fragment.NavHostFragment"
android:layout_width="0dp"
android:layout_height="0dp"
app:defaultNavHost="true"
app:layout_constraintBottom_toBottomOf="parent"
app:layout_constraintLeft_toLeftOf="parent"
app:layout_constraintRight_toRightOf="parent"
app:layout_constraintTop_toTopOf="parent"
app:navGraph="@navigation/nav_graph" />
</androidx.constraintlayout.widget.ConstraintLayout>

这里是我的CardListAdapter.java

package com.example.ourcards;
import android.content.Context;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;
import android.widget.TextView;
import androidx.annotation.NonNull;
import androidx.recyclerview.widget.RecyclerView;
import java.util.LinkedList;
public class CardListAdapter extends
RecyclerView.Adapter<CardListAdapter.CardViewHolder>  {
private final LinkedList<String> mCardList;
private final LayoutInflater mInflater;

class CardViewHolder extends RecyclerView.ViewHolder {
public final TextView cardItemView;
final CardListAdapter mAdapter;
public CardViewHolder(View itemView, CardListAdapter adapter) {
super(itemView);
cardItemView = itemView.findViewById(R.id.card);
this.mAdapter = adapter;
}
}
public CardListAdapter(Context context,
LinkedList<String> cardList) {
mInflater = LayoutInflater.from(context);
this.mCardList = cardList;
}
@NonNull
@Override
public CardListAdapter.CardViewHolder onCreateViewHolder(@NonNull ViewGroup parent, int viewType) {
View mItemView = mInflater.inflate(R.layout.cardlist_item,
parent, false);
return new CardViewHolder(mItemView, this);
}
@Override
public void onBindViewHolder(@NonNull CardListAdapter.CardViewHolder holder, int position) {
String mCurrent = mCardList.get(position);
holder.cardItemView.setText(mCurrent);
}
@Override
public int getItemCount() {
return mCardList.size();
}
}

这是我的FirstFragment.java

package com.example.ourcards;
import android.os.Bundle;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;
import androidx.annotation.NonNull;
import androidx.fragment.app.Fragment;
import androidx.navigation.fragment.NavHostFragment;
public class FirstFragment extends Fragment {
@Override
public View onCreateView(
LayoutInflater inflater, ViewGroup container,
Bundle savedInstanceState
) {
// Inflate the layout for this fragment
return inflater.inflate(R.layout.fragment_first, container, false);
}
public void onViewCreated(@NonNull View view, Bundle savedInstanceState) {
super.onViewCreated(view, savedInstanceState);
view.findViewById(R.id.button_first).setOnClickListener(new View.OnClickListener() {
@Override
public void onClick(View view) {
NavHostFragment.findNavController(FirstFragment.this)
.navigate(R.id.action_FirstFragment_to_SecondFragment);
}
});
}
}

Answer 1

这里是我的fragment_first.XML:的XML

请注意，您的文件名是fragment_first.xml。

和这里的MainActivity.java文件内容

此处的代码似乎与fragment_first.xml无关。例如，您的setContentView()调用正在加载activity_main.xml。

因此，要么您的RecyclerView需要在activity_main.xml中，要么setContentView()需要加载fragment_first，否则您需要同步两个代码段中的工作。