Android 4.1.1 - RecyclerView 上的 findViewById 在 MainActivity 中返回 null

Question

I'm following Android Fundamentals CodeLabs ( https://developer.android.com/codelabs/android-training-create-recycler-view?index=..%2F..%2Fandroid-training#3 ), with minor changes given我正在使用 Android Studio (4.1.1) 的更高级版本，并且在 MainActivity onCreate 方法中获得了 null recyclerView，即使视图存在。 我使用 BasicActivity 来启动这个项目。 我该如何解决？

这里我的 XML 为 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>

这里内容_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();
    }
}

这里是我的 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

这里我的 XML 为 fragment_first.xml：

请注意，您的文件名是fragment_first.xml 。

这里是 MainActivity.java 文件内容

您这里的代码似乎与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需要位于activity_main.xml中，要么您的setContentView()需要加载fragment_first ，否则您需要同步两个代码片段中的工作。