遇到在 null object 參考上調用虛擬方法 'void androidx.recyclerview.widget.RecyclerView.setHasFixedSize(boolean)' 的嘗試

Question

我正在做一個制作音樂流媒體應用程序的學校項目。 我決定在前面的屏幕上使用回收器視圖顯示歌曲。 但是，當我運行該應用程序時，我收到此錯誤並且該應用程序立即崩潰。 如果有人可以幫助我，我將不勝感激。 謝謝

錯誤日志：

    Process: com.example.musicstream, PID: 1641
    java.lang.RuntimeException: Unable to start activity ComponentInfo{com.example.musicstream/com.example.decibel.MainActivity}: java.lang.NullPointerException: Attempt to invoke virtual method 'void androidx.recyclerview.widget.RecyclerView.setHasFixedSize(boolean)' on a null object reference
        at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:3792)
        at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:3968)
        at android.app.servertransaction.LaunchActivityItem.execute(LaunchActivityItem.java:85)
        at android.app.servertransaction.TransactionExecutor.executeCallbacks(TransactionExecutor.java:135)
        at android.app.servertransaction.TransactionExecutor.execute(TransactionExecutor.java:95)
        at android.app.ActivityThread$H.handleMessage(ActivityThread.java:2307)
        at android.os.Handler.dispatchMessage(Handler.java:106)
        at android.os.Looper.loop(Looper.java:246)
        at android.app.ActivityThread.main(ActivityThread.java:8506)
        at java.lang.reflect.Method.invoke(Native Method)
        at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:602)
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:1130)
     Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'void androidx.recyclerview.widget.RecyclerView.setHasFixedSize(boolean)' on a null object reference
        at com.example.decibel.MainActivity.onCreate(MainActivity.java:30)
        at android.app.Activity.performCreate(Activity.java:8198)
        at android.app.Activity.performCreate(Activity.java:8182)
        at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1309)
        at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:3765)
        at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:3968) 
        at android.app.servertransaction.LaunchActivityItem.execute(LaunchActivityItem.java:85) 
        at android.app.servertransaction.TransactionExecutor.executeCallbacks(TransactionExecutor.java:135) 
        at android.app.servertransaction.TransactionExecutor.execute(TransactionExecutor.java:95) 
        at android.app.ActivityThread$H.handleMessage(ActivityThread.java:2307) 
        at android.os.Handler.dispatchMessage(Handler.java:106) 
        at android.os.Looper.loop(Looper.java:246) 
        at android.app.ActivityThread.main(ActivityThread.java:8506) 
        at java.lang.reflect.Method.invoke(Native Method) 
        at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:602) 
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:1130)

主要活動：

package com.example.decibel;
import androidx.appcompat.app.AppCompatActivity;
import androidx.recyclerview.widget.LinearLayoutManager;
import androidx.recyclerview.widget.RecyclerView;

import android.content.Intent;
import android.os.Bundle;
import android.util.Log;
import android.view.View;

import com.example.decibel.R;

import java.util.List;

public class MainActivity extends AppCompatActivity {
    private RecyclerView recyclerView;
    private RecyclerView.Adapter mAdapter;
    private RecyclerView.LayoutManager layoutManager;

    SongCollection songCollection = new SongCollection();
    List<Song> songList = songCollection.getSongList();


    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        recyclerView = findViewById(R.id.oneSongLayout);
        recyclerView.setHasFixedSize(true);

        layoutManager = new LinearLayoutManager(this);
        recyclerView.setLayoutManager(layoutManager);

        mAdapter = new RecycleViewAdapter(songList, MainActivity.this);
        recyclerView.setAdapter(mAdapter);


    }

    public void sendDataToActivity (int index){
        Intent intent = new Intent(this, PlaySongActivity.class);
        intent.putExtra("index", index);
        startActivity(intent);
    }

    public void handleSelection(View myView){
        String resourceId = getResources().getResourceEntryName(myView.getId());   //myView.getId() returns int that android makes for the ImgBtn, the rest of ir helps discover the String ID,(S1001, S1002)
        int currentArrayIndex = songCollection.searchSongById(resourceId);
        Log.d("temasek", "The id of the pressed ImageButton is: " + resourceId);
        sendDataToActivity(currentArrayIndex);
    }

}

RecyclerViewAdapter：

package com.example.decibel;

import android.content.Context;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;
import android.widget.ImageView;
import android.widget.TextView;

import androidx.annotation.NonNull;
import androidx.recyclerview.widget.RecyclerView;

import com.squareup.picasso.Picasso;

import org.jetbrains.annotations.NotNull;
import org.w3c.dom.Text;

import java.util.List;

public class RecycleViewAdapter extends RecyclerView.Adapter<RecycleViewAdapter.MyViewHolder> {

    List<Song> songList;
    Context context;

    public RecycleViewAdapter(List<Song> songList, Context context) {
        this.songList = songList;
        this.context = context;
    }

    @NonNull
    @NotNull
    @Override
    public MyViewHolder onCreateViewHolder(@NonNull @NotNull ViewGroup parent, int viewType) {

        View view = LayoutInflater.from(parent.getContext()).inflate(R.layout.one_song,parent,false);
        return new MyViewHolder(view);
    }

    @Override
    public void onBindViewHolder(@NonNull @NotNull RecycleViewAdapter.MyViewHolder holder, int position) {
        holder.songName.setText(songList.get(position).getTitle());
        holder.artistName.setText(songList.get(position).getArtist());
        Picasso.get().load(songList.get(position).getCoverArt()).into(holder.coverArt);
    }

    @Override
    public int getItemCount() {
        return songList.size();
    }

    public class MyViewHolder extends RecyclerView.ViewHolder {
        ImageView coverArt;
        TextView songName;
        TextView artistName;

        public MyViewHolder(@NonNull @org.jetbrains.annotations.NotNull View itemView) {
            super(itemView);
            coverArt = itemView.findViewById(R.id.coverArt);
            songName = itemView.findViewById(R.id.songName);
            artistName = itemView.findViewById(R.id.artistName);
        }
    }
}

歌曲合集：

package com.example.decibel;

import java.util.ArrayList;
import java.util.Arrays;
import java.util.List;

public class SongCollection {

    List<Song> songList = new ArrayList<Song>();

    private Song songs[] = new Song[25];

    public SongCollection(){
        Song theWayYouLookTonight = new Song(
                "S1001",
                "The Way You Look Tonight",
                "Michael Bublé",
                "pop",
                "https://p.scdn.co/mp3-preview/a5b8972e764025020625bbf9c1c2bbb06e394a60?cid=2afe87a64b0042dabf51f37318616965",
                "https://cdns-images.dzcdn.net/images/cover/a5dd92ae1b53ad19f39e3036dd510541/500x500-000000-80-0-0.jpg",
                4.66);


        Song billieJean = new Song(
                "S1002",
                "Billie Jean",
                "Michael Jackson",
                "pop",
                "https://p.scdn.co/mp3-preview/14a1ddedf05a15ad0ac11ce28b40ea1a15fabd20?cid=2afe87a64b0042dabf51f37318616965",
                "https://cdns-images.dzcdn.net/images/cover/ebeac32e9207c60877228ddc5bb37233/500x500-000000-80-0-0.jpg",
                4.9);


        Song dreamyNight = new Song(
                "S1003",
                "dreamy night",
                "LilyPichu",
                "lofi",
                "https://p.scdn.co/mp3-preview/9956e838630df9a0dfdb3753ad38ef1fc7cee5e7?cid=2afe87a64b0042dabf51f37318616965",
                "https://cdns-images.dzcdn.net/images/cover/6b2f8b3ff0e6cd136e685afa2940fcba/500x500-000000-80-0-0.jpg",
                4.1 );


        songs[0] = theWayYouLookTonight;
        songs[1] = billieJean;
        songs[2] = dreamyNight;

        songList.addAll(Arrays.asList());

    }

    public List<Song> getSongList() {
        return songList;
    }

    public Song getCurrentSong(int currentSongId){
        return songs[currentSongId];
    }

    //Finds song which has the same id and returns its position the the songs array
    public int searchSongById(String id){
        for (int index=0; index < songs.length; index++){
            Song tempSong = songs[index];
            if(tempSong.getId().equals(id)){
                return index;
            }
        }
        return -1;
    }

    public int getNextSong(int currentSongIndex) {
        if (currentSongIndex >= songs.length-1){
            return currentSongIndex;
        }
        else{
            return currentSongIndex + 1;
        }
    }

    public int getPrevSong(int currentSongIndex) {
        if (currentSongIndex <= 0){
            return currentSongIndex;
        }
        else{
            return currentSongIndex - 1;
        }
    }
}

回收商 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="com.example.decibel.MainActivity">

    <androidx.recyclerview.widget.RecyclerView
        android:id="@+id/songList"
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintHorizontal_bias="0.5"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toTopOf="parent" />
</androidx.constraintlayout.widget.ConstraintLayout>

布局 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"
    android:id="@+id/oneSongLayout">


    <ImageView
        android:id="@+id/coverArt"
        android:layout_width="100dp"
        android:layout_height="100dp"
        android:layout_marginStart="16dp"
        android:layout_marginTop="16dp"
        android:scaleType="centerCrop"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toTopOf="parent"
        app:srcCompat="@drawable/home1" />

    <TextView
        android:id="@+id/songName"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_marginStart="16dp"
        android:layout_marginTop="20dp"
        android:text="TextView"
        android:textAlignment="textStart"
        android:textSize="16sp"
        app:layout_constraintStart_toEndOf="@+id/coverArt"
        app:layout_constraintTop_toTopOf="@+id/coverArt" />

    <TextView
        android:id="@+id/artistName"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_marginStart="16dp"
        android:layout_marginTop="20dp"
        android:text="TextView"
        android:textSize="12sp"
        app:layout_constraintStart_toEndOf="@+id/coverArt"
        app:layout_constraintTop_toBottomOf="@+id/songName" />


</androidx.constraintlayout.widget.ConstraintLayout>

抱歉，如果問題不是很合適，我對 Android 中的編碼很陌生

Answer 1

在您的布局中，回收器的 ID @+id/songList ，但在代碼中，您正在執行findViewById(R.id.oneSongLayout) 。 oneSongLayout在布局中是不存在的，所以recyclerview是null。

將行更改為：

recyclerView = findViewById(R.id.songList);