java - 无法从 ListView 通过 Intent 正确传递数据

Question

我尝试将在 ListView 中选择的数据项传递到下一个 Activity。但我所能做的就是第二个 Activity 为空或 nullpoineException 错误。我用 RecycleView 做到了，但用 ListView 则出现了一些问题。

主要 Activity :

public class ListViewPlaces extends AppCompatActivity {

FirebaseFirestore mFirestore = FirebaseFirestore.getInstance();
CollectionReference placeRef = mFirestore.collection("Places");
Query query;
ListView mListView;
String name;
Places places;
ArrayList<Places> randomPlaceList;
PlaceAdapter mAdapter;

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_list_view_places);
    mListView = findViewById(R.id.place_list);

    randomPlaceList = new ArrayList<>();

    Boolean l_check1 = getIntent().getExtras().getBoolean("1");
    Boolean l_check2 = getIntent().getExtras().getBoolean("2");

    if (l_check2){
        query = placeRef.whereEqualTo("colour", "red");
    } else if (l_check1){
        query = placeRef.whereEqualTo("size", "1");
    } else {
        query = placeRef.whereEqualTo("size", "2");
    }
        query.get().addOnCompleteListener(new OnCompleteListener<QuerySnapshot>() {
            @Override
            public void onComplete(@NonNull Task<QuerySnapshot> task) {
                if (task.isSuccessful()) {
                    List<Places> placesList = new ArrayList<>();
                    for (DocumentSnapshot document : task.getResult()) {
                        Places place = document.toObject(Places.class);
                        placesList.add(place);
                    }
                    int placeCount = placesList.size();
                    Random randomGenerator = new Random();
                    ArrayList<Places> randomPlaceList = new ArrayList<>();
                    for (int i = 1; i <= 3; i++) {
                        randomPlaceList.add(placesList.get(randomGenerator.nextInt(placeCount)));
                    }
                    ListView mListView = (ListView) findViewById(R.id.place_list);
                    mAdapter = new PlaceAdapter(randomPlaceList, getBaseContext());
                    mListView.setAdapter(mAdapter);
                }
            }
        });


        mListView.setOnItemClickListener(new AdapterView.OnItemClickListener() {
            @Override
            public void onItemClick(AdapterView<?> parent, View view, int position, long id) {
                Places places = mAdapter.getItem(position);
                Intent intent = new Intent(getApplicationContext(), Card_activity.class);
                intent.putExtra("name", places.getName());
                startActivity(intent);
            }
        });

    }
}

我的模型类:

public class Places {
private String image, name;

public Places() {

}

public Places(String image, String name) {
    this.image = image;
    this.name = name;
}

public String getImage() { return image; }
public String getName() { return name; }
}

自定义适配器:

class PlaceAdapter extends ArrayAdapter<Places> {

private ArrayList<Places> dataSet;
Context mContext;

private static class ViewHolder {
    TextView name_text;
    ImageView image_text;
}

public PlaceAdapter(ArrayList<Places> dataSet, Context context ) {
    super(context, R.layout.item_list, dataSet);

    this.dataSet = dataSet;
    this.mContext = context;

}

@Override
public View getView(int position, View converView, ViewGroup parent) {

    Places places = getItem(position);
    ViewHolder viewHolder;

    final View result;

    if (converView == null) {
        viewHolder = new ViewHolder();
        LayoutInflater inflater = LayoutInflater.from(getContext());
        converView = inflater.inflate(R.layout.item_list, parent, false);
        viewHolder.name_text = (TextView) converView.findViewById(R.id.text_image_id);
        viewHolder.image_text = (ImageView) converView.findViewById(R.id.image_id);

        result = converView;

        converView.setTag(viewHolder);
    } else {
        viewHolder = (ViewHolder) converView.getTag();
        result = converView;
    }

    viewHolder.name_text.setText(places.getName());
    Glide.with(getContext()).load(places.getImage()).into(viewHolder.image_text);

    return converView;

第二个 Activity ，我传递数据:

public class Card_activity extends AppCompatActivity implements View.OnClickListener{

    TextView mTextView;

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

        mTextView = findViewById(R.id.head_name);
        String name = getIntent().getStringExtras("name")
        mTextView.setText(name);
}
}

已编辑:错误

java.lang.NullPointerException: Attempt to invoke virtual method 'java.lang.Object java.util.ArrayList.get(int)' on a null object reference

在这一行:

Places places = randomPlaceList.get(position);

错误:

Process: com.example.eugene.lafinalproduction, PID: 8352 java.lang.IllegalArgumentException: n <= 0: 0 at java.util.Random.nextInt(Random.java:182) at com.example.eugene.lafinalproduction.ListViewPlaces$1.onComplete(ListViewPlaces.java:63) at com.google.android.gms.tasks.zzj.run(Unknown Source) at android.os.Handler.handleCallback(Handler.java:815) at android.os.Handler.dispatchMessage(Handler.java:104) at android.os.Looper.loop(Looper.java:207) at android.app.ActivityThread.main(ActivityThread.java:5763) at java.lang.reflect.Method.invoke(Native Method) at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:789) at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:679)

这一行的问题:

randomPlaceList.add(placesList.get(randomGenerator.nextInt(placeCount)));

Answer 1

该错误意味着您的 randomPlaceList 为 null。

由于我看不到应用程序或您为获取错误而执行的逻辑，所以我只是根据我所看到的进行操作。

首先，如果您需要获取单击的项目，应该通过调用 adapter< 的 getItem(position) 方法来完成;您可以像下面这样轻松地实现它:

@Override
public Places getItem(int position) {
    if(dataSet != null && position < dataSet.size()){
       return dataSet.get(position);
    }
    return null;
}

现在，从您的 activity 中，您可以简单地执行如下调用:

Places places = mAdapter.getItem(position);

请记住在初始化适配器时添加对适配器的引用，方法是在 Activity 变量中添加 PlaceAdapter mAdapter; 并将初始化替换为

mAdapter = new PlaceAdapter(randomPlaceList, getBaseContext());
mListView.setAdapter(mAdapter);  

<小时/>

快速阅读代码，我也觉得这没有多大意义:

this.randomPlaceList = randomPlaceList;

但也许你删掉了一些代码，但我更愿意向你报告，以防万一:)

<小时/>

注意指定错误

错误就在这里，它与变量引用有关:

        query.get().addOnCompleteListener(new OnCompleteListener<QuerySnapshot>() {
            @Override
            public void onComplete(@NonNull Task<QuerySnapshot> task) {
                if (task.isSuccessful()) {
                    List<Places> placesList = new ArrayList<>();
                    for (DocumentSnapshot document : task.getResult()) {
                        Places place = document.toObject(Places.class);
                        placesList.add(place);
                    }                      
                    int placeCount = placesList.size();
                    Random randomGenerator = new Random();  

                    // In the line below, you specify a new ArrayList that is LOCAL, you don't refer to the Activity's object
                    ArrayList<Places> randomPlaceList = new ArrayList<>();
                    for (int i = 1; i <= 3; i++) {
                        randomPlaceList.add(placesList.get(randomGenerator.nextInt(placeCount)));
                    }
                    ListView mListView = (ListView) findViewById(R.id.place_list);
                    PlaceAdapter placeAdapter = new PlaceAdapter(randomPlaceList, getBaseContext());
                    mListView.setAdapter(placeAdapter);                    
            }
        });

您应该将其替换为:

        query.get().addOnCompleteListener(new OnCompleteListener<QuerySnapshot>() {
            @Override
            public void onComplete(@NonNull Task<QuerySnapshot> task) {
                if (task.isSuccessful()) {
                    List<Places> placesList = new ArrayList<>();
                    for (DocumentSnapshot document : task.getResult()) {
                        Places place = document.toObject(Places.class);
                        placesList.add(place);
                    }                      
                    int placeCount = placesList.size();
                    Random randomGenerator = new Random();

                    randomPlaceList = new ArrayList<>(); //this is the activity's list
                    for (int i = 1; i <= 3; i++) {
                        randomPlaceList.add(placesList.get(randomGenerator.nextInt(placeCount)));
                    }
                    ListView mListView = (ListView) findViewById(R.id.place_list);
                    PlaceAdapter placeAdapter = new PlaceAdapter(randomPlaceList, getBaseContext());
                    mListView.setAdapter(placeAdapter);                    
            }
        });

否则， Activity 的列表将始终为空，因为您从未初始化它

希望这有帮助