进度条NullPointerException

时间:2018-04-28 07:01:00

标签: java android nullpointerexception

当我尝试将进度条的可见性设置为VISIBLE时,我得到一个NPE(NullPointerException),即使我在调用导致应用程序崩溃的AsyncTask之前已经初始化它。 奇怪的是,我有另一个AsyncTask做同样的事情但是有效,我想因为我已经在RecyclerView上实现了一个onClickListener。 这是主要活动Java代码:

public class MainActivity extends AppCompatActivity {
private RecyclerView mTrainsRv;
private TrainAdapter mTrainAdapter;
ArrayList<Train> trains = new ArrayList<>();
ProgressBar mRvProgress;
ProgressBar mArrivalProgress;

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);
    Toolbar toolbar = (Toolbar) findViewById(R.id.toolbar);
    setSupportActionBar(toolbar);

    mArrivalProgress = findViewById(R.id.arrival_time_pb);
    mRvProgress = findViewById(R.id.rv_progress_bar);
    mTrainsRv = (RecyclerView) findViewById(R.id.train_rv);
    mTrainsRv.setLayoutManager(new LinearLayoutManager(this));
    mTrainAdapter = new TrainAdapter(createDefaultTrains());
    mTrainsRv.setAdapter(mTrainAdapter);

    FloatingActionButton fab = (FloatingActionButton) findViewById(R.id.fab);
    fab.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View view) {
            trains.add(new Train(4, "Berowra Platform 4", 12, "Late", "Beverly", "15:18"));
            mTrainAdapter.notifyDataSetChanged();
        }
    });

    mTrainAdapter.setClickListener(new ItemClickListener() {
        @Override
        public void onClick(View view, int position) {
            new RowUpdateAsync(position).execute();
            Toast.makeText(MainActivity.this, "YAS", Toast.LENGTH_SHORT).show();
        }
    });
}

private List<Train> createDefaultTrains() {
    trains.add(new Train(0, "Albion Park Platform 1", 3, "On Time", "Allawah", "14:11"));
    trains.add(new Train(1, "Arncliffe Platform 2", 4, "Late", "Central", "14:34"));
    trains.add(new Train(2, "Artarmion Platform 3", 7, "On Time", "Ashfield", "15:01"));
    trains.add(new Train(3, "Berowra Platform 4", 12, "Late", "Beverly", "15:18"));
    return trains;
}

@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.
    // Handle item selection
    switch (item.getItemId()) {
        case R.id.action_delete:
            trains.clear();
            mTrainAdapter.notifyDataSetChanged();
            return true;
        case R.id.action_refresh:
            new ViewUpdateAsync().execute();
            return true;
        case R.id.action_quit:
            Toast.makeText(this, R.string.quit_menu,
                    Toast.LENGTH_LONG).show();
            return true;
        default:
            return super.onOptionsItemSelected(item);
    }
}

private class ViewUpdateAsync extends AsyncTask<Void, Void, Integer[]> {

    @Override
    protected Integer[] doInBackground(Void... voids) {
        try {
            Thread.sleep(3000);
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
        Random r = new Random();
        Integer[] arr = new Integer[trains.size()];
        for (int i = 0; i < trains.size(); i++) {
            int min = 1;
            int max = 20;
            int ran = r.nextInt(max - min + 1) + min;
            arr[i] = ran;
        }
        return arr;
    }

    @Override
    protected void onPreExecute() {
        mRvProgress.setVisibility(View.VISIBLE);
        mTrainsRv.setAlpha(0);
        super.onPreExecute();
    }

    @Override
    protected void onPostExecute(Integer[] arrayList) {
        super.onPostExecute(arrayList);
        for (int i = 0; i < trains.size(); i++) {
            trains.get(i).setArrivalTime(arrayList[i]);
        }
        mTrainAdapter.notifyDataSetChanged();
        mTrainsRv.setAlpha(1);
        mRvProgress.setVisibility(View.GONE);
    }
}

private class RowUpdateAsync extends AsyncTask<Void, Void, Integer> {
    private int mPosition;

    private RowUpdateAsync(int position) {
        this.mPosition = position;
    }

    @Override
    protected Integer doInBackground(Void... voids) {
        try {
            Thread.sleep(2000);
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
        Random r = new Random();
        int min = 1;
        int max = 20;
        int ran = r.nextInt(max - min + 1) + min;
        return ran;
    }

    @Override
    protected void onPreExecute() {
        mArrivalProgress.setVisibility(View.VISIBLE);
        super.onPreExecute();
    }

    @Override
    protected void onPostExecute(Integer integer) {
        super.onPostExecute(integer);
        trains.get(mPosition).setArrivalTime(integer);
        mTrainAdapter.notifyDataSetChanged();
        mArrivalProgress.setVisibility(View.GONE);
    }
}

要获得进一步的帮助,这是Adapter类的代码:

public class TrainAdapter extends RecyclerView.Adapter<TrainAdapter.ViewHolder> {
private List<Train> mTrains;
public ItemClickListener mClickListener;

public TrainAdapter(List<Train> trains) {
    mTrains = trains;
}

@NonNull
@Override
public ViewHolder onCreateViewHolder(ViewGroup parent, int viewType) {
    View view = LayoutInflater.from(parent.getContext())
            .inflate(R.layout.train_item, parent, false);
    ViewHolder holder = new ViewHolder(view);

    return holder;
}

@Override
public void onBindViewHolder(@NonNull ViewHolder holder, int position) {
    Train train = mTrains.get(position);
    holder.mArrivalTimeTv.setText(String.valueOf(train.getArrivalTime()));
    holder.mPlatformTv.setText(train.getPlatform());
    holder.mStatusTv.setText(train.getStatus());
    holder.mDestinationTimeTv.setText(train.getDestinationTime());
    holder.mDestinationTv.setText(train.getDestination());
}

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

public void setClickListener(ItemClickListener itemClickListener) {
    mClickListener = itemClickListener;
}


class ViewHolder extends RecyclerView.ViewHolder implements View.OnClickListener{
    public TextView mArrivalTimeTv, mPlatformTv, mStatusTv, mDestinationTimeTv, mDestinationTv;
    public LinearLayout mArrivalLayout;

    public ViewHolder(View view) {
        super(view);

        mArrivalTimeTv = (TextView) view.findViewById(R.id.arrival_time);
        mPlatformTv = (TextView) view.findViewById(R.id.platform);
        mStatusTv = (TextView) view.findViewById(R.id.status);
        mDestinationTimeTv = (TextView) view.findViewById(R.id.destination_time);
        mDestinationTv = (TextView) view.findViewById(R.id.destination);
        mArrivalLayout = view.findViewById(R.id.time_layout);
        mArrivalLayout.setOnClickListener(this);
    }
    @Override
    public void onClick (View v){
        if (mClickListener != null) {
            mClickListener.onClick(v, getAdapterPosition());
        }
    }

}

崩溃日志:

04-28 16:51:56.064 4880-4880/com.mad.exercise5 E/AndroidRuntime: FATAL EXCEPTION: main
Process: com.mad.exercise5, PID: 4880
java.lang.NullPointerException: Attempt to invoke virtual method 'void android.widget.ProgressBar.setVisibility(int)' on a null object reference
    at com.mad.exercise5.MainActivity$RowUpdateAsync.onPreExecute(MainActivity.java:161)
    at android.os.AsyncTask.executeOnExecutor(AsyncTask.java:648)
    at android.os.AsyncTask.execute(AsyncTask.java:595)
    at com.mad.exercise5.MainActivity$2.onClick(MainActivity.java:55)
    at com.mad.exercise5.TrainAdapter$ViewHolder.onClick(TrainAdapter.java:69)
    at android.view.View.performClick(View.java:6294)
    at android.view.View$PerformClick.run(View.java:24770)
    at android.os.Handler.handleCallback(Handler.java:790)
    at android.os.Handler.dispatchMessage(Handler.java:99)
    at android.os.Looper.loop(Looper.java:164)
    at android.app.ActivityThread.main(ActivityThread.java:6494)
    at java.lang.reflect.Method.invoke(Native Method)
    at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:438)
    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:807)

1 个答案:

答案 0 :(得分:1)

由于它的空指针在使用之前尝试再次初始化asyntask中的进度条