java - 如何修复空对象引用上的 'void android.widget.TextView.setText(java.lang.CharSequence)'？

Question

我是安卓新手。我目前正在开发一个在 map 上显示多个位置的应用程序，该应用程序的标记位置取 self 已经设置的中央数据库，并且我知道正在工作如何解决我的问题。

这是我的 MainActivity.java 代码

package com.example.defiblocator;
import android.Manifest;
import android.content.pm.PackageManager;
import android.location.Location;
import android.os.Build;
import android.os.Bundle;
import android.widget.TextView;

import androidx.annotation.NonNull;
import androidx.core.content.ContextCompat;
import androidx.fragment.app.FragmentActivity;
import com.google.android.gms.common.ConnectionResult;
import com.google.android.gms.common.api.GoogleApiClient;
import com.google.android.gms.location.LocationListener;
import com.google.android.gms.location.LocationRequest;
import com.google.android.gms.location.LocationServices;
import com.google.android.gms.maps.CameraUpdateFactory;
import com.google.android.gms.maps.GoogleMap;
import com.google.android.gms.maps.OnMapReadyCallback;
import com.google.android.gms.maps.SupportMapFragment;
import com.google.android.gms.maps.model.BitmapDescriptorFactory;
import com.google.android.gms.maps.model.LatLng;
import com.google.android.gms.maps.model.Marker;
import com.google.android.gms.maps.model.MarkerOptions;

import org.w3c.dom.Text;


public class MainActivity extends FragmentActivity implements OnMapReadyCallback,
        LocationListener,GoogleApiClient.ConnectionCallbacks,
        GoogleApiClient.OnConnectionFailedListener {

    GoogleMap mapAPI;
    SupportMapFragment mapFragment;
    Location mLastLocation;
    Marker mCurrLocationMarker;
    GoogleApiClient mGoogleApiClient;
    LocationRequest mLocationRequest;
    public static TextView data;



    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        fetchData process = new fetchData();
        process.execute();
        mapFragment = (SupportMapFragment) getSupportFragmentManager()
                .findFragmentById(R.id.mapAPI);
        mapFragment.getMapAsync(this);

    }



    @Override
    public void onMapReady(GoogleMap googleMap) {

        mapAPI = googleMap;
        LatLng fox = new LatLng(52.187907,-0.143284);
        mapAPI.addMarker(new MarkerOptions().position(fox).title("test"));
        mapAPI.moveCamera(CameraUpdateFactory.newLatLng(fox));
        if (android.os.Build.VERSION.SDK_INT >= Build.VERSION_CODES.M) {
            if (ContextCompat.checkSelfPermission(this,
                    Manifest.permission.ACCESS_FINE_LOCATION)
                    == PackageManager.PERMISSION_GRANTED) {
                buildGoogleApiClient();
                mapAPI.setMyLocationEnabled(true);
            }
        } else {
            buildGoogleApiClient();
            mapAPI.setMyLocationEnabled(true);
        }

    }

    protected synchronized void buildGoogleApiClient() {
        mGoogleApiClient = new GoogleApiClient.Builder(this)
                .addConnectionCallbacks(this)
                .addOnConnectionFailedListener(this)
                .addApi(LocationServices.API).build();
        mGoogleApiClient.connect();
    }

    @Override
    public void onConnected(Bundle bundle) {

        mLocationRequest = new LocationRequest();
        mLocationRequest.setInterval(1000);
        mLocationRequest.setFastestInterval(1000);
        mLocationRequest.setPriority(LocationRequest.PRIORITY_BALANCED_POWER_ACCURACY);
        if (ContextCompat.checkSelfPermission(this,
                Manifest.permission.ACCESS_FINE_LOCATION)
                == PackageManager.PERMISSION_GRANTED) {
            LocationServices.FusedLocationApi.requestLocationUpdates(mGoogleApiClient, mLocationRequest, this);
        }

    }

    @Override
    public void onConnectionSuspended(int i) {

    }

    @Override
    public void onLocationChanged(Location location) {

        mLastLocation = location;
        if (mCurrLocationMarker != null) {
            mCurrLocationMarker.remove();
        }
        //Place current location marker
        LatLng latLng = new LatLng(location.getLatitude(), location.getLongitude());
        MarkerOptions markerOptions = new MarkerOptions();
        markerOptions.position(latLng);
        markerOptions.title("Current Position");
        markerOptions.icon(BitmapDescriptorFactory.defaultMarker(BitmapDescriptorFactory.HUE_AZURE));
        mCurrLocationMarker = mapAPI.addMarker(markerOptions);

        //move map camera
        mapAPI.moveCamera(CameraUpdateFactory.newLatLng(latLng));
        mapAPI.animateCamera(CameraUpdateFactory.zoomTo(11));

        //stop location updates
        if (mGoogleApiClient != null) {
            LocationServices.FusedLocationApi.removeLocationUpdates(mGoogleApiClient, this);
        }

    }

    @Override
    public void onConnectionFailed(@NonNull ConnectionResult connectionResult) {

    }

}

这是我从数据库获取数据并将其转换为 JSON 的代码。

fetchData.java

package com.example.defiblocator;

import android.os.AsyncTask;
import android.widget.TextView;

import org.json.JSONArray;
import org.json.JSONException;
import org.json.JSONObject;
import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStream;
import java.io.InputStreamReader;
import java.net.HttpURLConnection;
import java.net.MalformedURLException;
import java.net.URL;

public class fetchData extends AsyncTask <Void,Void,Void>{

    String data = "";
    String json_url;
    String dataParsed = "";
    String singleParsed = "";



    public Integer x = 0;
    @Override
    protected void onPreExecute(){
        json_url = "http://defiblocator.ml/json_get_data.php";
    }
    @Override
    protected Void doInBackground(Void... voids) {


        try {
            URL url = new URL(json_url);
            HttpURLConnection httpURLConnection = (HttpURLConnection)url.openConnection();
            InputStream inputStream = httpURLConnection.getInputStream();
            BufferedReader bufferedReader = new BufferedReader(new InputStreamReader(inputStream));

            String line = "";
            data = data + line;


            while(line != null){
                line = bufferedReader.readLine();
            }

            JSONArray JA = new JSONArray(data);
            for(int i =0 ; i <JA.length(); i++){
                JSONObject JO = (JSONObject) JA.get(i);
                singleParsed = JO.get("name") + "," + JO.get("lat") + "," +JO.get("lng") + ",";
                dataParsed = dataParsed + singleParsed ;
            }
        } catch (MalformedURLException e) {
            e.printStackTrace();
        } catch (IOException | JSONException e){
            e.printStackTrace();
        }

        return null;
    }

    @Override
    protected void onPostExecute(Void aVoid) {
        MainActivity.data.setText(dataParsed);

    }
}

当我尝试将转换后的数据发送回主 Activity 以处理名称、纬度和经度时，我的错误仅发生在 PostExecute() 中。如果有比我所分享的方法更简单的方法，我将使用 3 个单独的数组来完成此操作，但否则我们将不胜感激任何帮助，因为我相信我可以将 TextView 更改为字符串并将其从那里拆分为其组成部分。

Answer 1

正如评论中提到的，您需要使用接口(interface)实现回调才能使用异步任务的结果。首先创建一个接口(interface)。

New -> Jave Class -> Interface -> CallbackInterface .java. Define the method prototypes.

CallbackInterface.java

public interface CallbackInterface {
    public void onSuccess(String callbackData);
    public void onFailure();
}

在您的 MainActivity 中，定义一个实现该接口(interface)的类。请注意，方法必须在此处定义，并且必须包含使用异步任务结果的代码MainActivity.java

public class MainActivity extends AppCompatActivity {
    TextView data;
    Button performAsync;
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        //Add this without fail.
        data = findViewById(R.id.dataTV);

        performAsync = findViewById(R.id.performAsync);

        performAsync.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {


       //Recreating your "process"
            new AsyncTaskClass(new CallbackClass()).execute();
        }
    });

}

/*Create a class which implements the interface. It is a good practise to have
onSuccess and onFailure methods */
public class CallbackClass implements CallbackInterface {

    @Override
    public void onSuccess(String callbackData) {
        //do your callback functions here. In your case, setting the textview to result of parsed data.
        data.setText(callbackData);
    }

    @Override
    public void onFailure() {

    }
}

}

最后，在 fetchData 类(我的代码中为 AsyncTaskClass)中执行异步操作，最后在 postExecute 中使用适当的参数调用 onSuccess 方法。请注意，您应该将该类作为参数传递给 fetchData 类，并使用构造函数将其分配给接口(interface)的实例。AsyncTaskClass.java

public class AsyncTaskClass extends AsyncTask {
    CallbackInterface callbackInterface;
    String dataParsed;
    public AsyncTaskClass(CallbackInterface callbackInterface) {
        this.callbackInterface = callbackInterface;
    }

    @Override
    protected Object doInBackground(Object[] objects) {
        dataParsed = "Processed Data";
        return null;
    }

    @Override
    protected void onPreExecute() {
        dataParsed = "";
    }

    @Override
    protected void onPostExecute(Object o) {
        callbackInterface.onSuccess(dataParsed);
    }
}

<小时/>

虽然这看起来太多了，但这是 AsyncTaskClass 和使用它的类之间进行通信的唯一方法。如果您无法理解，请随时要求澄清。我在我的项目中检查了上面的答案，它运行得很好。