java - 如何修复GPS服务程序中的空指针异常？

Question

我对 Android 相当陌生，我想将 GPS 显示的坐标保存在文本文件中。我之前问过这个问题，我也得到了回复。然而，当我尝试代码时，我遇到了空指针异常。有谁对如何解决这个问题有任何想法？ (下面的大部分代码不是我的)。

链接到我之前的问题:How do I get coordinates from a GPS service into a text file?

当前代码:

public class MainActivity extends AppCompatActivity {

private Button btn_start, btn_stop;
public Button save;
BufferedWriter writer;

private BroadcastReceiver broadcastReceiver;
int delay = 1000; // delay for 1 sec.
int period = 10000; // repeat every 10 sec.
Timer timer = new Timer();
public EditText editText;
public TextView textView;

public String path = Environment.getExternalStorageDirectory().getAbsolutePath() + "/aaTutorial";





@Override
protected void onResume() {
    super.onResume();
    if(broadcastReceiver == null){
        broadcastReceiver = new BroadcastReceiver() {
            @Override
            public void onReceive(Context context, Intent intent)
                {

                    textView.append("\n" +intent.getExtras().get("coordinates"));

                    try {
                        writer.write((String) intent.getExtras().get("coordinates"));
                    } catch (IOException e) {
                        e.printStackTrace();
                    }
                    try {
                        writer.newLine();
                    } catch (IOException e) {
                        e.printStackTrace();
                    }

                }
            };
        };
        registerReceiver(broadcastReceiver,new IntentFilter("location_update"));
    }




protected void onDestroy() {
    super.onDestroy();
    try {
        writer.close();
    } catch (IOException e) {
        e.printStackTrace();
    }
    if(broadcastReceiver != null){
        unregisterReceiver(broadcastReceiver);
    }
}

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);
    try {
        writer = new BufferedWriter(new FileWriter("mnt/sdcard/gps.txt"));
    } catch (IOException e) {
        e.printStackTrace();
    }


    btn_start = (Button) findViewById(R.id.button);
    btn_stop = (Button) findViewById(R.id.button2);
    textView = (TextView) findViewById(R.id.textView);
 //   save = (Button) findViewById(R.id.save);
    File dir = new File(path);
    dir.mkdirs();

    if(!runtime_permissions())
        enable_buttons();

}

private void enable_buttons() {

    btn_start.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View view) {
            Intent i =new Intent(getApplicationContext(),GPS_Service.class);
            startService(i);
        }
    });




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

            Intent i = new Intent(getApplicationContext(),GPS_Service.class);
            stopService(i);

        }
    });

}

private boolean runtime_permissions() {
    if(Build.VERSION.SDK_INT >= 23 && ContextCompat.checkSelfPermission(this, Manifest.permission.ACCESS_FINE_LOCATION) != PackageManager.PERMISSION_GRANTED && ContextCompat.checkSelfPermission(this, Manifest.permission.ACCESS_COARSE_LOCATION) != PackageManager.PERMISSION_GRANTED){

        requestPermissions(new String[]{Manifest.permission.ACCESS_FINE_LOCATION, Manifest.permission.ACCESS_COARSE_LOCATION},100);

        return true;
    }
    return false;
}


@Override
public void onRequestPermissionsResult(int requestCode, @NonNull String[] permissions, @NonNull int[] grantResults) {
    super.onRequestPermissionsResult(requestCode, permissions, grantResults);
    if(requestCode == 100){
        if( grantResults[0] == PackageManager.PERMISSION_GRANTED && grantResults[1] == PackageManager.PERMISSION_GRANTED){
            enable_buttons();
        }else {
            runtime_permissions();
        }
    }
}
}

错误发生在OnResume方法下的第一次try/catch中。

与

Attempt to invoke virtual method 'void java.io.BufferedWriter.write(java.lang.String)' on a null object reference in the logcat.

Answer 1

您的写入器未初始化，因为您尝试写入的文件无效。首先，您省略了开头的 /，您的意思是 /mnt/sdcard。其次，不保证 SD 卡安装在何处。永远不要在android中使用sd卡的直接路径，使用getExternalDirectory来查找它。第三，在 kitkat 及以上版本上，这仍然会失败，因为您没有对 sdcard 根目录的写访问权限。