Android NullPointerException:println 需要一条消息

Question

我正在开发一个假电话应用程序，我已经提供了一个快速呼叫的选项。我在quickCall()方法中拥有此选项的所有逻辑。

这是MainActivity的代码

public class MainActivity extends AppCompatActivity {
public static final String LOG_TAG = "MainActivity";
ImageView imageView;
EditText number, name;
Button setTimeButton;
ImageButton imageButton;
TimePickerDialog timePicker;
long selectedTimeInMillis;
String enteredName, enteredNumber;
private static final int SELECT_IMAGE = 100;
public static final int REQUEST_CODE_PI = 1001;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
PreferenceManager.setDefaultValues(this, R.xml.settings, false);
imageView = findViewById(R.id.image_view);
setTimeButton = findViewById(R.id.set_time);
imageButton = findViewById(R.id.place_call_button);
number = findViewById(R.id.number);
name = findViewById(R.id.name);
imageView.setOnClickListener(new View.OnClickListener() {
@Override
public void onClick(View v) {
Intent galleryIntent = new Intent(Intent.ACTION_PICK);
galleryIntent.setType("image/*");
startActivityForResult(galleryIntent, SELECT_IMAGE);
}
});
setTimeButton.setOnClickListener(new View.OnClickListener() {
@Override
public void onClick(View v) {
final Calendar calendar = Calendar.getInstance();
int hour = calendar.get(Calendar.HOUR_OF_DAY);
int minute = calendar.get(Calendar.MINUTE);
timePicker = new TimePickerDialog(MainActivity.this, new TimePickerDialog.OnTimeSetListener() {
@Override
public void onTimeSet(TimePicker view, int hourOfDay, int minute) {
calendar.set(Calendar.HOUR_OF_DAY, hourOfDay);
calendar.set(Calendar.MINUTE, minute);
calendar.set(Calendar.SECOND, 0);
calendar.set(Calendar.MILLISECOND, 0);
selectedTimeInMillis = calendar.getTimeInMillis();
if (hourOfDay > 12) {
hourOfDay = hourOfDay - 12;
}
Toast.makeText(MainActivity.this, hourOfDay + ":" + minute, Toast.LENGTH_SHORT).show();
Log.v(LOG_TAG, "Selected time in millis:" + selectedTimeInMillis);
}
}, hour, minute, false);
timePicker.show();
}
});
imageButton.setOnClickListener(new View.OnClickListener() {
@Override
public void onClick(View v) {
enteredName = name.getText().toString().trim();
enteredNumber = number.getText().toString().trim();
if (enteredNumber.isEmpty()){
Toast.makeText(MainActivity.this, "Please enter  a number", Toast.LENGTH_LONG).show();
return;
}
Intent intent = new Intent(MainActivity.this, AlarmReciever.class);
intent.putExtra("FAKE_NAME", enteredName);
intent.putExtra("FAKE_NUMBER", enteredNumber);
PendingIntent pendingIntent = PendingIntent.getBroadcast(getApplicationContext(), REQUEST_CODE_PI, intent, 0);
AlarmManager alarmManager = (AlarmManager) getSystemService(ALARM_SERVICE);
if (alarmManager != null) {
alarmManager.set(AlarmManager.RTC_WAKEUP, selectedTimeInMillis, pendingIntent);
Toast.makeText(MainActivity.this, "Your call has been placed", Toast.LENGTH_SHORT).show();
} else {
Toast.makeText(MainActivity.this, "Error: something wrong", Toast.LENGTH_SHORT).show();
}
}
});
}
@Override
public boolean onCreateOptionsMenu(Menu menu) {
getMenuInflater().inflate(R.menu.menu_main, menu);
return true;
}
@Override
public boolean onOptionsItemSelected(MenuItem item) {
switch (item.getItemId()) {
case R.id.settings:
Intent intent = new Intent(MainActivity.this, SettingsActivity.class);
startActivity(intent);
return true;
case R.id.quick_call:
quickCall();
return true;
default:
return super.onOptionsItemSelected(item);
}
}
@Override
protected void onActivityResult(int requestCode, int resultCode, Intent data) {
super.onActivityResult(requestCode, resultCode, data);
if (resultCode == RESULT_OK) {
try {
Uri imageUri = data.getData();
InputStream imageStream = null;
if (imageUri != null) {
imageStream = getContentResolver().openInputStream(imageUri);
}
Bitmap selectedImage = BitmapFactory.decodeStream(imageStream);
imageView.setImageBitmap(selectedImage);
} catch (FileNotFoundException e) {
e.printStackTrace();
Toast.makeText(this, "Something went wrong", Toast.LENGTH_SHORT).show();
}
} else {
Toast.makeText(this, "You haven't picked any Image", Toast.LENGTH_SHORT).show();
}
}
public void quickCall(){
Calendar calendar = Calendar.getInstance();
calendar.get(Calendar.HOUR_OF_DAY);
calendar.get(Calendar.MINUTE);
long millis = calendar.getTimeInMillis();
Intent quickCallIntent = new Intent(MainActivity.this, AlarmReciever.class);
PendingIntent quickCallPendingIntent = PendingIntent.getBroadcast(getApplicationContext(), REQUEST_CODE_PI, quickCallIntent, 0);
AlarmManager alarmManager = (AlarmManager) getSystemService(ALARM_SERVICE);
if (alarmManager != null) {
alarmManager.set(AlarmManager.RTC_WAKEUP, millis + 10000, quickCallPendingIntent);
Toast.makeText(MainActivity.this, "Your call has been placed", Toast.LENGTH_SHORT).show();
} else {
Toast.makeText(MainActivity.this, "Error: something wrong", Toast.LENGTH_SHORT).show();
}
}
}

这是AlarmReceiver类

public class AlarmReciever extends BroadcastReceiver {
@Override
public void onReceive(Context context, Intent intent) {
String getFakeName = intent.getStringExtra("FAKE_NAME");
String getFakeNumber = intent.getStringExtra("FAKE_NUMBER");
Intent fakeRinger = new Intent();
fakeRinger.setClassName("com.example.mani.fakecall", "com.example.mani.fakecall.FakeRinger");
fakeRinger.addFlags(Intent.FLAG_ACTIVITY_NEW_TASK);
fakeRinger.putExtra("USER_FAKE_NAME", getFakeName);
fakeRinger.putExtra("USER_FAKE_NUMBER", getFakeNumber);
context.startActivity(fakeRinger);
Log.v("Fake name is", getFakeName);
Log.v("Fake number is", getFakeNumber);
}
}

这是FakeRinger活动

public class FakeRinger extends AppCompatActivity {
TextView displayName, displayNumber, displayCarrier;
ImageButton alarmOffButton;
String carrierName;
ImageView displayImage;
public static final int REQUEST_CODE = 1001;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_fake_ringer);
displayName = findViewById(R.id.display_name);
displayNumber = findViewById(R.id.display_number);
displayCarrier = findViewById(R.id.network_operator);
displayImage = findViewById(R.id.display_image);
TelephonyManager telephonyManager = (TelephonyManager) getSystemService(TELEPHONY_SERVICE);
if (telephonyManager != null) {
carrierName = telephonyManager.getNetworkOperatorName();
}
if (carrierName != null) {
displayCarrier.setText(getResources().getString(R.string.incoming_call, carrierName));
} else {
displayCarrier.setText(getResources().getString(R.string.incoming_call));
}
Intent intent = getIntent();
String userFakeName = intent.getExtras().getString("USER_FAKE_NAME");
displayName.setText(userFakeName);
String userFakeNumber = intent.getExtras().getString("USER_FAKE_NUMBER");
displayNumber.setText(userFakeNumber);
alarmOffButton = findViewById(R.id.hang_up_call);
alarmOffButton.setOnClickListener(new View.OnClickListener() {
@Override
public void onClick(View v) {
Intent intent = new Intent(getApplicationContext(), AlarmReciever.class);
PendingIntent pendingIntent = PendingIntent.getBroadcast(getApplicationContext(), REQUEST_CODE, intent, 0);
AlarmManager alarmManager = (AlarmManager) getSystemService(ALARM_SERVICE);
stopService(intent);
if (alarmManager != null) {
alarmManager.cancel(pendingIntent);
}
}
});
}
}

每当我单击快速呼叫选项时，我的应用程序都会崩溃。这是我得到的错误 -java.lang.RuntimeException: Unable to start receiver com.example.mani.fakecall.AlarmReciever: java.lang.NullPointerException: println needs a message

Answer 1

当您尝试在LogCat中打印值时，并且值为空，您会收到错误消息：

java.lang.NullPointerException： println 需要一条消息

因此，您可以验证空值：

Log.v("Fake name is", getFakeName!=null?getFakeName:"Not determinated");
Log.v("Fake number is", getFakeNumber!=null?getFakeNumber:"Not determinated");

或者只是

if(getFakeName != null){
Log.v("Fake name is", getFakeName);
}
if(getFakeNumber != null){
Log.v("Fake number is", getFakeNumber);
}

---

如果我们查看源代码Log.v()，消息需要一个字符串值(不是空值)：

public static int v(String tag, String msg, Throwable tr) {
return println(LOG_ID_MAIN, VERBOSE, tag, msg + 'n' + getStackTraceString(tr));
}

Answer 2

只需在将值打印到 logcat 之前添加检查：

if(getFakeName != null)
Log.v("Fake name is", getFakeName);
if(getFakeNumber != null)
Log.v("Fake number is", getFakeNumber);

你需要向Log.v()传递一个值，该值不能null

Answer 3

检查您是否在AndroidManifest.xml中声明了 AlarmReciever

https://developer.android.com/guide/components/broadcasts.html#receiving_broadcasts