Question

我是Android Studio的新手（也是Java的新手），我正在尝试创建一个应用程序，允许用户输入时间并让计时器在该时间发生时开始倒计时。在研究之后，似乎使用意图是最好的方法，但我在理解如何使用它时遇到了一些问题。现在，我的主活动文件中有以下代码（称为倒计时）：

public class Countdown extends AppCompatActivity {
EditText mHours;
EditText mMinutes;
TextView mDisplay;
public static long finalTime = 0;
private AlarmManager alarmMgr;
private PendingIntent alarmIntent;
Context context;

@Override
public void onCreate(Bundle savedInstanceState) {


    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_countdown);
    mHours = (EditText) findViewById(R.id.text1);
    mMinutes = (EditText) findViewById(R.id.text2);
    mDisplay = (TextView) findViewById(R.id.display);
    Button button = (Button) findViewById(R.id.button1);

    button.setOnClickListener(new View.OnClickListener() {
        public void onClick(View v) {
            Calendar calendar = Calendar.getInstance();
            calendar.setTimeInMillis(System.currentTimeMillis());
            calendar.set(Calendar.HOUR_OF_DAY, 11);
            calendar.set(Calendar.MINUTE, 22);
            Intent intent = new Intent(context, Countdown.class);
            alarmMgr = (AlarmManager) context.getSystemService(Context.ALARM_SERVICE);

            alarmIntent = PendingIntent.getBroadcast(context, 0, intent, 0);
            alarmMgr.set(AlarmManager.RTC, calendar.getTimeInMillis(), alarmIntent);

            final long hourInput = Long.parseLong(mHours.getText().toString());
            final long minuteInput = Long.parseLong(mMinutes.getText().toString());
            finalTime = (hourInput * 3600000) + (minuteInput *  60000);



    }
} );}}

在我的接收器文件中，我有：

public class CountdownStarter {
TextView mDisplay;
public void onRecieve() {

    final CountDownTimer aCounter = new CountDownTimer(Countdown.finalTime, 1000) {

        public void onTick(long millisUntilFinished) {
            mDisplay.setText("Seconds remaining: " + millisUntilFinished / 1000);
        }

        public void onFinish() {
            mDisplay.setText("Finished");
        }
    };
    aCounter.start();
}
}

现在，运行调试器我收到此消息：

E/AndroidRuntime: FATAL EXCEPTION: main
              Process: com.example.android.timerexample, PID: 32276
              java.lang.NullPointerException: Attempt to invoke virtual method 'java.lang.String android.content.Context.getPackageName()' on a null object reference
                  at android.content.ComponentName.<init>(ComponentName.java:128)
                  at android.content.Intent.<init>(Intent.java:5283)
                  at com.example.android.timerexample.Countdown$1.onClick(Countdown.java:42)
                  at android.view.View.performClick(View.java:5702)
                  at android.widget.TextView.performClick(TextView.java:10888)
                  at android.view.View$PerformClick.run(View.java:22533)
                  at android.os.Handler.handleCallback(Handler.java:739)
                  at android.os.Handler.dispatchMessage(Handler.java:95)
                  at android.os.Looper.loop(Looper.java:158)
                  at android.app.ActivityThread.main(ActivityThread.java:7229)
                  at java.lang.reflect.Method.invoke(Native Method)
                  at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:1230)
                  at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:1120)

我试图调查它，但我不明白为什么我会得到NullPointerException或如何解决它 - 我肯定在这里做了一些错误但我无法弄清楚是什么下一步是。任何帮助都会非常有帮助！

Answer 1

在代码段中没有启动上下文，如上所述，您不需要活动类中的上下文对象。

您可以替换此代码

Intent intent = new Intent(context, Countdown.class);
alarmMgr = (AlarmManager) context.getSystemService(Context.ALARM_SERVICE);

alarmIntent = PendingIntent.getBroadcast(context, 0, intent, 0);

带

Intent intent = new Intent(this, Countdown.class);
alarmMgr = (AlarmManager) getSystemService(Context.ALARM_SERVICE);

alarmIntent = PendingIntent.getBroadcast(this, 0, intent, 0);

Answer 2

Activity是Context，因此Context context不是必需的，您可以在此使用Countdown.this而不是context变量。

Intent intent = new Intent(context, Countdown.class); // here
alarmMgr = (AlarmManager) getSystemService(Context.ALARM_SERVICE);
alarmIntent = PendingIntent.getBroadcast(context, 0, intent, 0); // and here

或者您需要实际分配context

private Context;

@Override
public void onCreate(Bundle savedInstanceState) {

    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_countdown);
    context = this; // Here

使用意图创建警报并遇到一些问题

2 个答案: