Android：无法找到显式活动 class，即使我在 AndroidManifest.xml 中定义了它？

Question

I am writing a simple application that triggers an alarm 5 seconds after the app launch, in which another activity (AlarmDialog) is launched. However, when I run my app, I get the following error:

Caused by: android.content.ActivityNotFoundException: Unable to find explicit activity class {com.example.basicalarmsetter/AlarmDialog}; have you declared this activity in your AndroidManifest.xml?

This seems strange, considering that I believe I have declared the activity in my AndroidManifest.xml file :

<?xml version="1.0" encoding="utf-8"?>
<manifest xmlns:android="http://schemas.android.com/apk/res/android"
    package="com.example.basicalarmsetter">

    <uses-permission android:name="com.android.alarm.permission.SET_ALARM" />

    <application
        android:icon="@mipmap/ic_launcher"
        android:label="@string/app_name"
        android:theme="@style/AppTheme" >
        <activity
            android:name=".MainActivity"
            android:label="MainActivity" >
            <intent-filter>
                <action android:name="android.intent.action.MAIN" />

                <category android:name="android.intent.category.LAUNCHER" />
            </intent-filter>
        </activity>

        <activity
            android:name=".AlarmDialog"
            android:label="@string/app_name"
            android:theme="@style/Theme.AppCompat.Light.NoActionBar">
        </activity>

        <receiver android:name=".AlarmReceiver"/>
    </application>

</manifest>

Below is my code for the app's other classes:

MainActivity.java :

package com.example.basicalarmsetter;

import android.app.Activity;
import android.app.AlarmManager;
import android.app.PendingIntent;
import android.content.Context;
import android.content.Intent;
import android.os.Bundle;

public class MainActivity extends Activity {
    private Context context;

    private AlarmManager alarmMgr;
    private PendingIntent alarmIntent;

    // Creates the Pending Intent used to set off the alarm
    private PendingIntent generateAlarmPendingIntent(Context context) {
        Intent intent = new Intent(context, AlarmReceiver.class);

        // Each alarm requires a unique id
        int alarmId = (int) (Math.random() * (10000 - 1 + 1) + 1);
        System.out.println("alarmId = " + alarmId);

        PendingIntent alarmPendingIntent = PendingIntent.getBroadcast(context, alarmId, intent, 0);
        return alarmPendingIntent;
    }

    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        context = getApplicationContext();

        // Set an alarm 5 seconds after now
        alarmMgr = (AlarmManager) context.getSystemService(Context.ALARM_SERVICE);

        alarmMgr.setExactAndAllowWhileIdle(AlarmManager.RTC_WAKEUP, System.currentTimeMillis() + 5000,
                generateAlarmPendingIntent(context));
    }
}

AlarmReceiver :

package com.example.basicalarmsetter;

import android.content.BroadcastReceiver;
import android.content.Context;
import android.content.Intent;

public class AlarmReceiver extends BroadcastReceiver {
    @Override
    public void onReceive(Context k1, Intent k2) {
        System.out.println("Alarm received!");

        Intent i = new Intent();
        i.setClassName("com.example.basicalarmsetter", "AlarmDialog");
        i.setFlags(Intent.FLAG_ACTIVITY_NEW_TASK);
        k1.startActivity(i);
    }
}

AlarmDialog :

package com.example.basicalarmsetter;

import android.app.Activity;
import android.os.Bundle;

public class AlarmDialog extends Activity {
    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.alarm_dialog);
    }
}

File Structure : All classes MainActivity , AlarmDialog , and AlarmReceiver , are under the same directory.

Answer 1

Replace:

        Intent i = new Intent();
        i.setClassName("com.example.basicalarmsetter", "AlarmDialog");

with:

        Intent i = new Intent(k1, AlarmDialog.class);

This will use a better constructor, one that helps avoid the mistake in your hard-coded strings. As a bonus, if you use an IDE to rename the AlertDialog class, your Intent will be modified as well.