我写完整的可以运行的代码。使用一个Timepicker控件和一个imagebutton实现闹钟功能。点击按钮可切换闹钟的开和关。手机到timepicker上的时间时，闹钟触发并且循环播放MP3音乐，点击按钮可取消播放。

您好，我可以回答这个问题。以下是代码示例：

import android.app.AlarmManager;
import android.app.PendingIntent;
import android.content.Context;
import android.content.Intent;
import android.media.MediaPlayer;
import android.os.Bundle;
import android.view.View;
import android.widget.ImageButton;
import android.widget.TimePicker;
import android.widget.Toast;

import androidx.appcompat.app.AppCompatActivity;

import java.util.Calendar;

public class MainActivity extends AppCompatActivity {

    private TimePicker timePicker;
    private ImageButton imageButton;
    private MediaPlayer mediaPlayer;
    private PendingIntent pendingIntent;
    private AlarmManager alarmManager;

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

        timePicker = findViewById(R.id.timePicker);
        imageButton = findViewById(R.id.imageButton);
        mediaPlayer = MediaPlayer.create(this, R.raw.alarm);

        Intent intent = new Intent(MainActivity.this, AlarmReceiver.class);
        pendingIntent = PendingIntent.getBroadcast(MainActivity.this, , intent, );
        alarmManager = (AlarmManager) getSystemService(Context.ALARM_SERVICE);

        imageButton.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                if (alarmManager != null) {
                    if (imageButton.getTag() == null) {
                        int hour, minute;
                        if (android.os.Build.VERSION.SDK_INT >= android.os.Build.VERSION_CODES.M) {
                            hour = timePicker.getHour();
                            minute = timePicker.getMinute();
                        } else {
                            hour = timePicker.getCurrentHour();
                            minute = timePicker.getCurrentMinute();
                        }

                        Calendar calendar = Calendar.getInstance();
                        calendar.set(Calendar.HOUR_OF_DAY, hour);
                        calendar.set(Calendar.MINUTE, minute);
                        calendar.set(Calendar.SECOND, );
                        calendar.set(Calendar.MILLISECOND, );

                        long timeInMillis = calendar.getTimeInMillis();
                        long currentTimeInMillis = System.currentTimeMillis();

                        if (timeInMillis > currentTimeInMillis) {
                            alarmManager.setExact(AlarmManager.RTC_WAKEUP, timeInMillis, pendingIntent);
                            imageButton.setImageResource(R.drawable.ic_alarm_on);
                            imageButton.setTag("on");
                            Toast.makeText(MainActivity.this, "闹钟已开启", Toast.LENGTH_SHORT).show();
                        } else {
                            Toast.makeText(MainActivity.this, "请选择未来的时间", Toast.LENGTH_SHORT).show();
                        }
                    } else {
                        alarmManager.cancel(pendingIntent);
                        imageButton.setImageResource(R.drawable.ic_alarm_off);
                        imageButton.setTag(null);
                        mediaPlayer.stop();
                        Toast.makeText(MainActivity.this, "闹钟已关闭", Toast.LENGTH_SHORT).show();
                    }
                }
            }
        });
    }

    public static class AlarmReceiver extends BroadcastReceiver {
        @Override
        public void onReceive(Context context, Intent intent) {
            MediaPlayer mediaPlayer = MediaPlayer.create(context, R.raw.alarm);
            mediaPlayer.setLooping(true);
            mediaPlayer.start();
        }
    }
}

希望对您有所帮助！