<?xml version="1.0" encoding="utf-8"?>
<TimePicker xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:orientation="vertical"
    tools:context=".MainActivity">

    <TimePicker
        android:id="@+id/time"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"/>
    <Button
        android:id="@+id/button"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="设置闹钟"
        />

</TimePicker>

package com.example.myalarmi;

import androidx.appcompat.app.AlertDialog;
import androidx.appcompat.app.AppCompatActivity;

import android.content.DialogInterface;
import android.os.Bundle;

public class AlarmActivity extends AppCompatActivity {

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        AlertDialog alertDialog = new AlertDialog.Builder(this).create();
        alertDialog.setTitle("标题");
        alertDialog.setMessage("内容");
        alertDialog.setButton(
                DialogInterface.BUTTON_POSITIVE, "", new DialogInterface.OnClickListener() {
                    @Override
                    public void onClick(DialogInterface dialog, int which) { }
                }
        );
        alertDialog.show();
    }
}

package com.example.myalarmi;

import androidx.annotation.RequiresApi;
import androidx.appcompat.app.AlertDialog;
import androidx.appcompat.app.AppCompatActivity;

import android.app.AlarmManager;
import android.app.PendingIntent;
import android.content.Context;
import android.content.Intent;
import android.os.Build;
import android.os.Bundle;
import android.view.View;
import android.widget.Button;
import android.widget.TimePicker;
import android.widget.Toast;

import java.util.Calendar;
import java.util.Map;

public class MainActivity extends AppCompatActivity {

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

        final TimePicker timePicker = (TimePicker)findViewById(R.id.time);
        timePicker.setIs24HourView(true);
        Button button = (Button)findViewById(R.id.button);
        button.setOnClickListener(new View.OnClickListener() {
            @RequiresApi(api = Build.VERSION_CODES.M)
            @Override
            public void onClick(View v) {
                Intent intent = new Intent(
                    MainActivity.this, AlarmActivity.class
                );
                //获取显示闹钟的PendingIntent
                PendingIntent pendingIntent = PendingIntent.getActivity(
                        MainActivity.this,0,intent,0
                );
                AlarmManager alarmManager = (AlarmManager) getSystemService(Context.ALARM_SERVICE);
                Calendar c = Calendar.getInstance();
                c.set(Calendar.HOUR_OF_DAY,timePicker.getHour());
                c.set(Calendar.MINUTE,timePicker.getMinute());
                c.set(Calendar.SECOND,0);
                alarmManager.set(AlarmManager.RTC_WAKEUP,c.getTimeInMillis(),pendingIntent);
            }
        });

    }
}