CountDownActivity.java

import android.os.Bundle;
import android.os.Handler;
import android.widget.TextView;
import androidx.appcompat.app.AppCompatActivity;
import java.text.ParseException;
import java.text.SimpleDateFormat;
import java.util.Date;
import java.util.Locale;
import cn.com.smarthome.utils.DateHelper;

public class CountDownActivityextends AppCompatActivity {

  private TextViewmTvCountDown;

  private long mExpectTime;

  @Override
  protected void onCreate(Bundle savedInstanceState) {
      super.onCreate(savedInstanceState);
      setContentView(R.layout.activity_count_down);
      initView();
      initData();
      initCountDown();
  }

  private void initView() {
      mTvCountDown = findViewById(R.id.tv_countdown);
  }

  private void initData() {
      String dateStr = "2019-10-28 10:45"；
      SimpleDateFormat simpleDateFormat = new SimpleDateFormat("yyyy-MM-dd hh:mm", Locale.CHINA);
      try {
          Date desDate = simpleDateFormat.parse(dateStr);
          Date curDate = new Date();
          if (desDate != null) {
              mExpectTime = (desDate.getTime() - curDate.getTime()) / 1000;
          }
      } catch (ParseException e) {
          e.printStackTrace();
      }
  }
  Handler handler = new Handler();
  private void initCountDown() {
      Runnable runnable = new Runnable() {
          @Override
          public void run() {
              mExpectTime = mExpectTime - 1;
              mTvCountDown.setText(DateHelper.getDateStr(mExpectTime));
              handler.postDelayed(this,1000);
          }
      };
      handler.postDelayed(runnable,1000);
  }
}

DateHelper.java

public class DateHelper {
    public static String getDateStr(long seconds) {
      if (seconds <= 0) {
          return "00天00时00分00秒";
       }
      long day = seconds / (60 * 60 * 24);
      long hour = (seconds % (60 * 60 * 24)) / (60 *60);
      long minute = ((seconds % (60 * 60 * 24)) % (60 *60)) / 60;
      long second = ((seconds % (60 * 60 * 24)) % (60 *60)) % 60;
      return (day > 9 ? day : "0" + day) + "天" +
                (hour > 9 ? hour : "0" + hour) + "时" +
                (minute > 9 ? minute : "0" + minute) + "分" +
                (second > 9 ? second : "0" + second) + "秒";
    }
}

代码有些粗糙，凑合着看吧