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


    <LinearLayout
            android:orientation="vertical"
            android:layout_width="match_parent"
            android:layout_height="match_parent" tools:layout_editor_absoluteY="0dp"
            tools:layout_editor_absoluteX="0dp">
        <TextView
                android:text="00:00:00"
                android:textSize="60sp"
                android:gravity="center"
                android:layout_width="match_parent"
                android:layout_height="wrap_content" android:id="@+id/timeView"/>
        <Button
                android:text="start"
                android:onClick="onClickStart"
                android:layout_width="match_parent"
                android:layout_height="wrap_content" android:id="@+id/start"/>
        <Button
                android:text="stop"
                android:onClick="onClickStop"
                android:layout_width="match_parent"
                android:layout_height="wrap_content" android:id="@+id/stop"/>
        <Button
                android:text="reset"
                android:onClick="onClickReset"
                android:layout_width="match_parent"
                android:layout_height="wrap_content" android:id="@+id/reset"/>
    </LinearLayout>

</android.support.constraint.ConstraintLayout>

**
MainActivity**

package com.test;

import android.os.Handler;
import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.view.View;

import android.widget.TextView;

public class MainActivity extends AppCompatActivity {


    private int seconds = 0;
    private boolean running = false; //计时状态
    private boolean wasRunning = false; //保存running的状态

    //app进入后台，暂停计时
    @Override
    protected void onStop() {
        super.onStop();
        wasRunning = running;
        running = false;
    }

    //重新进入app，开始计时
    @Override
    protected void onStart() {
        super.onStart();
        if(wasRunning) running = true;
    }

    //失去焦点(如分屏)，暂停计时
    @Override
    protected void onPause() {
        super.onPause();
        wasRunning = running;
        running = false;
    }

    //获得焦点,重新开始计时
    @Override
    protected void onResume() {
        super.onResume();
        if(wasRunning) running = true;
    }

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        //获取保存的状态
        if(savedInstanceState!=null){
            seconds = savedInstanceState.getInt("seconds");
            running = savedInstanceState.getBoolean("running");
            wasRunning = savedInstanceState.getBoolean("wasRunning");
        }
        runTime();
    }

    /**
     *保存状态
     */
    @Override
    public void onSaveInstanceState(Bundle saveInstanceState) {

        super.onSaveInstanceState(saveInstanceState);
        saveInstanceState.putInt("seconds",seconds);
        saveInstanceState.putBoolean("running",running);
        saveInstanceState.putBoolean("wasRunning",wasRunning);
    }
    /**
     * 响应button的onClick事件
     * 方法名和onClick的值一致
     */
    public void onClickStart(View button){
        running = true;
    }
    public void onClickStop(View button){
       running = false;
    }
    public void onClickReset(View button){
        running = false;
        seconds = 0;
    }

     /**
     * 注意 ui线程不能被堵塞，因此不能在ui线程中调用sleep方法
     * 只允许ui线程更新界面，不能在后台线程更新界面
     *
     * ** 使用ui线程的Handler定时更新 **
     * 将任务封装到 Runnable的run方法中 ，通过Handler的
     * post(立即提交任务)或postDelayed(实现定时调度)方法提交到ui线程
     */
    private void runTime(){
        final Handler handler = new Handler();
        handler.post(new Runnable() {
            @Override
            public void run() {
                final TextView textView = findViewById(R.id.timeView);
                int hour = seconds /3600%24;
                int minute = seconds%3600/60;
                String time = String.format("%02d:%02d:%02d",hour,minute,seconds%60);
                textView.setText(time);
                if(running) seconds++;
                handler.postDelayed(this,1000);
               }
            }
        );

    }
}