用Android stdio写一个计时器，用三个按钮代表开始、暂停、重置，当按开始时，有通知开始计时；当按暂停时，有通知暂停计时；当按重置时，有通知重置计时，写出Java代码和xml代码，及具体步骤

好的，以下是用Android Studio编写计时器的具体步骤和Java代码、xml代码：

1. 创建一个新的Android项目，并在布局文件中添加一个TextView用于显示计时器的时间，三个Button分别代表开始、暂停、重置计时器的功能。xml代码如下：

<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    tools:context=".MainActivity">

    <TextView
        android:id="@+id/tv_timer"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="00:00:00"
        android:textSize="36sp"
        android:layout_centerHorizontal="true"
        android:layout_marginTop="32dp"/>

    <Button
        android:id="@+id/btn_start"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="开始"
        android:layout_below="@id/tv_timer"
        android:layout_marginTop="32dp"
        android:layout_alignParentStart="true"
        android:onClick="startTimer"/>

    <Button
        android:id="@+id/btn_pause"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="暂停"
        android:layout_below="@id/tv_timer"
        android:layout_marginTop="32dp"
        android:layout_centerHorizontal="true"
        android:onClick="pauseTimer"/>

    <Button
        android:id="@+id/btn_reset"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="重置"
        android:layout_below="@id/tv_timer"
        android:layout_marginTop="32dp"
        android:layout_alignParentEnd="true"
        android:onClick="resetTimer"/>

</RelativeLayout>

2. 在MainActivity中定义计时器的变量以及开始、暂停、重置计时器的方法。Java代码如下：

public class MainActivity extends AppCompatActivity {

    private int seconds = 0;
    private boolean running = false;
    private boolean wasRunning = false;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        if (savedInstanceState != null) {
            seconds = savedInstanceState.getInt("seconds");
            running = savedInstanceState.getBoolean("running");
            wasRunning = savedInstanceState.getBoolean("wasRunning");
        }
        runTimer();
    }

    public void onSaveInstanceState(Bundle savedInstanceState) {
        savedInstanceState.putInt("seconds", seconds);
        savedInstanceState.putBoolean("running", running);
        savedInstanceState.putBoolean("wasRunning", wasRunning);
    }

    public void startTimer(View view) {
        running = true;
    }

    public void pauseTimer(View view) {
        running = false;
    }

    public void resetTimer(View view) {
        running = false;
        seconds = 0;
    }

    private void runTimer() {
        final TextView tvTimer = (TextView) findViewById(R.id.tv_timer);
        final Handler handler = new Handler();
        handler.post(new Runnable() {
            @Override
            public void run() {
                int hours = seconds / 3600;
                int minutes = (seconds % 3600) / 60;
                int secs = seconds % 60;
                String time = String.format("%02d:%02d:%02d", hours, minutes, secs);
                tvTimer.setText(time);
                if (running) {
                    seconds++;
                }
                handler.postDelayed(this, 1000);
            }
        });
    }

    protected void onResume() {
        super.onResume();
        if (wasRunning) {
            running = true;
        }
    }

    protected void onPause() {
        super.onPause();
        wasRunning = running;
        running = false;
    }
}

3. 在AndroidManifest.xml文件中添加以下权限：

<uses-permission android:name="android.permission.VIBRATE" />

以上就是用Android Studio编写计时器的全部步骤和代码。