android中 双击系统返回键退出

------activity_main.xml

<?xml version="1.0" encoding="utf-8"?><LinearLayout    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"    tools:context="com.example.g160628_12_widget2.AActivity">    <TextView        android:layout_width="match_parent"        android:layout_height="match_parent"        android:text="这是a"        android:textSize="50dp"/></LinearLayout>

-------MainActivity.java

public class AActivity extends AppCompatActivity {    private long firstTime=0;    @Override    protected void onCreate(Bundle savedInstanceState) {        super.onCreate(savedInstanceState);        setContentView(R.layout.activity_main);    }    @Override    public boolean onKeyDown(int keyCode, KeyEvent event) {        if (keyCode == KeyEvent.KEYCODE_BACK && event.getAction() == KeyEvent.ACTION_DOWN) {            if (System.currentTimeMillis() - firstTime > 2000) {                Toast.makeText(this, "再按一次退出程序", Toast.LENGTH_SHORT).show();                firstTime = System.currentTimeMillis();            } else {                finish();                System.exit(0);            }            return true;        }        return super.onKeyDown(keyCode, event);    }}