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);
    }
}