android控制系统回退按钮再按一次退出程序

@Override
public boolean onKeyDown(int keyCode, KeyEvent event) {
if (keyCode == KeyEvent.KEYCODE_BACK) {
if ((System.currentTimeMillis() - mExitTime) > 2000) {
UIHelper.makeToast(this, "再按一次退出程序");
mExitTime = System.currentTimeMillis();
} else {
Intent home = new Intent(Intent.ACTION_MAIN);
home.addCategory(Intent.CATEGORY_HOME);
startActivity(home);
overridePendingTransition(R.anim.fade_in, R.anim.fade_out_bottom);
}
return true;
}
return super.onKeyDown(keyCode, event);

}fade_in.xml:<alpha xmlns:android="http://schemas.android.com/apk/res/android"android:duration="400"android:fromAlpha="0.0"android:interpolator="@android:anim/decelerate_interpolator"android:toAlpha="1.0" />

fade_out_bottom:

<translate xmlns:android="http://schemas.android.com/apk/res/android"android:duration="400"android:fromYDelta="0"android:interpolator="@android:anim/accelerate_interpolator"android:toYDelta="100%p"android:zAdjustment="top" />