按两次回退键退出系统

@Override

        public boolean onKeyDown(int keyCode, KeyEvent event) {

            if(keyCode==KeyEvent.KEYCODE_BACK){

                 if ((System.currentTimeMillis() - mExitTime) > 2000) {

                     new UIUtil(MainActivity.this).ShortToast("再按一次回退键退出程序");

                     mExitTime = System.currentTimeMillis();

                 } else {

                     MainActivity.this.finish();

                     overridePendingTransition(android.R.anim.fade_in,android.R.anim.fade_out);

                 }

             }

            return true;

        }