双击back按键退出程序！

long keydowmtime=0;//按键按下时的时间
long eventtime=0;//事件发生的时间
    @Override
    public boolean onKeyDown(int keyCode, KeyEvent event) {
        // TODO Auto-generated method stub
        eventtime=event.getEventTime();
        if(keydowmtime==0){
            keydowmtime=event.getDownTime();
        }else {
            if((eventtime-keydowmtime)<1000){//即两次点击时间小于1s时退出程序，否则，复位
                MainActivity.this.finish();
            }else{
                keydowmtime=0;
            }
        }
        if (keyCode == KeyEvent.KEYCODE_BACK && event.getRepeatCount() == 0) {
            Toast.makeText(MainActivity.this,"再按一次退出", Toast.LENGTH_SHORT).show();
        }
        return super.onKeyDown(keyCode, event);
    }
}