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);
}
}