Android 实现按两次返回键退出程序的功能；

@Override
public boolean onKeyDown(int keyCode, KeyEvent event) {
if (keyCode == KeyEvent.KEYCODE_BACK) {
if ((System.currentTimeMillis() - mExitTime) > 2000) {
Object mHelperUtils;

System.out.println(System.currentTimeMillis() - mExitTime+" 99999999999999");
Toast.makeText(this, "再按一次退出程序", 0).show();
mExitTime = System.currentTimeMillis();

} else {
finish();
System.exit(0);
}
return true;
}

return super.onKeyDown(keyCode, event);
}

private long mExitTime;