下面程序輸出是什麼

class J_Base
{
public int m_data=1;
public String mb_method()
{
return "2";
}
}
class J_Test extends J_Base
{
public int m_data=3;
public String mb_method()
{
return "4";
}
public static void main(String arg[])
{J_Base app=new J_Test();
System.out.println(app.m_data+app.mb_method());
}
}