父类封装后之类如何继承修改属性

Question

class phone{

private int cpu;

public void setCpu(int x){

this.cpu=x;

}

public int getCpu(){

return this.cpu;

}

}

class smartphone extends phone{

setCpu(2);/*problem*/

int num=getCpu();

public void out()

{

System.out.println(num);

}

}

public class telphone{

public static void main(String[] args){

smartphone s1 = new smartphone();

s1.out();

}

}

问题出在注释的那一行 怎么通过setCpu修改cpu的值呢