[求助]java 运行错误？？？

public interface cal
{
double square(double x);
}
public class father implements cal
{
public double square(double x)
{
rets=2*x*x;
return rets;
}
}
class son extends father
{
public static void main(String args[])
{
double p;
son so=new son();
p=so.square(9.0);
System.out.println("The double's square is:"+p);
}
}
为什么编译时会出现这样的错误如图

请高手解决一下，使其能运行，希望详细点，我刚学java新手

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you ren ma

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