//供参考
class A
{
    public void checkStr(String [] str)
    {
        try
        {
            for(String s:str)
            {
                if(s==null)
                    throw new RuntimeException("空字符串！");    
            }        
        }
        catch(Exception e)
        {
                e.printStackTrace();
        }
    }    
}

interface C
{
    public String compareLen(String [] str);    
}

class B extends A implements C
{
    public static void main(String[] args) 
    {
        B b = new B();
        String [] str = {"a","bb","ccc","dddd"};
        b.checkStr(str);
        System.out.println(b.compareLen(str));
    }
    
    public String compareLen(String [] str)
    {
        int max = 0;
        String maxStr = null;
        for(String s:str)
        {
            if(s.length()>max)
                maxStr = s;
        }    
        return maxStr;
    }
}

------解决方案--------------------
Java code

public class A extends Exception
{
    //输出异常信息
    public void print()
    {
        printStackTrace();
    }
}

class B extends A implements C
{
    @Override
    public String getMax(String a, String b, String c, String d)
    {
        String res = a;
        if(a.length() < b.length())
            res = b;
        if(res.length() < c.length())
            res = c;
        if(res.length() < d.length())
            res = d;
        return res;
    }
    
}

interface C
{
    //输入 4个字符串，返回长度最大的那个
    public String getMax(String a, String b, String c, String d);
}