java代码编译求助

public class FindDups {
    public static void main(String[] args) {
        Set<String> s = new HashSet<String>();
        for (String a : args)
            if (!s.add(a))
                System.out.println("Duplicate detected: " + a);
 
        System.out.println(s.size() + " distinct words: " + s);
    }
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    <span style="font-size:9pt;line-height:1.5;">}</span>
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<p>
    为什么Set<String> s = new HashSet<String>()无法通过编译，“cannot convert from HashSet<String> to Set<String><span>”，HashSet不是Set的实现吗？？怎么这样写不行的？？求助各位大神</span>
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