关于static的，求高手解答？

class B{
    int n;
  static int sum=0;
    void setN(int n){
        this.n=n;
        }
    int getSum(){
            for(int i=1;i<=n;i++)
            sum=sum+i;
            return sum;
            }
    }
    public class Add{
        public static void main(String args[]){
            B b1=new B(),b2=new B();
            b1.setN(3);
            b2.setN(4);
            int s1=b1.getSum();
            int s2=b2.getSum();
            System.out.println(s1+s2);
            }
        }
这个的结果怎么不是21啊？求解答，谢谢~~
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结果是27
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这个结果是22嘛。。。s1=6  s2=16
当 int s1 = b1.getSum();  完了后 sum = 6
到  int s2 = b2.getSum(); 时sum从6开始加。。完后 sum=16

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明白了，明白了~~~27的结果是b2.setN(5)
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