string1==string2,是值相等,而且内存地址也相等,是完全的相等 
string1.equals(string2)为true,只是值相同 

        String level;
        if(grade.equals("A"))
            level = "优秀";
        else if(grade.equals("B"))
            level = "良好";
        else if(grade.equals("C"))
            level = "及格";
        else if(grade.equals("D"))
            level = "不及格";
        else
            level = "未知等级";
        System.out.println(level);