package Dec2016;    import java.util.ArrayList;  import java.util.List;    public class Ques1229 {       public static void main(String[] args){         Student stu01 = new Student("001");         Student stu02 = new Student(null);           List
    
      students = new ArrayList
     
      ();         students.add(stu01);         students.add(stu02);         System.out.println("students是否包含001学号："+students.contains(stu01));         System.out.println("students是否包含null："+students.contains(stu02));     }  }    class Student{      private String stuId;      public Student(String stuId) {         super();         this.stuId = stuId;      }      public String getStuId() {          return stuId;      }      public void setStuId(String stuId) {          this.stuId = stuId;      }        @Override      public boolean equals(Object obj) {          if(obj instanceof Student){              Student stu = (Student) obj;              return stuId.equalsIgnoreCase(stu.getStuId().trim());          }          return false;      }   }
     
    

今日问题：