迭代两种不同类类型的arraylist的hashmap

Question

我必须创建一个 arraylist 的 hashmap，但 arraylist 可以是 StudentRecord 或 TeacherRecord 类型（它们是两个不同的类）我已经添加了两个记录，但我无法使用 display method.can 遍历 hashmap 的内容有人帮我定义显示功能吗？

public class StudentRecord 
    {
        String first_Name;
        String last_Name;
        String [] courses_Registered;
        String status;
        Date date;

    StudentRecord(String f_name,String l_name,String [] courses,String status,Date dat )
    {
        this.first_Name=f_name;
        this.last_Name=l_name;
        this.courses_Registered=courses;
        this.status=status;
        this.date=dat;
    }

}

---

public class TeacherRecord 
{
    String first_Name;
    String last_Name;
    String address;
    String phone;
    String specilization;
    String location;

    TeacherRecord(String f_name,String l_name,String addr,String number,String spec,String loc )
    {
        this.first_Name=f_name;
        this.last_Name=l_name;
        this.address=addr;
        this.phone=number;
        this.location=loc;
    }
}

---

public class Montreal_server
{

    String record_id;
    ArrayList<StudentRecord> student_arraylist=new ArrayList<StudentRecord>();
    ArrayList<TeacherRecord>teacher_arraylist=new ArrayList<TeacherRecord>();

    HashMap<String,ArrayList> map=new HashMap<String,ArrayList>();

    void createTRecord(String f_name,String l_name,String addr,String number,String spec,String loc)
    {
        TeacherRecord t=new TeacherRecord(f_name,l_name,addr,number,spec,loc);
        teacher_arraylist.add(t);
        map.put("x",teacher_arraylist);


    }

    void createSRecord(String f_name,String l_name,String [] courses,String status,Date dat)
    {
        StudentRecord s=new StudentRecord(f_name,l_name,courses,status,dat);
        student_arraylist.add(s);
        map.put("s",student_arraylist);




    }

    void display()
    {
        System.out.println("inside display");
        for(Entry<String, ArrayList> en : map.entrySet())
        {
            java.util.Iterator<StudentRecord> itr = ((List<StudentRecord>)student_arraylist).iterator();



            while(itr.hasNext())
            {
                Object c=(Object)itr.next();

                System.out.println(c.first_Name+" "+c.last_Name+" "+c.status+" "+c.date+" "+c.courses_Registered);
            }
        }

    }

}

Answer 1

一个简单的解决方案是在每个类上覆盖 toString()：

public class StudentRecord {
    // fields, constructor etc.

    @Override
    public String toString() {
        return first_Name+" "+last_Name+" "+status+" "+date+" "+Arrays.toString(courses_Registered);  
    }
}

为TeacherRecord 创建一个类似的方法。现在，在您的循环中，只需调用 System.out.println(c);，它将隐式打印您的 toString() 实现的结果。

此外，很多混乱是由于试图将包含不相关类型的列表强制转换为单个映射的结果。最简单的解决方案是单独打印每个列表：

void display() {
    student_arraylist.forEach(System.out::println);
    teacher_arraylist.forEach(System.out::println);
}

Answer 2

让两个类都实现一个通用接口，例如

public interface PersonRecord {
String firstName();
String lastName();
...
}

和

public class StudentRecord implements PersonRecord
    { ... }

允许

List<PersonRecord> students = new List<>();
students.add(new StudentRecord());