无法从链接列表中删除特定元素

Question

I have been set an assignment to create a small register based program written in Java, in the form of a linked list. I started by creating a student class, and then a tester file for the class. Following that, in the registry file I have set out my methods, and a constructor, and am in the process of writing a tester file to test all my methods.

However I am having trouble when trying to remove a specific element from my linkedlist, I want to be able to remove a student, referencing them by their individual studentID, but am not sure how to do this.

Whilst trying to solve the problem I came across the removeFirstOccurrence(Object o) method. Is this the right method to use?

Any help would be much appreciated.

STUDENT FILE CODE:

public class Student {

private String foreName;
private String surName;
private int studentID;

//declaring the variables needed for my student


public Student (String foreName, String surName, int studentID)
{
    this.foreName = foreName;
    this.surName = surName;
    this.studentID = studentID;
}
//constructor to set out what a student needs



public String getForeName() {
    return foreName;
}

public String getSurName() {
    return surName;
}

public int getStudentID() {
    return studentID;
}

public void setForeName(String foreName) {
    this.foreName = foreName;
}

public void setSurName(String surName) {
    this.surName = surName;
}

public void setStudentID(int studentID) {
    this.studentID = studentID;
}

// getters and setters for my variables

public String toString ()
{

    return getClass().getName() + "foreName = " + foreName + "surName = " + surName + "studentID = " + studentID;
}
//my toString method

}

REGISTRY FILE CODE:

import java.util.*;
public class Registry {


LinkedList<String> studentList
        = new LinkedList<String>();
//setting my type parameter


public Registry() {}
//empty constructor to hold arguements

public void addStudent(String aStudent)
{
    this.studentList.addLast(aStudent);
}



public void deleteStudent(int studentID) 
{
    //????         
}


@Override public String toString()
{
  return "Registry";
}



public String format()
{
}

REGISTRY TESTER FILE CODE:

import java.util.*;

public class RegistryTester {
public static void main (String[] args)
{
    LinkedList<String> studentList
            = new LinkedList<String>();




    System.out.println("Test 1");
    System.out.println("Methods tested: addStudent, constructor");
    System.out.println("********************");

    studentList.add("Joe Perkins 123");        
    studentList.addLast("Shilpa Gupta 1234");
    studentList.addLast("Seany Ray 12345");
    // adding 3 students to my list

    System.out.println(studentList);

}

}

Answer 1

1. Define the List as a List of Student

    LinkedList<Student> studentList = new LinkedList<Student>(); 

2. Override the hashCode() and equals() method in Student class.

    public boolean equals(Object obj) { if (obj instanceof Student) { return studentID == ((Student)obj).getStudentID(); } return false; } public int hashCode() { return studentID; } 

3. Define methods:

    public void addStudent(Student aStudent) { this.studentList.addLast(aStudent); } public void deleteStudent(Student astudent) { this.studentList.remove(aStudent) } 

Answer 2

First you should make your LinkedList for type Student :

LinkedList<Student>

Then to remove a student, you could :

• go through the list, find out the student object with same id, remove it

• override equals() and hashcode() method in your Student class, then

   public void deleteStudent(int studentID) { //getStudent object (stu) By the given ID studentList.remove(stu); } 

• use Map , ( HashMap or LinkedHashMap ) instead of LinkedList , key is the studentId, value is the studentObject. This will make add/remove easier.

Answer 3

If you have a Collection (in your case a LinkedList ) of Student s and call remove(studentToRemove) on it, Java will compare each object with studentToRemove by using its equals() method.

In your case, you haven't written an equals() method, hence the one for Object is used. If indeed a student is uniquely identified by its studentId (that is, two Student instances with the same studentId are always the same student) you should override the equals() metod and check for equality using that field.

Read about equals() and hashCode() .

Another alternative would be to iterate the list until you find the match, and then remove the student from the list.