Java：从ID中删除链接列表中的学生不起作用

Question

我对此方法有问题，我想从链接列表中删除学生信息，但它不起作用，并且输出也与所需的一个不同 你能发现错误吗？

 public void deleteStudent (String id){
        Node help_ptr = head;

        if (head == null) {
            if (head == tail) {
                if (head.student.id.equals(id))
                    System.out.println(head.student.id + " Deleted!");
                head = tail = null;
                return;


            } else if (head.student.id.equals(id)) {
                // if “value” is in the first node
                System.out.println(head.student.id + " Deleted!");
                head = head.next;
                head.prev = null;
                return;


            } else if (tail.student.id.equals(id)) {
                // if “value” is in the last node tail = tail.prev;
                System.out.println(tail.student.id + " Deleted!");
                tail.next = null;
                return;

            } else {

                while (help_ptr != null) {
                    if (help_ptr.student.id.equals(id)) {
                        System.out.println(help_ptr.student.id + " Deleted!");
                        help_ptr.prev.next = help_ptr.next;
                        help_ptr.next.prev = help_ptr.prev;
                        return;
                    }
                    help_ptr = help_ptr.next;
                }

            }

所需的输出：

********* Delete Student **********

0000001 Deleted

********* Delete Student **********

0000003 Deleted

********* Delete Student **********

00000023 Not found in the list

我获得的输出：

************** Delete Student **************

************** Delete Student **************

************** Delete Student **************

主类：

 package classreport;
import java.util.*;
import java.io.*;


public class ClassReport {
        /*
         * @param args the command line arguments
         */
        public static void main(String[] args) {
            String id;
            String name;
            Double gpa;
            StudentList list = new StudentList();
            try {
                // create file object
                File f = new File("studentsList.txt");
                // create scanner object that reads from file
                Scanner sf = new Scanner(f);
                // start reading
                String line;
                while (sf.hasNextLine()) {
                    while ((line = sf.nextLine().trim()).isEmpty())
                        ;
                    name = line;
                    id = sf.nextLine().trim();
                    gpa = Double.parseDouble(sf.nextLine().trim());
                    // System.out.println("name: " + name + ", id: " + id + ", gpa: " + gpa);
                    list.add(name, id, gpa);
                }

                sf.close();
                // commands file
                File commands = new File("commands.txt");
                Scanner sc = new Scanner(commands);
                // output file
                File output = new File("output.txt");
 

               PrintWriter out = new PrintWriter(output);
                 System.out.println("*********************************************************\n"
                            + "\t\tData Structures Final Report\n"
                            + "******

***************************************************");

                // start reading
                while (sc.hasNext()) {
                    String command = sc.next().trim();
                    // System.out.println("command: " + command);
                    if (command.equals("TOTALSTUDENT")) {
                        int total = list.getTotal();
                        System.out.println("\nTotal number of students in the class = " + total);
                    } else if (command.equals("PRINTLIST")) {
                        System.out.println("\n************** Students List **************");
                        list.printList();
                    } else if (command.equals("ADDSTUDENT")) {
                        String newName = sc.nextLine();
                        newName = newName.trim();
                        String newId = sc.nextLine().trim();
                        Double newGpa = Double.parseDouble(sc.nextLine().trim());
                        System.out.println("\n************** Add Student **************");
                        list.addStudent(newName, newId, newGpa);
                    } else if (command.equals("STUDENTINFO")) {
                        String idToSearch = sc.next().trim();
                       System.out.println("\n************** Student Info **************");
                        list.studentInfo(idToSearch);
                    } else if (command.equals("DELETESTUDENT")) {
                        String idToDelete = sc.next().trim();
                        System.out.println("\n************** Delete Student **************");
                        list.deleteStudent(idToDelete);
                    } else if (command.equals("REVERSEPRINTLIST")) {
                        System.out.println("\n************** Reverse List **************");
                        list.printReverse();
                    } else if (command.equals("FINDLARGEST")) {
                        System.out.println("\n************** Highest GPA **************");
                        list.findLargest();
                    } else if (command.equals("QUIT")){
                        System.out.println(" ");
                        System.out.println("Quit");
                        System.out.println("*********************************************************");
                    }
                    else {
                        System.out.print("*********************************************************");
                        out.flush();
                        out.close();
                        sc.close();
                        sf.close();
                        System.exit(0);
                    }
                }
            } catch (IOException ex) {
                System.out.println(ex.getMessage());
            }
        }
    }

studentList.java类： 包班级；

导入java.io..printwriter;

公共班级学生列表{ printwriter out;

void setOut(PrintWriter out) {
    this.out = out;
}

    Node head;
    Node tail;

    static class Node {
        Student student;
        Node next;
        Node prev;

        Node(Student s) {
            student = s;
            next = null;
        }
    }

    StudentList() {
        head = null;
    }

    StudentList(Student student) {
        head = new Node(student);
    }

    void printStudent (Student student){
        System.out.println(student.name);
       System.out.println(student.id);
        System.out.println(student.gpa);
    }

    void addStudent (String studentName, String id, Double GPA){
        Node newStudent = new Node(new Student(studentName, id, GPA));
        Node helpPtr = head;


        while (helpPtr != null && helpPtr.next != null) {
            helpPtr = helpPtr.next;
        }
        if (helpPtr != null) {
            helpPtr.next = newStudent;
            printStudent(helpPtr.next.student);
        } else
            head = newStudent;

        System.out.println("Student added successfully!");
    }
    void add (String studentName, String id, Double GPA){
        Node newStudent = new Node(new Student(studentName, id, GPA));
        Node temp = head;
        while (temp != null && temp.next != null) {
            temp = temp.next;
        }
        if (temp != null)
            temp.next = newStudent;
        else
            head = newStudent;

    }

    int getTotal () {
        // Counter for number of student.
        int numStudent = 0;
        // HelpPtr object for Node.
        Node helpPtr = head;
        // Check if head is empty or not.
        if (head == null) {
            // Return Counter.
            return numStudent;
        }
        // Loop for count student.
        while (helpPtr != null) {
            // If helpPtr not equal to null, then increment counter.
            numStudent++;
            // For get next node of student.
            helpPtr = helpPtr.next;
        } // End loop.
        // return counter.
        return numStudent;
    }

    void studentInfo (String id){
        Node helpPtr = head;
        while (helpPtr != null) {
            if (helpPtr.student.id.equals(id)) {
                printStudent(helpPtr.student);
                return;
            }
            helpPtr = helpPtr.next;
        }
        System.out.println(id + " Not found in the list");
    }

    public void deleteStudent (String id){
        Node help_ptr = head;

        if (head == null) {
            if (head == tail) {
                if (head.student.id.equals(id))
                    System.out.println(head.student.id + " Deleted!");
                head = tail = null;
                return;


            } else if (head.student.id.equals(id)) {
                // if “value” is in the first node
                System.out.println(head.student.id + " Deleted!");
                head = head.next;
                head.prev = null;
                return;


            } else if (tail.student.id.equals(id)) {
                // if “value” is in the last node tail = tail.prev;
                System.out.println(tail.student.id + " Deleted!");
                tail.next = null;
                return;

            } else {

                while (help_ptr != null) {
                    if (help_ptr.student.id.equals(id)) {
                        System.out.println(help_ptr.student.id + " Deleted!");
                        help_ptr.prev.next = help_ptr.next;
                        help_ptr.next.prev = help_ptr.prev;
                        return;
                    }
                    help_ptr = help_ptr.next;
                }

            }
            if (help_ptr == null) {
                System.out.println("There is no student with id " + id + " in the system.");
                return;
            }
        }


    }

    void findLargest () {
        Student largestGPA = null;
        Node helpPtr = head;
        if (head == null) {
            System.out.println("Not found in the list");
            return;
        }


        while (helpPtr != null && head != null) {
            if (largestGPA == null) {
                largestGPA = helpPtr.student;
            } else {
                if (largestGPA.gpa < helpPtr.student.gpa) {
                    largestGPA = helpPtr.student;
                }
            }
            helpPtr = helpPtr.next;
        }
        System.out.println("Student having highest GPA:");
        printStudent(largestGPA);
    }

    void printList () {
        Node helpPtr = head;
        while (helpPtr != null && head != null) {
            printStudent(helpPtr.student);
            helpPtr = helpPtr.next;
        }
    }


    public void printReverseLinkedList (Node head){
        if (null == head) {

            return;
        }
        printReverseLinkedList(head.next);
        printStudent(head.student);
    }
    public void printReverse () {
        printReverseLinkedList(head);
    }

}

Student.Java课：

    package classreport;

public class Student {
    String name;
    String id;
    Double gpa;

    Student(String name, String id, Double gpa) {
        this.name = name;
        this.id = id;
        this.gpa = gpa;
}

}

Answer 1

public void deleteStudent (String id){
        Node help_ptr = head;

        if (head == null) {

这种情况应该

if(head!=null)

不允许其余的代码执行