ArrayList.remove() 不删除对象 [英] ArrayList.remove() is not removing an object
问题描述
我知道这是一个凌乱的实现，但我基本上有这个代码(我写了所有的)，我需要能够在使用适当的菜单选项时从列表中删除学生或教师.代码中的其他所有内容都有效，只是菜单选项 3 和 4 无效.尝试删除时，我为对象输入了完全相同的信息.这是代码.所有三个类都在下面.
I know this is a messy implementation, but I basically have this code (I wrote all of it), and I need to be able to remove a student or instructor from the list when using the appropriate menu choice. Everything else in the code works, just not menu options 3 and 4. I'm entering the exact same information for the object when trying to delete. Here's the code. All three classes are below.
驱动程序类:
import java.util.ArrayList;
import java.util.Scanner;
public class Driver {
private ArrayList<Student> students;
private ArrayList<Instructor> instructors;
public static void main(String[] args) {
Driver aDriver = new Driver();
aDriver.run();
}
public Driver() {
students = new ArrayList<Student>();
instructors = new ArrayList<Instructor>();
}
private void run() {
Student aStudent;
Instructor anInstructor;
Scanner inp = new Scanner(System.in);
int choice = -1;
String str = "Enter a menu option:\n";
str += " 0: Quit\n";
str += " 1: Add new student\n";
str += " 2: Add new instructor\n";
str += " 3: Delete existing student\n";
str += " 4: Delete existing instructor\n";
str += " 5: Print list of students\n";
str += " 6: Print list of instructors\n";
str += "Your choice: ";
do {
System.out.print(str);
choice = inp.nextInt();
switch(choice) {
case 0:
System.out.println("Thanks! Have a great day!");
break;
case 1:
aStudent = getStudentInfo();
addStudent(aStudent);
break;
case 2:
anInstructor = getInstructorInfo();
addInstructor(anInstructor);
break;
case 3:
aStudent = getStudentInfo();
deleteStudent(aStudent);
break;
case 4:
anInstructor = getInstructorInfo();
deleteInstructor(anInstructor);
break;
case 5:
printStudents();
break;
case 6:
printInstructors();
break;
default:
System.out.println("Invalid menu item " + choice);
}
}
while(choice != 0);
}
public Student getStudentInfo() {
Student aStudent;
String name = null;
String id = null;
double GPA = 0.0;
Scanner inp = new Scanner(System.in);
System.out.print("\n\nEnter the student's name: ");
name = inp.nextLine();
System.out.print("Enter the student's ID: ");
id = inp.nextLine();
System.out.print("Enter the student's GPA: ");
GPA = inp.nextDouble();
aStudent = new Student(name, id, GPA);
return aStudent;
}
public Instructor getInstructorInfo() {
Instructor anInstructor;
String name = null;
String id = null;
String dept = null;
String email = null;
Scanner inp = new Scanner(System.in);
System.out.print("\n\nEnter the instructor's name: ");
name = inp.nextLine();
System.out.print("Enter the instructor's ID: ");
id = inp.nextLine();
System.out.print("Enter the instructor's department: ");
dept = inp.nextLine();
System.out.print("Enter the instructor's email address: ");
email = inp.nextLine();
anInstructor = new Instructor(name, id, dept, email);
return anInstructor;
}
public void addStudent(Student aStudent) {
students.add(aStudent);
}
public void addInstructor(Instructor anInstructor) {
instructors.add(anInstructor);
}
public void deleteStudent(Student aStudent) {
students.remove(aStudent);
}
public void deleteInstructor(Instructor anInstructor) {
instructors.remove(anInstructor);
}
public void printStudents() {
System.out.println("\n\n" + Student.printHeader());
for(int i = 0; i < students.size(); i++) {
System.out.print(students.get(i));
}
System.out.print("\n\n");
}
public void printInstructors() {
System.out.print("\n\n" + Instructor.printHeader());
for(int i = 0; i < instructors.size(); i++) {
System.out.print(instructors.get(i));
}
System.out.print("\n\n");
}
}
学生类:
public class Student {
private String name;
private String id; //String to allow for the possibility of leading zeroes
private double GPA;
public Student() {
name = "TestFirst TestLast";
id = "00000";
GPA = -1.00;
}
public Student(String name1, String id1, double GPA1) {
name = name1;
id = id1;
GPA = GPA1;
}
public static String printHeader() {
String str = String.format("%-25s%-7s%-6s\n", "Name", "ID", "GPA");
return str;
}
public String toString() {
String str = String.format("%-25s%-7s%-6.3f\n", name, id, GPA);
return str;
}
public String getName() {
return name;
}
public void setGPA(double GPA2) {
GPA = GPA2;
}
}
讲师班:
public class Instructor {
private String name;
private String id;
private String dept;
private String email;
public Instructor() {
name = "TestFirst TestLast";
id = "-00001";
dept = "TestDept";
email = "test@test.net";
}
public Instructor(String name1, String id1, String dept1, String email1) {
name = name1;
id = id1;
dept = dept1;
email = email1;
}
public static String printHeader() {
String str = String.format("%-30s%-6s%-15s%-15s\n", "Name", "ID", "Department", "Email Address");
return str;
}
public String toString() {
String str = String.format("%-30s%-6s%-15s%-15s\n", name, id, dept, email);
return str;
}
public String getName() {
return name;
}
}
推荐答案
您必须正确覆盖 Student 和 Instructor 的 equals() 方法代码>类.
You must correctly override the equals() method for both Student and Instructor classes.
当覆盖 equals 时，最好也覆盖 hashCode().新学生(姓名，身份证，GPA);
When overriding equals, it is good to override hashCode() as well.
new Student(name, id, GPA);
例如，像这样:
public boolean equals(Object o) {
if (!(o instanceof Student)) {
return false;
}
Student other = (Student) o;
return name.equals(other.name) && id.equals(other.id) && GPA == other.GPA;
}
public int hashCode() {
return name.hashCode();
}
这样，您就有机会让 ArrayList 找出与您在删除时作为参数传递的对象相对应的对象.如果您不覆盖上述方法，它将使用 Object 中的默认实现，当您删除一个新的 Student 对象时，它会比较内存地址肯定不同.
This way, you give a chance to the ArrayList figure out which object correspond to the one you passed as a parameter when deleting. If you don't override the above methods, it will use the default implementations in Object, which compare memory addresses which are definitely different as you remove a new Student object.
您可以在 Object 的 javadoc 中阅读有关这 2 种方法的更多信息.
You can read even more information about the 2 methods in the javadocs for Object.
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