我知道这是一个混乱的实现,但我基本上有这个代码(我写了所有这些),并且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,而不是菜单选项3和4.我在尝试删除时输入的对象信息完全相同。这是代码。所有三个班级都在下面。
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");
}
}
学生班:
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;
}
}
答案 0 :(得分:16)
您必须正确覆盖equals()
和Student
类的Instructor
方法。
当覆盖等于时,最好也覆盖hashCode()
。
新学生(姓名,身份证,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,您有机会ArrayList
找出哪个对象与您在删除时作为参数传递的对象相对应。如果你没有覆盖上面的方法,它将使用Object
中的默认实现,它们比较在删除新的Student
对象时肯定不同的内存地址。
您可以在Object
的javadoc中阅读有关2种方法的更多信息。
答案 1 :(得分:7)
您需要覆盖集合的equals和hashcode方法才能正常工作。
@Override
public boolean equals(Object obj) {
if (obj == null)
return false;
if (!(obj instanceof Student))
return false;
Student other = (Student) obj;
return id == null ? false : id.equals(other.id);//Compare Id if null falseF
}
由于您只使用ArrayList,因此不会使用hashcode方法,但提供它仍然是一种好习惯。
@Override
public int hashCode() {
return id == null ? 0 : id.hashCode();
}
答案 2 :(得分:6)
您没有覆盖Student
和Instructor
的方法equals
。
ArrayList
使用此方法检查2个对象是否相同。如果没有自定义实现,它只会检查引用,因为它们是两个不同的对象,因此它们会有所不同。
要提供自定义相等性,您必须检查所涉及类的所有字段是否相同。这可以通过在实例变量上调用equals
来递归完成。
答案 3 :(得分:3)
覆盖学生和教师的equals方法将起作用:
以下是Student类的示例:
public boolean equals(Object other){
if(other == null) return false;
if(other == this) return true;
if(!(other instanceof Student)) return false;
Student otherStudent = (Student)other;
return otherStudent.id.equals(this.id);
}
您可能还想覆盖hashCode()
:
public String hashCode(){
return new HashCodeBuilder(17, 31).
append(name).
append(id).
toHashCode();
}