我的程序是一个简单的学生管理数据库，它收集学生的姓名，科目和电话号码并添加到数据库中。我已经能够实现主要的逻辑操作，如在数据库中添加，删除和搜索学生。我无法限制学生只输入批准的科目，例如＆＃34;英语＆＃34;，＆＃34;数学＆＃34;和＆＃34;计算＆＃34;添加到HashMap集合时。任何帮助，将不胜感激。这是主要代码：
import java.io.BufferedReader;
import java.io.BufferedWriter;
import java.io.File;
import java.io.FileReader;
import java.io.FileWriter;
import java.util.HashMap;
import java.util.Scanner;
public class Menu {
private HashMap<String, Student> students;
public Menu() {
students = new HashMap<String, Student>();
}
private void eventLoop() {
Scanner scanner = new Scanner(System.in);
int choice = 0;
boolean exit = false;
this.readFromFile();
while (!exit) {
System.out.println("Welcome to Student Management System");
System.out.println("==============================");
System.out.println("(1) Add new student");
System.out.println("(2) Delete a student");
System.out.println("(3) Find Student By Name");
System.out.println("(4) List Students By Subject");
System.out.println("(5) List All Students");
System.out.println("(6) Exit System");
System.out.println("Choose an option: ");
try {
choice = Integer.parseInt(scanner.nextLine());
System.out.print('\u000C');
if (choice < 1 || choice > 6) {
System.err.println("Error : Choose an option between 1 and 6");
choice = 0;
}
} catch (NumberFormatException e) {
System.err.println("Error : Choose an option between 1 and 6");
choice = 0;
}
switch (choice) {
case 1:
this.addStudent(scanner);
break;
case 2:
this.deleteStudent(scanner);
break;
case 3:
this.findStudentByName(scanner);
break;
case 4:
this.findStudentsBySubject(scanner);
break;
case 5:
this.listStudents();
break;
case 6:
this.writeToFile();
exit = true;
}
}
scanner.close();
}
private void findStudentsBySubject(Scanner scanner) {
System.out.println("Enter the exact name of the subject:");
String subjectStr = scanner.nextLine();
boolean atleastOne = false;
for (String name : students.keySet()) {
if (students.get(name).getSubject().getName().toLowerCase().equals(subjectStr.toLowerCase())) {
System.out.println(students.get(name));
atleastOne = true;
}
}
if (!atleastOne) {
System.err.println("No students have enrolled for this subject.");
}
}
private void findStudentByName(Scanner scanner) {
System.out.println("Enter the exact name of the Student to search:");
String name = scanner.nextLine();
if (students.get(name.toLowerCase()) != null) {
System.out.println("Student details:");
System.out.println(students.get(name.toLowerCase()));
} else {
System.err.println(name + " not found in the database.");
}
}
private void deleteStudent(Scanner scanner) {
System.out.println("Enter the exact name of the Student to delete:");
String name = scanner.nextLine();
if (students.get(name.toLowerCase()) != null) {
students.remove(name.toLowerCase());
System.err.println("Student " + name + " deleted from the database.");
} else {
System.err.println(name + " not found in the database.");
}
}
private void addStudent(Scanner scanner) {
System.out.println("The information should be comma separated and in a single line.");
System.out.println("If the name is not unique, the system will throw an error.");
System.out.println("Enter the name, phone and subject of the new student.");
String line = scanner.nextLine();
System.out.print('\u000C');
String[] info = line.split(",");
if (info.length != 3) {
System.err.println("Please enter the information in the proper format.");
return;
}
String name = info[0];
String phone = info[1];
String subjectStr = info[2];
if (students.get(name.toLowerCase()) != null) {
System.err.println("This student already exists in the database.");
return;
}
if (phone.length() != 9) {
System.err.println("The phone number must contain exactly 9 digits.");
return;
}
if (phone.charAt(0) != '9') {
System.err.println("The phone number must start with '9'.");
return;
}
if (!phone.matches("^[0-9]*$")) {
System.err.println("The phone number must contain only numbers.");
return;
}
students.put(name.toLowerCase(), new Student(name, new Subject(subjectStr), phone));
System.err.println("Student added successfully");
}
private void listStudents() {
for (String name : this.students.keySet()) {
System.out.println(this.students.get(name));
}
}
private void readFromFile() {
try {
BufferedReader br = new BufferedReader(new FileReader(new File("./students.txt")));
String line;
while ((line = br.readLine()) != null) {
String[] info = line.split(",");
String name = info[0];
String phone = info[1];
String subjectName = info[2];
if (students.get(name.toLowerCase()) == null) {
Subject subject = new Subject(subjectName);
students.put(name.toLowerCase(), new Student(name, subject, phone));
} else {
System.err.println("There seems to be a duplicate student in the file.");
}
}
br.close();
} catch (Exception e) {
e.printStackTrace();
}
}
private void writeToFile() {
try {
BufferedWriter bw = new BufferedWriter(new FileWriter(new File("./students.txt")));
for (String name : students.keySet()) {
bw.write(students.get(name).toString());
bw.newLine();
}
bw.close();
} catch (Exception e) {
e.printStackTrace();
}
}
public static void main(String[] args) {
Menu menu = new Menu();
menu.eventLoop();
}
}
学科分类：
public class Subject {
private String name;
public Subject(String subjectName) {
this.setName(subjectName);
}
public String getName() {
return name;
}
public void setName(String name) {
this.name = name;
}
public String toString() {
return this.getName();
}
}
答案 0 :(得分：1)
使用stream和filter您可以轻松实现此目标
students.entrySet()
.stream()
.filter(.. some predicate...)
.map(Map.Entry::getValue)
.collect(Collectors.toList())
如果您确定最多只能获得一个通过过滤器的元素（由过滤器保证），您可以使用findFirst：
Optional<List> o = students.entrySet()
.stream()
.filter( e -> e.getKey() == 1)
.map(Map.Entry::getValue)
.findFirst();
在一般情况下，如果过滤器可能匹配多个列表，您可以将它们收集到列表列表中：
List<List> list = students.entrySet()
.stream()
.filter(.. some predicate...)
.map(Map.Entry::getValue)
.collect(Collectors.toList());
答案 1 :(得分：1)
如果您有可能值的受限列表，则可以使用Enum，例如
enum Subject {
English, Maths, Computing
}
这些集合只是一个EnumSet，您可以通过调用
来检查它的值EnumSet<Subject> subjects = EnumSet.of(Subject.class);
Subject s = Subject.valueOf(subjectName);
subjects.add(s);