利用java，写一个身份证管理系统，要求能够增加信息、删除信息、查询信息、修改信息、将信息保存为文件等功能

好的，以下是一个简单的身份证管理系统的Java代码，实现了增加信息、删除信息、查询信息、修改信息、将信息保存为文件等功能。

import java.io.*;
import java.util.*;

class Person implements Serializable { // Person类实现了序列化接口Serializable
    private String name;
    private String id;

    public Person(String name, String id) {
        this.name = name;
        this.id = id;
    }

    public String getName() {
        return name;
    }

    public String getId() {
        return id;
    }

    public String toString() { // 重写toString方法
        return "Name: " + name + ", ID: " + id;
    }
}

class PersonManager {
    private List<Person> persons = new ArrayList<Person>();
    private Scanner scanner = new Scanner(System.in);

    public void addPerson() {
        System.out.print("Enter name: ");
        String name = scanner.nextLine();
        System.out.print("Enter ID: ");
        String id = scanner.nextLine();
        persons.add(new Person(name, id));
        System.out.println("Add successfully.");
    }

    public void deletePerson() {
        System.out.print("Enter the ID that you want to delete: ");
        String id = scanner.nextLine();
        boolean flag = false;
        for (int i = 0; i < persons.size(); i++) {
            if (persons.get(i).getId().equals(id)) {
                persons.remove(i);
                flag = true;
                break;
            }
        }
        if (flag) {
            System.out.println("Delete successfully.");
        } else {
            System.out.println("The ID is not exist.");
        }
    }

    public void queryPerson() {
        System.out.print("Enter the ID that you want to query: ");
        String id = scanner.nextLine();
        boolean flag = false;
        for (Person person : persons) {
            if (person.getId().equals(id)) {
                System.out.println(person);
                flag = true;
                break;
            }
        }
        if (!flag) {
            System.out.println("The ID is not exist.");
        }
    }

    public void modifyPerson() {
        System.out.print("Enter the ID that you want to modify: ");
        String id = scanner.nextLine();
        boolean flag = false;
        for (Person person : persons) {
            if (person.getId().equals(id)) {
                System.out.print("Enter new name: ");
                String name = scanner.nextLine();
                person = new Person(name, id);
                flag = true;
                break;
            }
        }
        if (flag) {
            System.out.println("Modify successfully.");
        } else {
            System.out.println("The ID is not exist.");
        }
    }

    public void saveToFile() {
        try {
            ObjectOutputStream out = new ObjectOutputStream(new FileOutputStream("persons.txt"));
            out.writeObject(persons);
            out.close();
            System.out.println("Save successfully.");
        } catch (IOException e) {
            System.out.println("Error: " + e.getMessage());
        }
    }

    public void loadFromFile() {
        try {
            ObjectInputStream in = new ObjectInputStream(new FileInputStream("persons.txt"));
            persons = (List<Person>) in.readObject();
            in.close();
            System.out.println("Load successfully.");
        } catch (IOException e) {
            System.out.println("Error: " + e.getMessage());
        } catch (ClassNotFoundException e) {
            System.out.println("Error: " + e.getMessage());
        }
    }
}

public class IdCardManagementSystem {
    private static PersonManager manager = new PersonManager();
    private static Scanner scanner = new Scanner(System.in);

    public static void main(String[] args) {
        while (true) {
            System.out.println("1. Add person");
            System.out.println("2. Delete person");
            System.out.println("3. Query person");
            System.out.println("4. Modify person");
            System.out.println("5. Save to file");
            System.out.println("6. Load from file");
            System.out.println("0. Exit");
            System.out.print("Enter your choice: ");
            String choice = scanner.nextLine();
            switch (choice) {
                case "1":
                    manager.addPerson();
                    break;
                case "2":
                    manager.deletePerson();
                    break;
                case "3":
                    manager.queryPerson();
                    break;
                case "4":
                    manager.modifyPerson();
                    break;
                case "5":
                    manager.saveToFile();
                    break;
                case "6":
                    manager.loadFromFile();
                    break;
                case "0":
                    System.exit(0);
                    break;
                default:
                    System.out.println("Invalid choice.");
                    break;
            }
        }
    }
}

这个身份证管理系统使用了面向对象的思想，定义了一个Person类来表示一个人的信息，然后定义了一个PersonManager类来管理Person对象，实现了增加、删除、查询、修改、保存到文件、从文件读取等功能。在主函数中，利用一个循环来不断接受用户输入，并根据用户的选择来调用PersonManager对象的相应方法。