根据类图，实现java代码：@startuml class BankAccount { -accountNumber : String -balance : double -ownerName : String +BankAccount(accountNumber:String, ownerName:String) +getAccountNumber():String +getBalance():double +getOwnerName():String +deposit(amount:double):void +withdraw(amount:double):void } class BankAccountDAO { -accounts : Map<String, BankAccount> -fileName : String +BankAccountDAO(fileName:String) +getAccount(accountNumber:String):BankAccount +getAllAccounts():List<BankAccount> +addAccount(account:BankAccount):void +updateAccount(account:BankAccount):void +deleteAccount(accountNumber:String):void -saveToFile():void } BankAccount -> BankAccountDAO @enduml

import java.util.*;

class BankAccount {
    private String accountNumber;
    private double balance;
    private String ownerName;

    public BankAccount(String accountNumber, String ownerName) {
        this.accountNumber = accountNumber;
        this.ownerName = ownerName;
        this.balance = 0.0;
    }

    public String getAccountNumber() {
        return this.accountNumber;
    }

    public double getBalance() {
        return this.balance;
    }

    public String getOwnerName() {
        return this.ownerName;
    }

    public void deposit(double amount) {
        this.balance += amount;
    }

    public void withdraw(double amount) {
        if (balance >= amount) {
            this.balance -= amount;
        } else {
            System.out.println("Insufficient funds!");
        }
    }
}

class BankAccountDAO {
    private Map<String, BankAccount> accounts;
    private String fileName;

    public BankAccountDAO(String fileName) {
        this.accounts = new HashMap<>();
        this.fileName = fileName;
        // TODO: load accounts from file into accounts map
    }

    public BankAccount getAccount(String accountNumber) {
        return accounts.get(accountNumber);
    }

    public List<BankAccount> getAllAccounts() {
        return new ArrayList<>(accounts.values());
    }

    public void addAccount(BankAccount account) {
        accounts.put(account.getAccountNumber(), account);
        saveToFile();
    }

    public void updateAccount(BankAccount account) {
        accounts.replace(account.getAccountNumber(), account);
        saveToFile();
    }

    public void deleteAccount(String accountNumber) {
        accounts.remove(accountNumber);
        saveToFile();
    }

    private void saveToFile() {
        // TODO: write accounts map to file
    }
}