如何使用Java Stream查找匹配元素并优雅处理存在与否的情况

public void findVehicleTraditional(List<Vehicle> vehicles, String rego) {
    System.out.println("Input a vehicle rego: " + rego);
    for (int i = 0; i < vehicles.size(); i++) {
        if (vehicles.get(i).getRego().equals(rego)) {
            System.out.println(vehicles.get(i).toString());
            return; // 找到即返回
        }
    }
    System.out.println("The vehicle does not exist."); // 未找到
}

public void findVehicleStreamInitial(List<Vehicle> vehicles, String rego) {
    System.out.println("Input a vehicle rego: " + rego);
    vehicles.stream()
            .filter(vehicle -> vehicle.getRego().equals(rego.toUpperCase()))
            .forEach(System.out::println);
    // 问题：如何处理未找到的情况？forEach会遍历所有匹配项，不符合“找到即返回”的需求
}

import java.util.List;
import java.util.Optional;
import java.util.Scanner;

// 假设有一个Vehicle类，包含getRego()和toString()方法
class Vehicle {
    private String rego;
    private String model;

    public Vehicle(String rego, String model) {
        this.rego = rego;
        this.model = model;
    }

    public String getRego() {
        return rego;
    }

    @Override
    public String toString() {
        return "Vehicle [rego=" + rego + ", model=" + model + "]";
    }
}

public class VehicleFinder {

    private List<Vehicle> vehicles; // 假设这是已填充的车辆列表

    public VehicleFinder(List<Vehicle> vehicles) {
        this.vehicles = vehicles;
    }

    public void findVehicleOptimizedStream() {
        System.out.println("Input a vehicle rego: ");
        Scanner in = new Scanner(System.in);
        String rego = in.nextLine();

        Optional<Vehicle> foundVehicle = vehicles.stream()
                .filter(v -> v.getRego().equalsIgnoreCase(rego)) // 忽略大小写匹配
                .findFirst(); // 返回Optional<Vehicle>

        // 处理Optional的结果
        if (foundVehicle.isPresent()) {
            System.out.println(foundVehicle.get());
        } else {
            System.out.println("The vehicle does not exist.");
        }
    }

    public static void main(String[] args) {
        // 示例数据
        List<Vehicle> vehicleList = List.of(
            new Vehicle("ABC123", "Model S"),
            new Vehicle("XYZ789", "Model 3"),
            new Vehicle("DEF456", "Model X")
        );
        VehicleFinder finder = new VehicleFinder(vehicleList);
        finder.findVehicleOptimizedStream();
    }
}

import java.util.List;
import java.util.Scanner;

// Vehicle类定义同上

public class VehicleFinderAdvanced {

    private List<Vehicle> vehicles;

    public VehicleFinderAdvanced(List<List<Vehicle>> vehicles) {
        this.vehicles = vehicles.stream().flatMap(List::stream).collect(java.util.ArrayList::new, java.util.ArrayList::addAll, java.util.ArrayList::addAll);
    }

    public void findVehicleWithIfPresentOrElse() {
        System.out.println("Input a vehicle rego: ");
        Scanner in = new Scanner(System.in);
        String rego = in.nextLine();

        vehicles.stream()
                .filter(v -> v.getRego().equalsIgnoreCase(rego)) // 筛选匹配项
                .findFirst() // 获取第一个匹配项，返回Optional
                .ifPresentOrElse( // 如果Optional包含值，执行第一个Lambda；否则执行第二个Lambda
                    System.out::println, // 值存在时执行
                    () -> System.out.println("The vehicle does not exist.") // 值不存在时执行
                );
    }

    public static void main(String[] args) {
        // 示例数据，与问题答案保持一致，假设vehicles是一个List<List<Vehicle>>
        List<List<Vehicle>> vehicleData = List.of(
            List.of(new Vehicle("ABC123", "Model S")),
            List.of(new Vehicle("XYZ789", "Model 3"), new Vehicle("XYZ789", "Model 3-duplicate")), // 包含重复项以测试findFirst
            List.of(new Vehicle("DEF456", "Model X"))
        );
        VehicleFinderAdvanced finder = new VehicleFinderAdvanced(vehicleData);
        finder.findVehicleWithIfPresentOrElse();

        System.out.println("\n--- Testing a non-existent vehicle ---");
        finder.findVehicleWithIfPresentOrElse(); // 再次调用，输入一个不存在的注册号
    }
}