Java单元测试实战：应对System.in交互式输入的挑战

@Override
public void addBill(Bill bill, Menu menu) {
    Scanner scanner = new Scanner(System.in);
    // ...
    System.out.print("Insert menu you want to be served: ");
    option = scanner.nextLine(); // 此处等待用户输入
    // ...
}

import java.io.InputStream;
import java.util.Scanner;

public class BillingService {

    public void addBill() {
        Scanner scanner = new Scanner(System.in);

        System.out.print("Insert menu you want to be served: ");
        var option = scanner.nextLine(); // 第一次输入

        String menuOption = ""; // 初始化 menuOption
        if (!"e".equals(option)) { // 避免在退出时再次读取
            System.out.print("Choose menu you want to eat/drink: ");
            menuOption = scanner.nextLine(); // 第二次输入
        }

        try {
            switch (option) {
                case "1":
                    System.out.println("Food Menu Selected.");
                    switch (menuOption) {
                        case "1" -> System.out.println("Breakfast selected.");
                        case "2" -> System.out.println("Lunch selected.");
                        case "3" -> System.out.println("Dinner selected.");
                        default -> System.out.println("No food option found !!!");
                    }
                    break;
                case "2":
                    System.out.println("Drink Menu Selected.");
                    switch (menuOption) {
                        case "1" -> System.out.println("Alcohol selected.");
                        case "2" -> System.out.println("Soft Drink selected.");
                        default -> System.out.println("No drink option found !!!");
                    }
                    break;
                case "e":
                    System.out.println("Exiting bill service.");
                    break;
                default:
                    System.out.println("No main option found !!!");
                    break;
            }
        } catch (NullPointerException exception) {
            System.out.println("Error: " + exception.getMessage());
        } finally {
            // 在实际应用中，不应关闭System.in关联的Scanner
            // 但在测试中，由于我们替换了System.in，这里通常可以忽略或由测试框架处理
        }
    }
}

import org.junit.jupiter.api.AfterEach;
import org.junit.jupiter.api.BeforeEach;
import org.junit.jupiter.api.Test;
import org.junit.jupiter.api.extension.ExtendWith;
import org.mockito.junit.jupiter.MockitoExtension;

import java.io.ByteArrayInputStream;
import java.io.ByteArrayOutputStream;
import java.io.InputStream;
import java.io.PrintStream;

import static org.junit.jupiter.api.Assertions.assertTrue;

@ExtendWith(MockitoExtension.class)
class BillingServiceTest {

    private BillingService billingService;

    // 保存原始的System.in和System.out
    private final InputStream systemIn = System.in;
    private final PrintStream systemOut = System.out;

    // 用于模拟输入的流
    private ByteArrayInputStream testIn;
    // 用于捕获输出的流
    private ByteArrayOutputStream testOut;

    @BeforeEach
    public void init() {
        billingService = new BillingService();

        // 在每个测试开始前，重定向System.out到testOut
        testOut = new ByteArrayOutputStream();
        System.setOut(new PrintStream(testOut));
    }

    @AfterEach
    public void restoreSystemInputOutput() {
        // 在每个测试结束后，恢复原始的System.in和System.out
        System.setIn(systemIn);
        System.setOut(systemOut);
    }

    // 辅助方法：提供模拟输入
    private void provideInput(String data) {
        testIn = new ByteArrayInputStream(data.getBytes());
        System.setIn(testIn);
    }

    @Test
    void whenChooseFoodMenuAndBreakfast_thenOutputCorrectMessage() {
        // 模拟用户输入 "1" (选择食物菜单) 和 "1" (选择早餐)
        provideInput("1\n1");
        billingService.addBill();

        // 验证输出是否包含预期的消息
        String actualOutput = testOut.toString();
        assertTrue(actualOutput.contains("Food Menu Selected."), "Output should contain 'Food Menu Selected.'");
        assertTrue(actualOutput.contains("Breakfast selected."), "Output should contain 'Breakfast selected.'");
    }

    @Test
    void whenChooseDrinkMenuAndSoftDrink_thenOutputCorrectMessage() {
        // 模拟用户输入 "2" (选择饮料菜单) 和 "2" (选择软饮)
        provideInput("2\n2");
        billingService.addBill();

        // 验证输出是否包含预期的消息
        String actualOutput = testOut.toString();
        assertTrue(actualOutput.contains("Drink Menu Selected."), "Output should contain 'Drink Menu Selected.'");
        assertTrue(actualOutput.contains("Soft Drink selected."), "Output should contain 'Soft Drink selected.'");
    }

    @Test
    void whenChooseExitOption_thenOutputExitMessage() {
        // 模拟用户输入 "e" (退出)
        provideInput("e\n"); // 即使只输入一个，也应提供换行符
        billingService.addBill();

        // 验证输出是否包含预期的消息
        String actualOutput = testOut.toString();
        assertTrue(actualOutput.contains("Exiting bill service."), "Output should contain 'Exiting bill service.'");
    }

    @Test
    void whenChooseInvalidOption_thenOutputErrorMessage() {
        // 模拟用户输入 "x" (无效选项) 和任意后续输入
        provideInput("x\nany_input\n");
        billingService.addBill();

        String actualOutput = testOut.toString();
        assertTrue(actualOutput.contains("No main option found !!!"), "Output should contain 'No main option found !!!'");
    }
}