Project 1: Build an Expense Tracker CLI in Plain Java

$ expenses add 12.50 food "lunch with Sam" --date 2026-09-03Added #1: 2026-09-03 12.50 FOOD$ expenses summary --month 2026-09Summary for 2026-09  FOOD               12.50  TRANSPORT          45.00  HOUSING          1200.00  TOTAL            1257.50

   command line        |        v   +---------+      +----------------+      +--------------------+   |   Cli   | ---> | ExpenseService | ---> |  ExpenseRepository |  (interface)   +---------+      +----------------+      +--------------------+   text in/out       rules: dates,                 ^            ^                     filtering, totals             |            |                                         CsvExpenseRepository  InMemoryExpenseRepository                                         (the real program)    (the tests)

expense-tracker/├── pom.xml├── .gitignore├── README.md└── src/    ├── main/java/com/example/expenses/    └── test/java/com/example/expenses/

<?xml version="1.0" encoding="UTF-8"?><project xmlns="http://maven.apache.org/POM/4.0.0"         xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"         xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 https://maven.apache.org/xsd/maven-4.0.0.xsd">  <modelVersion>4.0.0</modelVersion>  <groupId>com.example</groupId>  <artifactId>expense-tracker</artifactId>  <version>1.0.0</version>  <packaging>jar</packaging>  <properties>    <maven.compiler.release>21</maven.compiler.release>    <project.build.sourceEncoding>UTF-8</project.build.sourceEncoding>    <junit.version>6.1.3</junit.version>  </properties>  <dependencies>    <dependency>      <groupId>org.junit.jupiter</groupId>      <artifactId>junit-jupiter</artifactId>      <version>${junit.version}</version>      <scope>test</scope>    </dependency>  </dependencies>  <build>    <plugins>      <plugin>        <groupId>org.apache.maven.plugins</groupId>        <artifactId>maven-surefire-plugin</artifactId>        <version>3.5.6</version>      </plugin>      <plugin>        <groupId>org.apache.maven.plugins</groupId>        <artifactId>maven-jar-plugin</artifactId>        <version>3.4.2</version>        <configuration>          <archive>            <manifest>              <mainClass>com.example.expenses.Main</mainClass>            </manifest>          </archive>        </configuration>      </plugin>    </plugins>  </build></project>

target/.idea/*.iml.vscode/

# Expense Tracker CLIA command-line expense tracker. Build with `mvn clean verify`, run with`java -jar target/expense-tracker-1.0.0.jar`. Set EXPENSE_TRACKER_FILE to choose the data file.

// File: src/main/java/com/example/expenses/InvalidInputException.javapackage com.example.expenses;/** The user supplied something we cannot accept. The message is written for the user. */public class InvalidInputException extends RuntimeException {    public InvalidInputException(String message) {        super(message);    }}

// File: src/main/java/com/example/expenses/Category.javapackage com.example.expenses;import java.util.Arrays;import java.util.Locale;public enum Category {    FOOD, TRANSPORT, HOUSING, UTILITIES, HEALTH, ENTERTAINMENT, OTHER;    public static Category parse(String text) {        try {            return valueOf(text.trim().toUpperCase(Locale.ROOT));        } catch (IllegalArgumentException e) {            throw new InvalidInputException(                    "unknown category '" + text + "', choose one of " + Arrays.toString(values()));        }    }}

// File: src/main/java/com/example/expenses/Money.javapackage com.example.expenses;import java.math.BigDecimal;import java.util.Locale;/** Amounts are held as a whole number of cents, never as double. */public final class Money {    private Money() {    }    public static long parseCents(String text) {        BigDecimal value;        try {            value = new BigDecimal(text.trim());        } catch (NumberFormatException e) {            throw new InvalidInputException("'" + text + "' is not a valid amount");        }        if (value.signum() <= 0) {            throw new InvalidInputException("amount must be greater than zero");        }        if (value.stripTrailingZeros().scale() > 2) {            throw new InvalidInputException("amount can have at most 2 decimal places");        }        try {            return value.movePointRight(2).longValueExact();        } catch (ArithmeticException e) {            throw new InvalidInputException("amount is too large");        }    }    public static String format(long cents) {        return String.format(Locale.ROOT, "%d.%02d", cents / 100, cents % 100);    }}

// File: src/main/java/com/example/expenses/Expense.javapackage com.example.expenses;import java.time.LocalDate;import java.util.Objects;public record Expense(long id, LocalDate date, long amountCents, Category category, String description) {    public static final int MAX_DESCRIPTION_LENGTH = 100;    public Expense {        Objects.requireNonNull(date, "date");        Objects.requireNonNull(category, "category");        if (id <= 0) {            throw new IllegalArgumentException("id must be positive: " + id);        }        if (amountCents <= 0) {            throw new InvalidInputException("amount must be greater than zero");        }        description = description == null ? "" : description.strip();        if (description.length() > MAX_DESCRIPTION_LENGTH) {            throw new InvalidInputException("description is longer than " + MAX_DESCRIPTION_LENGTH + " characters");        }        if (description.indexOf('\n') >= 0 || description.indexOf('\r') >= 0) {            throw new InvalidInputException("description must be a single line");        }    }}

// File: src/test/java/com/example/expenses/MoneyTest.javapackage com.example.expenses;import static org.junit.jupiter.api.Assertions.assertEquals;import static org.junit.jupiter.api.Assertions.assertThrows;import org.junit.jupiter.params.ParameterizedTest;import org.junit.jupiter.params.provider.CsvSource;import org.junit.jupiter.params.provider.ValueSource;class MoneyTest {    @ParameterizedTest(name = "\"{0}\" is {1} cents")    @CsvSource({            "12.50,   1250",            "12.5,    1250",            "12,      1200",            "0.01,    1",            "' 3.20 ', 320",            "12.500,  1250"    })    void parsesValidAmounts(String text, long expectedCents) {        assertEquals(expectedCents, Money.parseCents(text));    }    @ParameterizedTest(name = "\"{0}\" is rejected")    @ValueSource(strings = {"0", "-5", "abc", "", "12.505", "1.2.3", "99999999999999999999"})    void rejectsInvalidAmounts(String text) {        assertThrows(InvalidInputException.class, () -> Money.parseCents(text));    }    @ParameterizedTest(name = "{0} cents is shown as {1}")    @CsvSource({"1250, 12.50", "1, 0.01", "100, 1.00", "123456, 1234.56"})    void formatsCents(long cents, String expected) {        assertEquals(expected, Money.format(cents));    }}

// File: src/main/java/com/example/expenses/ExpenseRepository.javapackage com.example.expenses;import java.time.LocalDate;import java.util.List;/** Where expenses are stored. The rest of the program depends only on this interface. */public interface ExpenseRepository {    Expense add(LocalDate date, long amountCents, Category category, String description);    List<Expense> findAll();    boolean deleteById(long id);}

// File: src/main/java/com/example/expenses/InMemoryExpenseRepository.javapackage com.example.expenses;import java.time.LocalDate;import java.util.ArrayList;import java.util.List;import java.util.TreeMap;public class InMemoryExpenseRepository implements ExpenseRepository {    private final TreeMap<Long, Expense> byId = new TreeMap<>();    @Override    public Expense add(LocalDate date, long amountCents, Category category, String description) {        long id = byId.isEmpty() ? 1 : byId.lastKey() + 1;        Expense expense = new Expense(id, date, amountCents, category, description);        byId.put(id, expense);        return expense;    }    @Override    public List<Expense> findAll() {        return new ArrayList<>(byId.values());    }    @Override    public boolean deleteById(long id) {        return byId.remove(id) != null;    }}

// File: src/main/java/com/example/expenses/CsvExpenseRepository.javapackage com.example.expenses;import java.io.IOException;import java.io.UncheckedIOException;import java.nio.charset.StandardCharsets;import java.nio.file.Files;import java.nio.file.Path;import java.nio.file.StandardCopyOption;import java.time.LocalDate;import java.util.ArrayList;import java.util.List;/** Stores expenses in a UTF-8 CSV file. Every operation reads or rewrites the whole file. */public class CsvExpenseRepository implements ExpenseRepository {    static final String HEADER = "id,date,amountCents,category,description";    private final Path file;    public CsvExpenseRepository(Path file) {        this.file = file;    }    @Override    public Expense add(LocalDate date, long amountCents, Category category, String description) {        List<Expense> all = load();        long id = all.stream().mapToLong(Expense::id).max().orElse(0) + 1;        Expense expense = new Expense(id, date, amountCents, category, description);        all.add(expense);        store(all);        return expense;    }    @Override    public List<Expense> findAll() {        return load();    }    @Override    public boolean deleteById(long id) {        List<Expense> all = load();        boolean removed = all.removeIf(e -> e.id() == id);        if (removed) {            store(all);        }        return removed;    }    private List<Expense> load() {        if (!Files.exists(file)) {            return new ArrayList<>();        }        try {            List<String> lines = Files.readAllLines(file, StandardCharsets.UTF_8);            List<Expense> expenses = new ArrayList<>();            for (int i = 1; i < lines.size(); i++) {           // line 0 is the header                if (!lines.get(i).isBlank()) {                    expenses.add(parse(lines.get(i), i + 1));                }            }            return expenses;        } catch (IOException e) {            throw new UncheckedIOException("cannot read " + file, e);        }    }    private Expense parse(String line, int lineNumber) {        try {            List<String> f = splitCsv(line);            if (f.size() != 5) {                throw new IllegalArgumentException("expected 5 fields but found " + f.size());            }            return new Expense(Long.parseLong(f.get(0)), LocalDate.parse(f.get(1)),                    Long.parseLong(f.get(2)), Category.valueOf(f.get(3)), f.get(4));        } catch (RuntimeException e) {            throw new IllegalStateException(                    "data file " + file + " is damaged at line " + lineNumber + ": " + e.getMessage(), e);        }    }    private void store(List<Expense> expenses) {        try {            Path directory = file.toAbsolutePath().getParent();            Files.createDirectories(directory);            List<String> lines = new ArrayList<>();            lines.add(HEADER);            for (Expense e : expenses) {                lines.add(e.id() + "," + e.date() + "," + e.amountCents() + "," + e.category()                        + "," + escape(e.description()));            }            Path temp = Files.createTempFile(directory, "expenses", ".tmp");            Files.write(temp, lines, StandardCharsets.UTF_8);            Files.move(temp, file, StandardCopyOption.REPLACE_EXISTING);   // the old file is never half-written        } catch (IOException e) {            throw new UncheckedIOException("cannot write " + file, e);        }    }    static String escape(String field) {        if (field.contains(",") || field.contains("\"")) {            return "\"" + field.replace("\"", "\"\"") + "\"";        }        return field;    }    static List<String> splitCsv(String line) {        List<String> fields = new ArrayList<>();        StringBuilder current = new StringBuilder();        boolean quoted = false;        for (int i = 0; i < line.length(); i++) {            char c = line.charAt(i);            if (quoted) {                if (c == '"') {                    if (i + 1 < line.length() && line.charAt(i + 1) == '"') {                        current.append('"');                         // "" inside quotes is one quote                        i++;                    } else {                        quoted = false;                    }                } else {                    current.append(c);                }            } else if (c == '"') {                quoted = true;            } else if (c == ',') {                fields.add(current.toString());                current.setLength(0);            } else {                current.append(c);            }        }        fields.add(current.toString());        return fields;    }}

// File: src/test/java/com/example/expenses/CsvExpenseRepositoryTest.javapackage com.example.expenses;import static org.junit.jupiter.api.Assertions.assertEquals;import static org.junit.jupiter.api.Assertions.assertFalse;import static org.junit.jupiter.api.Assertions.assertThrows;import static org.junit.jupiter.api.Assertions.assertTrue;import java.io.IOException;import java.nio.file.Files;import java.nio.file.Path;import java.time.LocalDate;import java.util.List;import org.junit.jupiter.api.Test;import org.junit.jupiter.api.io.TempDir;class CsvExpenseRepositoryTest {    @TempDir    Path directory;    @Test    void dataSurvivesANewRepositoryInstance() {        Path file = directory.resolve("expenses.csv");        new CsvExpenseRepository(file).add(LocalDate.of(2026, 3, 1), 1250, Category.FOOD, "lunch");        List<Expense> loaded = new CsvExpenseRepository(file).findAll();        assertEquals(List.of(new Expense(1, LocalDate.of(2026, 3, 1), 1250, Category.FOOD, "lunch")), loaded);    }    @Test    void descriptionsWithCommasAndQuotesRoundTrip() {        Path file = directory.resolve("expenses.csv");        CsvExpenseRepository repository = new CsvExpenseRepository(file);        repository.add(LocalDate.of(2026, 3, 1), 100, Category.OTHER, "tea, milk and \"biscuits\"");        assertEquals("tea, milk and \"biscuits\"", repository.findAll().get(0).description());    }    @Test    void missingFileMeansNoExpensesAndTheFileAndFolderAreCreatedOnFirstAdd() {        Path file = directory.resolve("nested").resolve("expenses.csv");        CsvExpenseRepository repository = new CsvExpenseRepository(file);        assertTrue(repository.findAll().isEmpty());        repository.add(LocalDate.of(2026, 3, 1), 100, Category.OTHER, "");        assertTrue(Files.exists(file));    }    @Test    void deleteRemovesOnlyTheNamedExpense() {        CsvExpenseRepository repository = new CsvExpenseRepository(directory.resolve("expenses.csv"));        Expense first = repository.add(LocalDate.of(2026, 3, 1), 100, Category.FOOD, "a");        Expense second = repository.add(LocalDate.of(2026, 3, 2), 200, Category.FOOD, "b");        assertTrue(repository.deleteById(first.id()));        assertFalse(repository.deleteById(first.id()));        assertEquals(List.of(second), repository.findAll());    }    @Test    void aDamagedFileIsReportedWithItsLineNumber() throws IOException {        Path file = directory.resolve("expenses.csv");        Files.writeString(file, CsvExpenseRepository.HEADER + "\n1,2026-03-01,not-a-number,FOOD,lunch\n");        IllegalStateException e = assertThrows(IllegalStateException.class,                () -> new CsvExpenseRepository(file).findAll());        assertTrue(e.getMessage().contains("damaged at line 2"), e.getMessage());    }    @Test    void splitsQuotedFieldsCorrectly() {        assertEquals(List.of("1", "a,b", "say \"hi\"", ""),                CsvExpenseRepository.splitCsv("1,\"a,b\",\"say \"\"hi\"\"\","));    }}

// File: src/main/java/com/example/expenses/ExpenseService.javapackage com.example.expenses;import java.time.Clock;import java.time.LocalDate;import java.time.YearMonth;import java.util.Comparator;import java.util.EnumMap;import java.util.List;import java.util.Map;public class ExpenseService {    public record Summary(Map<Category, Long> byCategory, long totalCents) {    }    private final ExpenseRepository repository;    private final Clock clock;    public ExpenseService(ExpenseRepository repository, Clock clock) {        this.repository = repository;        this.clock = clock;    }    /** Adds an expense. A null date means today. */    public Expense add(long amountCents, Category category, String description, LocalDate date) {        LocalDate today = LocalDate.now(clock);        LocalDate when = date == null ? today : date;        if (when.isAfter(today)) {            throw new InvalidInputException("date is in the future: " + when);        }        return repository.add(when, amountCents, category, description);    }    /** Lists expenses oldest first. A null month means all months. */    public List<Expense> list(YearMonth month) {        return repository.findAll().stream()                .filter(e -> month == null || YearMonth.from(e.date()).equals(month))                .sorted(Comparator.comparing(Expense::date).thenComparingLong(Expense::id))                .toList();    }    public Summary summary(YearMonth month) {        Map<Category, Long> totals = new EnumMap<>(Category.class);        long total = 0;        for (Expense e : list(month)) {            totals.merge(e.category(), e.amountCents(), Long::sum);            total += e.amountCents();        }        return new Summary(totals, total);    }    public boolean delete(long id) {        return repository.deleteById(id);    }}

// File: src/test/java/com/example/expenses/ExpenseServiceTest.javapackage com.example.expenses;import static org.junit.jupiter.api.Assertions.assertEquals;import static org.junit.jupiter.api.Assertions.assertFalse;import static org.junit.jupiter.api.Assertions.assertThrows;import static org.junit.jupiter.api.Assertions.assertTrue;import java.time.Clock;import java.time.Instant;import java.time.LocalDate;import java.time.YearMonth;import java.time.ZoneOffset;import java.util.List;import org.junit.jupiter.api.BeforeEach;import org.junit.jupiter.api.Test;class ExpenseServiceTest {    private static final Clock FIXED = Clock.fixed(Instant.parse("2026-03-15T10:00:00Z"), ZoneOffset.UTC);    private ExpenseService service;    @BeforeEach    void setUp() {        service = new ExpenseService(new InMemoryExpenseRepository(), FIXED);    }    @Test    void usesTodayWhenNoDateIsGiven() {        Expense saved = service.add(450, Category.FOOD, "lunch", null);        assertEquals(LocalDate.of(2026, 3, 15), saved.date());        assertEquals(1, saved.id());    }    @Test    void rejectsADateInTheFuture() {        InvalidInputException e = assertThrows(InvalidInputException.class,                () -> service.add(450, Category.FOOD, "lunch", LocalDate.of(2026, 3, 16)));        assertEquals("date is in the future: 2026-03-16", e.getMessage());    }    @Test    void acceptsTodayAsTheLatestValidDate() {        Expense saved = service.add(450, Category.FOOD, "lunch", LocalDate.of(2026, 3, 15));        assertEquals(LocalDate.of(2026, 3, 15), saved.date());    }    @Test    void listsOldestFirstAndFiltersByMonth() {        service.add(1000, Category.FOOD, "b", LocalDate.of(2026, 3, 10));        service.add(2000, Category.HOUSING, "a", LocalDate.of(2026, 2, 28));        service.add(500, Category.FOOD, "c", LocalDate.of(2026, 3, 2));        List<String> all = service.list(null).stream().map(Expense::description).toList();        List<String> march = service.list(YearMonth.of(2026, 3)).stream().map(Expense::description).toList();        assertEquals(List.of("a", "c", "b"), all);        assertEquals(List.of("c", "b"), march);    }    @Test    void summarisesTotalsPerCategory() {        service.add(1000, Category.FOOD, "", LocalDate.of(2026, 3, 10));        service.add(250, Category.FOOD, "", LocalDate.of(2026, 3, 11));        service.add(4000, Category.TRANSPORT, "", LocalDate.of(2026, 3, 12));        service.add(9999, Category.HOUSING, "", LocalDate.of(2026, 2, 1));        ExpenseService.Summary summary = service.summary(YearMonth.of(2026, 3));        assertEquals(1250L, summary.byCategory().get(Category.FOOD));        assertEquals(4000L, summary.byCategory().get(Category.TRANSPORT));        assertFalse(summary.byCategory().containsKey(Category.HOUSING));        assertEquals(5250, summary.totalCents());    }    @Test    void deleteReportsWhetherAnythingWasRemoved() {        Expense saved = service.add(450, Category.FOOD, "", null);        assertTrue(service.delete(saved.id()));        assertFalse(service.delete(saved.id()));    }}

// File: src/main/java/com/example/expenses/Cli.javapackage com.example.expenses;import java.io.PrintStream;import java.io.UncheckedIOException;import java.time.LocalDate;import java.time.YearMonth;import java.time.format.DateTimeParseException;import java.util.ArrayList;import java.util.Arrays;import java.util.List;import java.util.Locale;/** Turns command-line arguments into service calls and prints the result. */public class Cli {    private static final String USAGE = """            Usage:              expenses add <amount> <category> [description] [--date yyyy-MM-dd]              expenses list [--month yyyy-MM]              expenses summary [--month yyyy-MM]              expenses delete <id>              expenses categories            """;    private final ExpenseService service;    private final PrintStream out;    private final PrintStream err;    public Cli(ExpenseService service, PrintStream out, PrintStream err) {        this.service = service;        this.out = out;        this.err = err;    }    /** Runs one command and returns the process exit code: 0 success, 1 bad input, 2 storage problem. */    public int run(String[] args) {        if (args.length == 0) {            err.print(USAGE);            return 1;        }        List<String> rest = new ArrayList<>(Arrays.asList(args).subList(1, args.length));        try {            switch (args[0]) {                case "add" -> add(rest);                case "list" -> list(rest);                case "summary" -> summary(rest);                case "delete" -> delete(rest);                case "categories" -> categories(rest);                default -> {                    err.println("error: unknown command '" + args[0] + "'");                    err.print(USAGE);                    return 1;                }            }            return 0;        } catch (InvalidInputException e) {            err.println("error: " + e.getMessage());            return 1;        } catch (UncheckedIOException | IllegalStateException e) {            err.println("error: " + e.getMessage());            return 2;        }    }    private void add(List<String> args) {        String dateText = takeOption(args, "--date");        if (args.size() < 2 || args.size() > 3) {            throw new InvalidInputException("add needs <amount> <category> and an optional description");        }        long cents = Money.parseCents(args.get(0));        Category category = Category.parse(args.get(1));        String description = args.size() == 3 ? args.get(2) : "";        LocalDate date = dateText == null ? null : parseDate(dateText);        Expense saved = service.add(cents, category, description, date);        out.println("Added #" + saved.id() + ": " + saved.date() + " " + Money.format(saved.amountCents())                + " " + saved.category());    }    private void list(List<String> args) {        YearMonth month = parseMonth(takeOption(args, "--month"));        noMoreArguments(args);        List<Expense> expenses = service.list(month);        if (expenses.isEmpty()) {            out.println("No expenses found.");            return;        }        out.printf(Locale.ROOT, "%4s  %-10s  %10s  %-13s  %s%n", "ID", "DATE", "AMOUNT", "CATEGORY", "DESCRIPTION");        long total = 0;        for (Expense e : expenses) {            out.printf(Locale.ROOT, "%4d  %-10s  %10s  %-13s  %s%n",                    e.id(), e.date(), Money.format(e.amountCents()), e.category(), e.description());            total += e.amountCents();        }        out.printf(Locale.ROOT, "%d expense(s), total %s%n", expenses.size(), Money.format(total));    }    private void summary(List<String> args) {        YearMonth month = parseMonth(takeOption(args, "--month"));        noMoreArguments(args);        ExpenseService.Summary summary = service.summary(month);        out.println(month == null ? "Summary (all time)" : "Summary for " + month);        summary.byCategory().forEach((category, cents) ->                out.printf(Locale.ROOT, "  %-13s %10s%n", category, Money.format(cents)));        out.printf(Locale.ROOT, "  %-13s %10s%n", "TOTAL", Money.format(summary.totalCents()));    }    private void delete(List<String> args) {        if (args.size() != 1) {            throw new InvalidInputException("delete needs exactly one <id>");        }        long id;        try {            id = Long.parseLong(args.get(0));        } catch (NumberFormatException e) {            throw new InvalidInputException("'" + args.get(0) + "' is not a valid id");        }        if (service.delete(id)) {            out.println("Deleted #" + id);        } else {            throw new InvalidInputException("no expense with id " + id);        }    }    private void categories(List<String> args) {        noMoreArguments(args);        for (Category category : Category.values()) {            out.println(category);        }    }    /** Removes "--name value" from the arguments and returns the value, or null if absent. */    private static String takeOption(List<String> args, String name) {        int index = args.indexOf(name);        if (index < 0) {            return null;        }        if (index + 1 >= args.size()) {            throw new InvalidInputException(name + " needs a value");        }        String value = args.get(index + 1);        args.remove(index + 1);        args.remove(index);        return value;    }    private static void noMoreArguments(List<String> args) {        if (!args.isEmpty()) {            throw new InvalidInputException("unexpected arguments: " + args);        }    }    private static LocalDate parseDate(String text) {        try {            return LocalDate.parse(text);        } catch (DateTimeParseException e) {            throw new InvalidInputException("'" + text + "' is not a valid date, use yyyy-MM-dd");        }    }    private static YearMonth parseMonth(String text) {        if (text == null) {            return null;        }        try {            return YearMonth.parse(text);        } catch (DateTimeParseException e) {            throw new InvalidInputException("'" + text + "' is not a valid month, use yyyy-MM");        }    }}

// File: src/test/java/com/example/expenses/CliTest.javapackage com.example.expenses;import static org.junit.jupiter.api.Assertions.assertEquals;import static org.junit.jupiter.api.Assertions.assertTrue;import java.io.ByteArrayOutputStream;import java.io.PrintStream;import java.nio.charset.StandardCharsets;import java.nio.file.Path;import java.time.Clock;import java.time.Instant;import java.time.ZoneOffset;import java.util.List;import org.junit.jupiter.api.BeforeEach;import org.junit.jupiter.api.Test;import org.junit.jupiter.api.io.TempDir;/** Drives the whole program except the real clock and the real home directory. */class CliTest {    private static final Clock FIXED = Clock.fixed(Instant.parse("2026-03-15T10:00:00Z"), ZoneOffset.UTC);    @TempDir    Path directory;    private ByteArrayOutputStream outBytes;    private ByteArrayOutputStream errBytes;    private Cli cli;    @BeforeEach    void setUp() {        outBytes = new ByteArrayOutputStream();        errBytes = new ByteArrayOutputStream();        ExpenseService service = new ExpenseService(                new CsvExpenseRepository(directory.resolve("expenses.csv")), FIXED);        cli = new Cli(service,                new PrintStream(outBytes, true, StandardCharsets.UTF_8),                new PrintStream(errBytes, true, StandardCharsets.UTF_8));    }    private int run(String... args) {        return cli.run(args);    }    private List<String> out() {        return outBytes.toString(StandardCharsets.UTF_8).lines().toList();    }    private List<String> err() {        return errBytes.toString(StandardCharsets.UTF_8).lines().toList();    }    @Test    void addThenListShowsTheExpense() {        assertEquals(0, run("add", "12.50", "food", "lunch with Sam", "--date", "2026-03-10"));        assertEquals("Added #1: 2026-03-10 12.50 FOOD", out().get(0));        assertEquals(0, run("list"));        List<String> lines = out();        assertTrue(lines.get(2).contains("lunch with Sam"), lines.toString());        assertEquals("1 expense(s), total 12.50", lines.get(lines.size() - 1));    }    @Test    void summaryTotalsByCategoryForAMonth() {        run("add", "10", "food", "--date", "2026-03-01");        run("add", "2.50", "food", "--date", "2026-03-02");        run("add", "40", "transport", "--date", "2026-03-03");        run("add", "99", "housing", "--date", "2026-02-03");        outBytes.reset();        assertEquals(0, run("summary", "--month", "2026-03"));        assertEquals(List.of(                "Summary for 2026-03",                "  FOOD               12.50",                "  TRANSPORT          40.00",                "  TOTAL              52.50"), out());    }    @Test    void anInvalidAmountIsReportedWithExitCode1() {        assertEquals(1, run("add", "abc", "food"));        assertEquals(List.of("error: 'abc' is not a valid amount"), err());    }    @Test    void anUnknownCategoryListsTheValidOnes() {        assertEquals(1, run("add", "5", "snacks"));        assertTrue(err().get(0).startsWith("error: unknown category 'snacks', choose one of ["), err().toString());    }    @Test    void aFutureDateIsRejected() {        assertEquals(1, run("add", "5", "food", "--date", "2026-03-16"));        assertEquals(List.of("error: date is in the future: 2026-03-16"), err());    }    @Test    void deletingAnUnknownIdFails() {        assertEquals(1, run("delete", "7"));        assertEquals(List.of("error: no expense with id 7"), err());    }    @Test    void anUnknownCommandPrintsTheUsage() {        assertEquals(1, run("remove", "1"));        assertEquals("error: unknown command 'remove'", err().get(0));        assertEquals("Usage:", err().get(1));    }    @Test    void noArgumentsPrintsTheUsage() {        assertEquals(1, run());        assertEquals("Usage:", err().get(0));    }}

// File: src/main/java/com/example/expenses/Main.javapackage com.example.expenses;import java.nio.file.Path;import java.time.Clock;public class Main {    public static void main(String[] args) {        Path file = dataFile(System.getenv("EXPENSE_TRACKER_FILE"), System.getProperty("user.home"));        ExpenseService service = new ExpenseService(new CsvExpenseRepository(file), Clock.systemDefaultZone());        int exitCode = new Cli(service, System.out, System.err).run(args);        System.exit(exitCode);    }    /** The file named by EXPENSE_TRACKER_FILE, or ~/.expense-tracker/expenses.csv. */    static Path dataFile(String override, String home) {        if (override != null && !override.isBlank()) {            return Path.of(override);        }        return Path.of(home, ".expense-tracker", "expenses.csv");    }}

mvn clean verify

alias expenses='java -jar target/expense-tracker-1.0.0.jar'

$ expenses categoriesFOODTRANSPORTHOUSINGUTILITIESHEALTHENTERTAINMENTOTHER$ expenses add 12.50 food "lunch with Sam" --date 2026-09-03Added #1: 2026-09-03 12.50 FOOD$ expenses add 45 transport "monthly metro pass" --date 2026-09-05Added #2: 2026-09-05 45.00 TRANSPORT$ expenses add 1200 housing "rent" --date 2026-09-01Added #3: 2026-09-01 1200.00 HOUSING$ expenses add 3.20 food "coffee, and a muffin" --date 2026-10-02Added #4: 2026-10-02 3.20 FOOD$ expenses list  ID  DATE            AMOUNT  CATEGORY       DESCRIPTION   3  2026-09-01     1200.00  HOUSING        rent   1  2026-09-03       12.50  FOOD           lunch with Sam   2  2026-09-05       45.00  TRANSPORT      monthly metro pass   4  2026-10-02        3.20  FOOD           coffee, and a muffin4 expense(s), total 1260.70$ expenses list --month 2026-09  ID  DATE            AMOUNT  CATEGORY       DESCRIPTION   3  2026-09-01     1200.00  HOUSING        rent   1  2026-09-03       12.50  FOOD           lunch with Sam   2  2026-09-05       45.00  TRANSPORT      monthly metro pass3 expense(s), total 1257.50$ expenses summary --month 2026-09Summary for 2026-09  FOOD               12.50  TRANSPORT          45.00  HOUSING          1200.00  TOTAL            1257.50$ expenses summarySummary (all time)  FOOD               15.70  TRANSPORT          45.00  HOUSING          1200.00  TOTAL            1260.70$ expenses add 12.505 fooderror: amount can have at most 2 decimal places(exit code 1)$ expenses add 5 snackserror: unknown category 'snacks', choose one of [FOOD, TRANSPORT, HOUSING, UTILITIES, HEALTH, ENTERTAINMENT, OTHER](exit code 1)$ expenses add 5 food --date 2099-01-01error: date is in the future: 2099-01-01(exit code 1)$ expenses delete 2Deleted #2$ expenses delete 2error: no expense with id 2(exit code 1)$ expenses list --month 2026-09  ID  DATE            AMOUNT  CATEGORY       DESCRIPTION   3  2026-09-01     1200.00  HOUSING        rent   1  2026-09-03       12.50  FOOD           lunch with Sam2 expense(s), total 1212.50$ cat "$EXPENSE_TRACKER_FILE"id,date,amountCents,category,description1,2026-09-03,1250,FOOD,lunch with Sam3,2026-09-01,120000,HOUSING,rent4,2026-10-02,320,FOOD,"coffee, and a muffin"$ expenses Usage:  expenses add <amount> <category> [description] [--date yyyy-MM-dd]  expenses list [--month yyyy-MM]  expenses summary [--month yyyy-MM]  expenses delete <id>  expenses categories(exit code 1)