java-tdd-guide

name	java-tdd-guide
description	Bernard Ladenthin's personal Java Test-Driven Development skill — Red → Green → Refactor workflow with project-independent conventions. Baselines on Java 8 + JUnit Jupiter; BAF-specific Java 21 idioms live in the optional supplement.

Java TDD Skill — Test-Driven Development

Author: Bernard Ladenthin License: Apache 2.0

This is the canonical Java TDD skill for the four sibling repos (BitcoinAddressFinder, streambuffer, java-llama.cpp, llamacpp-ai-index-maven-plugin). It describes the workflow and conventions that are actually in use across the codebases — verified by reading the test sources, not invented.

Baseline assumed by this skill: Java 8 + JUnit Jupiter 6.1.0 + Hamcrest 3.0. Java 21 idioms (records, switch expressions, text blocks) are documented separately in ../../../guides/src/CODE_WRITING_GUIDE-21.md; only BitcoinAddressFinder targets Java 21 today.

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TDD Workflow — Red → Green → Refactor

Follow the Red → Green → Refactor cycle rigorously. Every new behaviour must be covered by a failing test before the production code is written.

1 — Red (failing test first)

Write one test that precisely describes the next desired behaviour. The test must compile but must fail when run. Do not write any production code yet.

2 — Green (minimum production code)

Write the smallest change to production code that makes the failing test pass. Do not add code that is not driven by a test.

3 — Refactor

Improve the implementation and the test code without changing observable behaviour. All tests must stay green.

Repeat for each behaviour increment.

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Test Framework — the actually-used stack

Concern	Choice	Evidence
Runner	JUnit Jupiter 6.1.0 (org.junit.jupiter.api.*)	pom.xml in all four repos
Assertions	Hamcrest 3.0 (assertThat(actual, is(equalTo(expected))))	universally adopted; assertThrows is the one Jupiter assertion that's always allowed
Parameterized	@ParameterizedTest + @MethodSource(SourceClass.CONSTANT_NAME)	BitHelperTest.java:21,33; StreamBufferTest.java:42-47
Mocking	Mockito (mock(), when(), verify(), ArgumentCaptor)	BAF + plugin; not used by sb/jllama
Temp filesystem	@TempDir Path folder (JUnit Jupiter native)	universal
Property-based (optional)	jqwik pinned ≤ 1.9.3	see policies/jqwik-prompt-injection.md

Never use:

• org.junit.Test / org.junit.Before / org.junit.Rule (JUnit 4) — banned via Maven Enforcer
• @RunWith(DataProviderRunner.class) + com.tngtech.java.junit.dataprovider — not used in any repo
• TestNG
• assertEquals etc. from org.junit.jupiter.api.Assertions when Hamcrest can express it more clearly. Two narrow exceptions are accepted in practice and visible in StreamBufferTest: (a) assertThrows/assertDoesNotThrow for exception assertion, (b) assertArrayEquals when comparing byte arrays.

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Test File Structure

File header — SPDX

Every test file must start with the SPDX-format license header:

// SPDX-FileCopyrightText: <YEAR-RANGE> Bernard Ladenthin <[e-mail]>
//
// SPDX-License-Identifier: Apache-2.0
package net.ladenthin.<repo>;

• <YEAR-RANGE> is the file's actual lifespan (e.g. 2017-2026, 2014-2026).
• No // @formatter:off / // @formatter:on wrapper — the SPDX block is three single-line comments and does not need formatter exemption.
• jllama uses MIT (SPDX-License-Identifier: MIT) with a dual-copyright line for the upstream Konstantin Herud attribution; everywhere else is Apache-2.0.

Class declaration

public class FooTest {

No runner annotation — JUnit Jupiter auto-discovers @Test / @ParameterizedTest methods.

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Two equally valid grouping styles

Owner's actual codebases use two different patterns to group related tests within a class. Both are accepted in workspace canonical; pick the style that matches the surrounding file rather than mixing.

Style A — NetBeans <editor-fold> (used in BAF, plugin)

// <editor-fold defaultstate="collapsed" desc="getKillBits">
@ParameterizedTest
@MethodSource(CommonDataProvider.DATA_PROVIDER_KILL_BITS)
public void getKillBits_bitsGiven_killBitsEqualsExpectation(int bits, BigInteger killBits) {
    // ...
}
// </editor-fold>

Rules: desc equals the method name (or a short feature label); defaultstate="collapsed" is mandatory; one fold per method under test.

Style B — @Nested + @DisplayName (used in streambuffer)

@Nested
@DisplayName("roundtrip")
class RoundtripTests {
    @DisplayName("simple round trip")
    @Test
    public void testSimpleRoundTrip() throws IOException {
        // ...
    }
}

Rules: each @Nested class groups tests for a related behaviour; @DisplayName provides the human-readable label.

The two styles are not interchangeable inside one class. Stay consistent within a file.

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Test Method Naming

Pattern: methodUnderTest_inputOrCondition_expectedBehavior

getKillBits_bitsGiven_killBitsEqualsExpectation
convertBitsToSize_bitsGiven_sizeEqualsExpectation
assertBatchSizeInBitsIsInRange_bitsGivenBelowMinimum_exceptionThrown

Verified in BAF: BitHelperTest.java:22,34,45,54,66.

Rules:

• All three segments are required and separated by underscores.
• Use camelCase within each segment.
• The expected segment describes the observable outcome (_returnsZero, _throwsException, _exceptionThrown, _skipsFile, _roundtripsCorrectly).
• Exception tests end with _throwsException or _exceptionThrown.
• No-op / smoke tests use _noExceptionThrown.

Inside @Nested classes (streambuffer style), naming may relax to testFoo because @DisplayName provides the readable label; do not mix the relaxed naming into a non-@Nested class.

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Test Body — AAA Structure

Every test body must follow the Arrange / Act / Assert structure with explicit section comments. Verified in BAF (323+ matches across 10 files), sb (658 matches in one file), plugin (129 matches). The combined // act, assert comma form is acceptable when the act IS the assertion (BitHelperTest.java:26,38,49,58,70):

@Test
public void getKillBits_bitsGiven_killBitsEqualsExpectation(int bits, BigInteger killBits) {
    // arrange
    BitHelper bitHelper = new BitHelper();

    // act, assert
    assertThat(bitHelper.getKillBits(bits), is(equalTo(killBits)));
}

For separate act + assert:

@Test
public void example() {
    // arrange
    Foo foo = new Foo();

    // act
    int result = foo.doThing();

    // assert
    assertThat(result, is(equalTo(42)));
}

// pre-assert — two valid positions

// pre-assert is a named section that asserts a condition without it being the primary assertion of the test. Use it (a) before // act to verify a precondition, or (b) between // act and // assert as a null-guard before accessing fields.

// act
final Foo result = sut.compute();

// pre-assert
assertThat(result, is(notNullValue()));

// assert
assertThat(result.value(), is(equalTo("expected")));

Rules:

• // arrange may be omitted only when there is genuinely nothing to arrange.
• Keep the act to a single method call whenever possible.
• Do not use Objects.requireNonNull(...) as a guard in tests; use a // pre-assert with assertThat(x, is(notNullValue())).

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Assertions — Hamcrest Style

// equality
assertThat(result, is(equalTo(expected)));

// null / not null
assertThat(result, is(nullValue()));
assertThat(result, is(notNullValue()));

// boolean
assertThat(flag, is(true));
assertThat(flag, is(false));

// negation
assertThat(result, is(not(equalTo(unexpected))));

// strings
assertThat(message, containsString("substring"));
assertThat(output, not(emptyOrNullString()));

// collections
assertThat(list, hasSize(3));
assertThat(list, is(empty()));

// numbers
assertThat(count, is(greaterThan(0)));

Imports to use:

import static org.hamcrest.MatcherAssert.assertThat;
import static org.hamcrest.Matchers.*;
// or specific:
import static org.hamcrest.Matchers.equalTo;
import static org.hamcrest.Matchers.is;

Two narrow exceptions where Jupiter assertions are acceptable:

import static org.junit.jupiter.api.Assertions.assertThrows;
import static org.junit.jupiter.api.Assertions.assertArrayEquals;  // byte-array compare

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Exception Testing

@Test
public void doIt_invalidInput_throwsException() {
    // arrange
    Foo foo = new Foo();

    // act, assert
    assertThrows(IllegalArgumentException.class, () -> foo.doIt(-1));
}

// With message check:
@Test
public void doIt_invalidInput_throwsExceptionWithReason() {
    // arrange
    Foo foo = new Foo();

    // act
    IllegalArgumentException e = assertThrows(IllegalArgumentException.class,
            () -> foo.doIt(-1));

    // assert
    assertThat(e.getMessage(), containsString("must be >= 0"));
}

Verified in BAF BitHelperTest.java:50,59.

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Parameterized Tests — @MethodSource

The owner's pattern stores method-source names as named string constants on a CommonDataProvider (or sibling provider) class, then references them symbolically from @MethodSource. This prevents typos and makes provider methods easy to find via "find usages".

// In CommonDataProvider.java:
public static final String DATA_PROVIDER_KILL_BITS = "killBitsArguments";

static Stream<Arguments> killBitsArguments() {
    return Stream.of(
            Arguments.of(1, BigInteger.valueOf(1L)),
            Arguments.of(8, BigInteger.valueOf(255L))
    );
}

// In BitHelperTest.java:
@ParameterizedTest
@MethodSource(CommonDataProvider.DATA_PROVIDER_KILL_BITS)
public void getKillBits_bitsGiven_killBitsEqualsExpectation(int bits, BigInteger killBits) {
    // ...
}

Verified in BitHelperTest.java:21,33,65.

For sources local to the test class (the streambuffer pattern), the provider method lives on the test class itself:

static Stream<Arguments> writeMethods() {
    return Stream.of(
            Arguments.of(WriteMethod.BYTE_ARRAY),
            Arguments.of(WriteMethod.INT),
            Arguments.of(WriteMethod.BYTE_ARRAY_WITH_PARAMETER));
}

Verified in StreamBufferTest.java:42-47.

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Mocking the Logger

For Maven plugins (org.apache.maven.plugin.logging.Log) — plugin only:

@BeforeEach
public void setUp() {
    mockLog = mock(Log.class);
}

@Test
public void indexSourceRoot_missingFile_logsWarning() {
    // arrange
    final SourceFileIndexer indexer = new SourceFileIndexer(mockLog, ...);

    // act
    indexer.indexSourceRoot(sourceRoot);

    // assert
    verify(mockLog, atLeastOnce()).warn(contains("Skipping"));
}

For SLF4J-based repos (BAF, jllama, streambuffer), production code typically uses the static logger idiom:

private static final Logger LOG = LoggerFactory.getLogger(Foo.class);

In tests, use LogCaptor to verify expected log output without mocking the static field:

try (LogCaptor captor = LogCaptor.forClass(Foo.class)) {
    // act
    foo.doSomething();

    // assert
    assertThat(captor.getInfoLogs(), hasItem(containsString("started")));
}

Verified in BAF (LogCaptor 2.12.6 declared at pom.xml:84,847; used in 5 test files) and jllama (LoggingSmokeTest).

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Production-side conventions referenced by tests

When TDD drives production code, these are the conventions tests assume and reinforce:

• Named constants with Javadoc. Every meaningful string/number/flag literal must be a public static final or private static final named constant with a Javadoc comment. Verified in BAF: 141 static final declarations across 29 source files.
• Custom domain exceptions. Throw a specific named exception type rather than generic IllegalArgumentException / RuntimeException when a domain meaning is involved. BAF examples: KeyProducerIdNullException, PrivateKeyTooLargeException.
• Constructor injection. Dependencies arrive through the constructor (with private final fields) so tests can substitute fakes. Loggers are an exception in SLF4J-using repos (static-field Logger); inject the Log when using Maven Log.
• @VisibleForTesting (Guava annotation) marks members that exist only to support tests. Used in BAF (16 sites); see policies/code-quality-todos.md for the design-fit review.

For the Java 21 idioms (records, switch expressions, text blocks, pattern matching) that apply to BitcoinAddressFinder but not the Java-8 repos, see ../../../guides/src/CODE_WRITING_GUIDE-21.md.

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What NOT To Do

Anti-pattern	Correct alternative
import org.junit.Test (JUnit 4)	import org.junit.jupiter.api.Test
@RunWith(DataProviderRunner.class)	@ParameterizedTest + @MethodSource
@Rule public TemporaryFolder folder = new TemporaryFolder()	@TempDir Path folder
Assert.assertEquals(expected, actual)	assertThat(actual, is(equalTo(expected)))
Assert.assertTrue(condition)	assertThat(condition, is(true))
Assert.assertNotNull(x)	assertThat(x, is(notNullValue()))
Objects.requireNonNull(x) as guard in tests	// pre-assert with assertThat(x, is(notNullValue()))
System.out.println(...) in tests	Remove; use LogCaptor / mock-Log assertions instead
Missing // arrange / act / assert comments	Add the section comments always
Mixing <editor-fold> and @Nested styles in one file	Pick one and stay consistent
/* license */ block instead of SPDX lines	Use the SPDX single-line form

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Preserving Existing Comments

When modifying existing test code:

• Keep all existing inline comments that are correct and descriptive.
• Only remove a comment if it is factually wrong, misleading, or describes code that no longer exists.
• Add new comments where added code is not self-explanatory.
• When adding AAA section comments, place them around existing inline comments — do not replace them.

The goal is to minimize the diff to only lines that actually need changing.