---

name: java-patterns description: This skill should be used for Java/Spring patterns, dependency injection, streams, Optional, Kotlin, Spring Boot, Maven, Gradle, JVM backend whenToUse: Java code, Spring Boot, JUnit, Kotlin, .java files, .kt files, Maven, Gradle, JVM backend, Spring framework, Jakarta EE whenNotToUse: Non-JVM code, Android-specific (use Android docs) seeAlso:

• skill: testing-strategies when: JUnit test architecture
• skill: api-design when: Spring REST APIs
• skill: architecture-patterns when: Java application structure

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Java Patterns

Idiomatic Java/Spring patterns for Java 17+.

Records

Copypublic record User(String name, String email) {}

Optional

CopyOptional.ofNullable(user)
    .map(User::getEmail)
    .orElse("default@example.com");

Streams

CopyList<String> names = users.stream()
    .filter(u -> u.isActive())
    .map(User::getName)
    .collect(Collectors.toList());

Spring Dependency Injection

Copy@Service
public class UserService {
    private final UserRepository repo;

    public UserService(UserRepository repo) {
        this.repo = repo;
    }
}

JUnit 5

Copy@Test
void shouldReturnUser() {
    User user = service.findById(1L);
    assertThat(user.getName()).isEqualTo("test");
}

@ParameterizedTest
@ValueSource(strings = {"a", "b", "c"})
void shouldValidate(String input) {
    assertTrue(validator.isValid(input));
}