Java / Lombok Interview questions
1. What is Lombok?
Lombok is a Java library that removes boilerplate code — getters, setters, constructors, equals / hashCode , toString , and more — by generating it automatically at compile time from simple annotations, rather than you writing it by hand. import lombok.Getter ; import lombok.Setter ; ...
2. What is the purpose of Lombok in Java?
Lombok exists to cut down the sheer amount of repetitive, mechanical code that plain Java classes typically require — particularly simple data-holding classes (often called POJOs) that need getters, setters, constructors, and standard Object method overrides just to function correctly with ...
3. How do you add Lombok to a Maven project?
Lombok is added as a regular dependency, typically scoped as provided since it's only needed at compile time — the generated code becomes part of your compiled classes, so Lombok itself doesn't need to be on the runtime classpath.
4. How do you add Lombok to a Gradle project?
Gradle uses the compileOnly and annotationProcessor configurations together for Lombok — compileOnly makes the annotations available while writing code, and annotationProcessor tells Gradle to actually run Lombok during compilation to generate the boilerplate. dependencies { compileOnly 'or...
5. What is @Getter used for?
@Getter generates a standard getter method for a field (or every field, when applied at the class level), following the usual JavaBeans naming convention — getFieldName() for most types, or isFieldName() for a boolean field. public class Person { @Getter private String name; @Getter private...
6. What is @Setter used for?
@Setter generates a standard setter method for a field, following the JavaBeans convention of setFieldName(Type value) , and like @Getter , it can be applied per-field or at the class level to cover every field at once. public class Person { @Setter private String name; } // generates: // public ...
7. What is @ToString used for?
@ToString generates a toString() override that prints the class name followed by each field's name and value, giving a readable default representation instead of Java's default ClassName@hashcode output. @ToString public class Person { private String name; private int age; } // toString() produce...
8. What is @EqualsAndHashCode used for?
@EqualsAndHashCode generates proper equals() and hashCode() overrides based on the class's fields, following the standard contract Java expects — two objects with equal field values are considered equal and produce the same hash code, which is essential for correct behavior in collections l...
9. What is @NoArgsConstructor used for?
@NoArgsConstructor generates a public, no-argument constructor for the class — useful for frameworks that require a default constructor to instantiate objects via reflection, such as JPA entities or JSON deserialization libraries like Jackson. @NoArgsConstructor public class Person { privat...
10. What is @AllArgsConstructor used for?
@AllArgsConstructor generates a constructor that takes one parameter for every field in the class, in the order the fields are declared — a quick way to get a fully-populating constructor without writing it by hand. @AllArgsConstructor public class Person { private String name; private int ...
11. What is @RequiredArgsConstructor used for?
@RequiredArgsConstructor generates a constructor that includes only the fields Lombok considers "required": final fields, and fields annotated @NonNull that don't already have a default value assigned. @RequiredArgsConstructor public class Person { private final String name; // required: included...
12. What is @Data used for?
@Data is a convenience annotation that bundles several others together: @Getter , @Setter , @ToString , @EqualsAndHashCode , and @RequiredArgsConstructor , all applied at once with a single annotation on the class. @Data public class Person { private final String id; private String name; private ...
13. What is @Value used for?
@Value is the immutable counterpart to @Data : it makes the class and all its fields final by default, generates getters (but no setters), a constructor covering every field, toString() , and equals() / hashCode() — producing a fully immutable value object from a single annotation. @Value p...
14. What is @Builder used for?
@Builder generates the builder pattern for a class — a fluent, chainable way to construct an object field by field, particularly useful for classes with many fields (especially optional ones) where a single large constructor call would be hard to read and easy to get wrong. @Builder public ...
15. What is @Slf4j used for?
@Slf4j generates a private, static logger field named log , pre-configured for the SLF4J logging facade, saving you from writing the same one-line logger declaration in every class that needs logging. @Slf4j public class OrderService { public void placeOrder(Order o) { log . info( "Placing order ...
16. What is @NonNull used for?
Placing @NonNull on a field, constructor parameter, or method parameter tells Lombok to generate a null check at the start of the relevant generated (or annotated) method, throwing a NullPointerException immediately with a clear message if the value is null , rather than letting a null slip throu...
17. What is @Cleanup used for?
@Cleanup automatically calls a resource's close() method (or another method you specify) at the end of the enclosing block, functioning as a Lombok-era predecessor to Java's built-in try-with-resources for ensuring resources get released. @Cleanup InputStream in = new FileInputStream( "data.txt" ...
18. What is @SneakyThrows used for?
@SneakyThrows lets a method throw a checked exception without declaring it in a throws clause and without wrapping it in a try/catch — Lombok generates bytecode that throws the checked exception directly, exploiting the fact that the JVM itself doesn't actually enforce checked exceptions th...
19. What is @Synchronized used for?
@Synchronized is Lombok's safer alternative to Java's built-in synchronized keyword on a method: instead of locking on this (or the class object for a static method), which exposes the lock to any external code that also happens to synchronize on the same object, Lombok generates a private, dedic...
20. What is @With used for?
@With generates a "wither" method — a method that returns a new instance of the class with one field changed and every other field copied unchanged from the original, which is the standard pattern for "updating" an immutable object without mutating it. @Value @With public class Point { int ...
21. Describe how Lombok works under the hood (annotation processing)?
Lombok hooks into the Java compilation process as an annotation processor , a standard extension point the javac compiler supports for running code during compilation that can inspect and modify the program being compiled. Unlike most annotation processors, which are restricted to only generating...
22. How do you install the Lombok plugin in an IDE?
Because Lombok's generated methods don't exist in your actual source file, an IDE that only parses the source as written would show errors for calls to getters/setters that "don't exist" — the IDE plugin teaches the IDE's own code analysis to understand Lombok's annotations and treat the ge...
23. What are the types of Lombok annotations?
Lombok's annotations fall into two broad groups, reflected in their package names. Core (lombok.*) Experimental (lombok.experimental.*) Stable, well-established annotations: @Getter, @Setter, @Data, @Builder, @ToString, @EqualsAndHashCode, and similar. Newer or more situational features whose API...
24. How do you exclude a field from @ToString?
Annotating a specific field with @ToString.Exclude removes just that field from the generated toString() output, while every other field is still included as usual. @ToString public class User { private String username; @ToString . Exclude private String password; } // toString() produces "User(u...
25. How do you exclude a field from @EqualsAndHashCode?
@EqualsAndHashCode.Exclude on a field removes it from both the equality comparison and the hash code calculation, while every other (non-excluded) field is still used as normal. @EqualsAndHashCode public class Session { private String userId; @EqualsAndHashCode . Exclude private long lastAccessed...
26. What is @Builder.Default used for?
Normally, a field's default field initializer (like private int retries = 3; ) is silently ignored when using @Builder , because the builder constructs the object through its own constructor logic rather than the field's normal initialization path — without @Builder.Default , an unset field...
27. Why does IDE support matter for using Lombok effectively?
Since Lombok's generated methods only exist after annotation processing runs during compilation, an IDE without Lombok awareness sees only the source file as literally written — no getters, no setters, no generated constructor — and will flag every call to a Lombok-generated method as...
28. What is the difference between @Data and @Value?
Both are convenience bundles that save you from applying several annotations individually, but they target opposite design intents: @Data produces a mutable class; @Value produces an immutable one. @Data @Value Fields stay mutable by default (non-final unless you mark them so). Fields become fina...
29. What is the difference between @NoArgsConstructor, @AllArgsConstructor, and @RequiredArgsConstructor?
All three generate a constructor, but they differ in which fields end up as parameters. @NoArgsConstructor @RequiredArgsConstructor @AllArgsConstructor No parameters at all. Only final fields and @NonNull fields without a default. Every field, in declaration order. Useful for frameworks needing a...
30. When should you use @Builder instead of a constructor?
Reach for @Builder once a class has several fields, especially a mix of required and optional ones, or multiple fields of the same type where positional constructor arguments become easy to mix up (two adjacent String parameters, for instance, where swapping them compiles fine but is wrong). // e...
31. How does @EqualsAndHashCode handle inheritance by default?
By default, @EqualsAndHashCode does not call the superclass's equals / hashCode — it only considers the fields declared in the annotated class itself, which can silently produce incorrect equality if a subclass has meaningful fields inherited from a parent class that should also factor into...
32. Why can @Data be risky on JPA entity classes?
@Data bundles @EqualsAndHashCode and @ToString across every field, which interacts poorly with several JPA/Hibernate specifics: lazy-loaded associations can trigger unwanted database queries the moment toString() or equals() touches them, and including a mutable, database-generated id field in eq...
33. How do you combine Lombok's @Builder with inheritance?
Plain @Builder doesn't naturally handle a subclass adding its own fields on top of a parent class's builder — by default, each class's @Builder only knows about that class's own fields. Lombok's @SuperBuilder annotation exists specifically to solve this, generating a builder that's aware of...
34. What is the difference between @Getter(lazy = true) and a normal @Getter?
A normal @Getter just returns the field's current value directly. @Getter(lazy = true) is meant for a field whose value is expensive to compute: it generates a getter that computes the value only the first time it's called, caches the result, and returns the cached value on every subsequent call....
35. Why should you be careful using @EqualsAndHashCode on Hibernate entities with lazy-loaded proxies?
Hibernate sometimes returns a lazy-loading proxy object standing in for the real entity, rather than the actual entity instance itself — and a proxy's runtime class differs from the real entity's class (it's a dynamically generated subclass). A naive equals() comparing this.getClass() == ot...
36. What is @Accessors used for and how does it change fluent-style access?
@Accessors customizes the naming and chaining style of Lombok's generated getters/setters, moving away from strict JavaBeans conventions toward a more fluent, chainable style some codebases prefer. @Accessors (fluent = true, chain = true) @Getter @Setter public class Person { private String name;...
37. How does Lombok handle @ToString with circular references?
Lombok's @ToString doesn't have built-in circular reference detection — if class A has a @ToString -included field pointing to class B, and B has a field pointing back to A, calling toString() on either one recurses infinitely (A's toString calls B's toString, which calls A's toString, and ...
38. What is @FieldDefaults used for?
@FieldDefaults (an experimental annotation) lets you set a default access level and/or final -ness for every field in a class in one line, rather than repeating private final on each field individually. @FieldDefaults (makeFinal = true, level = AccessLevel . PRIVATE) public class Point { int x; /...
39. What is @Delegate used for?
@Delegate (experimental) generates wrapper methods on a class that forward calls to a field's methods, implementing the composition-based delegation pattern without hand-writing a pass-through method for every method you want to expose. public class SortedList { @Delegate private final List < Str...
40. Why is Lombok's approach considered a "compiler hack" and what are the implications?
Lombok works by using internal, officially unsupported compiler APIs (from javac 's and Eclipse's own compiler internals) to directly rewrite a class's abstract syntax tree during compilation — something the standard, documented annotation processing API doesn't actually permit (it only sup...
41. How do you debug generated Lombok code?
Since the generated methods don't appear in your source file, stepping through them in a debugger or inspecting exactly what was generated requires a couple of specific techniques rather than just reading the .java file. # view the actual generated code as plain Java: delombok src / main / java -...
42. What issues can Lombok cause with static code analysis tools?
Static analysis tools that operate on source code (rather than compiled bytecode) may not understand Lombok's annotations at all, and so analyze the class as if the generated methods simply don't exist — this can produce both false positives (flagging a field as "never read" when it actuall...
43. What are the risks of using @Data with mutable collections?
A field like private List
44. How does delombok work and when would you use it?
delombok runs Lombok's own transformation logic but, instead of feeding the result to the compiler, writes out the fully expanded, plain-Java equivalent source code — every getter, setter, constructor, and other generated method spelled out explicitly, with no Lombok annotations remaining. ...
45. Why might Lombok cause issues in multi-module Maven builds with annotation processor ordering?
Annotation processors run in whatever order the build tool discovers and applies them, and if a project uses multiple processors together (Lombok alongside MapStruct, or a custom code-generating processor), the order they run in can matter — particularly since Lombok's generated methods nee...
46. What is @UtilityClass used for?
@UtilityClass (experimental) converts a class into a static utility class: it makes the class final , generates a private constructor that throws if somehow invoked via reflection, and marks every field and method in the class as static automatically — the standard boilerplate for a class t...
47. What is the difference between Lombok's @Builder and the traditional Gang-of-Four Builder pattern?
The classic Gang-of-Four Builder pattern is typically hand-written as a separate, standalone class (often with its own interface) explicitly responsible for assembling a complex object step by step, sometimes supporting different construction algorithms or representations via a "Director" that or...
48. How do you use Lombok with Java records - is Lombok still needed?
Java's built-in record type (since Java 16) already generates a canonical constructor, accessors, equals() , hashCode() , and toString() automatically — covering a large chunk of what @Value or @Data used to provide for immutable classes, without needing Lombok at all. // Java record - no L...
