Understanding Lombok @Builder Default Value Issue and Its Solution

When using Lombok's @Builder annotation in Java, fields that have default values defined in the class are not initialized with those defaults when the builder creates an instance.

Example code shows a class Demo with fields boolean bTest1 = true and Boolean bTest2 = true . Creating an object with new DemoBuilder().build() yields a Demo instance where both fields are false/null because the defaults are ignored.

Inspecting the byte‑code‑generated class reveals that the builder simply calls the all‑args constructor without applying the field initializers.

To fix this, Lombok provides the @Builder.Default annotation. Adding it to the fields causes Lombok to generate static methods (e.g., $default$bTest1() ) that return the specified default values, and the builder's build() method uses these methods when a field has not been explicitly set.

The resulting compiled class contains the static default methods and modified builder logic that checks a “set” flag for each field, applying the defaults only when necessary. This resolves the unexpected behavior of missing default values.

Although @Builder.Default solves the initialization issue, it introduces additional complexity, which will be explored in future articles.