Planned lessonJNR-028-L06

Entity equality

Lesson 6 of 8
Junior

On This Lesson 4 topics

In this lesson
Planned topics
Draft outline details
Practice

This lesson is planned. The outline below lists the topics it will cover. The full lesson has not been published yet.

In this lesson

write equals/hashCode that survive the lifecycle.

Planned topics

Default equals compares memory identity; two loads in different transactions are different objects.
Using all fields (as Lombok @Data does) breaks when fields change and can load lazy relations.
The generated id is null before saving, so id-based hashCode changes after persist if the entity is in a HashSet.
Safe options: a stable business key (such as SKU) or the id-based pattern with a constant hashCode.
Never use @Data on entities.

Draft outline details

Demonstration

java

@Override public boolean equals(Object o) {
    if (this == o) return true;
    if (!(o instanceof Product other)) return false;
    return id != null && id.equals(other.id);
}
@Override public int hashCode() { return getClass().hashCode(); }

Check your understanding

Why is @Data risky on an entity?

Show answer

Its equals/hashCode use all fields, which change and may trigger lazy loading.

Practice

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