What methods or fields can be used to identify an entity in a system?

Unique identifiers are usually used to identify entities in a system. In Java, these can be:

• Fields of type id with unique values, such as UUID or auto-incrementing numbers.
• Methods equals() and hashCode(), which are overridden for correct comparison of objects by identifier.

Example:

import java.util.UUID;

public class Entity {
    private UUID id;

    public Entity() {
        this.id = UUID.randomUUID();
    }

    public UUID getId() {
        return id;
    }

    @Override
    public boolean equals(Object o) {
        if (this == o) return true;
        if (o == null || getClass() != o.getClass()) return false;
        Entity entity = (Entity) o;
        return id.equals(entity.id);
    }

    @Override
    public int hashCode() {
        return id.hashCode();
    }
}

Thus, a unique identifier and correctly implemented comparison methods allow for unambiguous identification of an entity.