Illegal attempt to map a non collection as a @onetomany, @manytomany or @collectionofelements:

Error: Illegal attempt to map a non-collection as a @OneToMany, @ManyToMany, or @CollectionOfElements:

This error occurs when trying to map a non-collection type property as a @OneToMany, @ManyToMany, or @CollectionOfElements relationship in an ORM (Object-Relational Mapping) framework like Hibernate or JPA (Java Persistence API).

To understand the error better, let’s look at each of these annotations and their purpose:

• @OneToMany: Specifies a one-to-many relationship between two entities. It is used when one entity can have multiple instances of another entity.
• @ManyToMany: Specifies a many-to-many relationship between two entities. It is used when multiple instances of one entity can be associated with multiple instances of another entity.
• @CollectionOfElements: Specifies a collection of basic types (like String, Integer, etc.) or embeddable objects. It is used when a property contains a collection of non-entity objects.

In all of these annotations, the targeted property should be a collection type like List, Set, or Map. The error occurs when the targeted property is not a collection type.

Example:

    
      public class Employee {
          // Incorrect usage of @OneToMany
          @OneToMany
          private int employeeId;
      }
    
  

In the above example, the employeeId property is of type int which is not a collection type. Therefore, it cannot be mapped as a @OneToMany relationship.

To fix this error, you need to ensure that the targeted property is a collection type. For example:

    
      public class Employee {
          // Correct usage of @OneToMany
          @OneToMany
          private List<Address> addresses;
      }
    
  

In the corrected example, the addresses property is of type List