@Entity
private class Parent {
   
    @Id
    @GeneratedValue(GenerationType.AUTO)
    @Column(name="PARENT_ID")
    private Integer id;
   
    // Notice mappedBy attribute's value refers Parent property OR field in Child class
    @OneToMany(
       mappedBy="parent",
       cascade={
           CascadeType.PERSIST,
           CascadeType.MERGE
       })
    @JoinColumn(name="PARENT_ID", nullable=false)
    private List<Child> childList = new ArrayList<Child>();

    public void addChild(Child child) {
        childList.add(child);

        // It is important because mappedBy="parent"
        child.setParent(this);
    }

}

@Entity
public class Child {

    @Id
    @GeneratedValue(GenerationType.AUTO)
    @Column(name="CHILD_ID")
    private Integer id;   

    @ManyToOne
    @JoinColumn(name="PARENT_ID", nullable=false)
    private Parent parent;

}

Parent parent = new Parent();

parent.addChild(new Child());
parent.addChild(new Child());

session.save(parent); // Both parent and childs will be saved

Parent parent = new Parent();

Child updatableChild; // Notice child id should be set up

parent.addChild(updatableChild);

session.save(parent); // Parent will be saved and Child will be updatable