@Entity
public class Parent {
    // ...

    @OneToMany(mappedBy = "parent", fetch = FetchType.LAZY, cascade = CascadeType.REMOVE)
    private Set<Child> children = new HashSet<Child>();

    // ...
}

@Entity
public class Child {
    // ...

    @ManyToOne(fetch = FetchType.LAZY)
    private Parent parent;

    // ...
}

Parent parent = new Parent();
 em.persist(parent);

 // ...

 Child child = new Child();
 child.setParent(parent);
 em.persist(child);

 parent.getChildren().size(); // returns 0

public void addChild(Child child) {
    child.setParent0(this);
    getChildren().add(individualNeed);
 }

public void setParent(Parent parent) {
   parent.addChild(child);
}

public void setParent0(Parent parent) {
   this.parent = parent;
}

@Entity
public class Parent {
   // ...

   @OneToMany(mappedBy = "parent", fetch = FetchType.LAZY,
      cascade = {CascadeType.REMOVE, CascadeType.PERSIST})
   @Cascade({org.hibernate.annotations.CascadeType.SAVE_UPDATE})
   private Set<Child> children = new HashSet<Child>();

   // ...
}

@Entity
public class Child {
    // ...

    @ManyToOne(fetch = FetchType.LAZY, cascade = CascadeType.PERSIST)
    @Cascade({org.hibernate.annotations.CascadeType.SAVE_UPDATE})
    private Parent parent;

    // ...
}

Parent parent = new Parent();
em.persist(parent);

// ...

Child child = new Child();
child.setParent(parent);
em.persist(child); //will cascade update to parent

parent.getChildren().size(); // returns 1

Parent parent = new Parent();
Child child = new Child();
parent.setChild(parent);
em.persist(parent); //will cascade update to child

child.getParent(); // returns the parent