package test02;

import java.io.Serializable;
import java.util.ArrayList;
import java.util.List;

import javax.persistence.*;

@Entity
public class A implements Serializable {

   private static final long serialVersionUID = 1L;

   @Id
   private int id;

   @OneToMany(mappedBy = "a") // YThis does not work as expected
   private List<B> b = new ArrayList<B>();

   public A() {
      super();
   }

   public A(int id) {
      this.id = id;
   }

   public int getId() {
      return this.id;
   }

   public void setId(int id) {
      this.id = id;
   }

   public void setB(List<B> b) {
      this.b = b;
   }

// When using this annotation (and commenting the above one), things work as expected (by me)
//   @OneToMany(mappedBy = "a")
   public List<B> getB() {
      return b;
   }   
}

package test02;

import java.io.Serializable;
import javax.persistence.*;

@Entity
public class B implements Serializable {

   private static final long serialVersionUID = 1L;

   @Id
   private int id;

   @ManyToOne
   private A a;

   public B() {
      super();
   }

   public B(int id) {
      this.id = id;
   }

   public int getId() {
      return this.id;
   }

   public void setId(int id) {
      this.id = id;
   }

   public A getA() {
      return a;
   }

   public void setA(A a) {
      this.a = a;
   }
}