JPA - @OneToOne Examples

@Entity
public class EntityA {
    @Id
    @GeneratedValue
    private int myIdA;
    @OneToOne
    private EntityB entityB;
    .............
}

@Entity
public class EntityB {
    @Id
    @GeneratedValue
    private int myIdB;
    private String stringB;
    .............
}

@Entity
public class EntityC {
    @Id
    @GeneratedValue
    private int myIdC;
    @OneToOne(cascade = CascadeType.PERSIST)
    private EntityD entityD;
    .............
}

@Entity
public class EntityD {
    @Id
    @GeneratedValue
    private int myIdD;
    private String stringD;
    .............
}

@Entity
public class EntityA {
    @Id
    @GeneratedValue
    private int myIdA;
    @OneToOne(optional = false)
    private EntityB entityB;
    .............
}

@Entity
public class EntityB {
    @Id
    @GeneratedValue
    private int myIdB;
    private String stringB;
    .............
}

@Entity
public class EntityA {
    @Id
    @GeneratedValue
    private int myIdA;
    @OneToOne
    private EntityB refEntityB;
    .............
}

@Entity
public class EntityB {
    @Id
    @GeneratedValue
    private int myIdB;
    @OneToOne(mappedBy = "refEntityB")
    private EntityA refEntityA;
    private String str;
    .............
}

@Entity
public class EntityA {
    @Id
    @GeneratedValue
    private int myIdA;
    private String stringA;
    @OneToOne
    @JoinTable(name = "MY_JOIN_TABLE",
            joinColumns = {
                    @JoinColumn(name = "ENTITYA_FK", referencedColumnName = "myIdA")
            },
            inverseJoinColumns = {
                    @JoinColumn(name = "ENTITYB_FK", referencedColumnName = "myIdB", unique = true)
            }
    )
    private EntityB entityB;
    .............
}

@Entity
public class EntityB {
    @Id
    @GeneratedValue
    private int myIdB;
    private String stringB;
    .............
}

@Entity
public class EntityA {
    @Id
    @GeneratedValue
    private int myIdA;
    private String stringA;
    @OneToOne
    @JoinTable(name = "MY_JOIN_TABLE",
            joinColumns = {
                    @JoinColumn(name = "ENTITYA_FK", referencedColumnName = "myIdA")
            },
            inverseJoinColumns = {
                    @JoinColumn(name = "ENTITYB_FK", referencedColumnName = "myIdB", unique = true)
            }
    )
    private EntityB entityB;
    .............
}

@Entity
public class EntityB {
    @Id
    @GeneratedValue
    private int myIdB;
    private String stringB;

    @OneToOne(mappedBy = "entityB")
    private EntityA entityA;
    .............
}

import javax.persistence.Entity;
import javax.persistence.Id;
import javax.persistence.IdClass;
import javax.persistence.OneToOne;
import java.util.Date;

@Entity
@IdClass(CompositeTaskId.class)
public class Task {
    @Id
    private long taskId;
    @Id
    @OneToOne
    private Employee employee;
    private String taskName;
    private Date date;

    public Task() {
    }

    public Task(Employee employee, long taskId) {
        this.employee = employee;
        this.taskId = taskId;
    }

    public String getTaskName() {
        return taskName;
    }

    public void setTaskName(String taskName) {
        this.taskName = taskName;
    }

    public Date getDate() {
        return date;
    }

    public void setDate(Date date) {
        this.date = date;
    }

    @Override
    public String toString() {
        return "Task{" +
                "taskId=" + taskId +
                ", employee=" + employee +
                ", taskName='" + taskName + '\'' +
                ", date=" + date +
                '}';
    }
}

import java.io.Serializable;
import java.util.Objects;

public class CompositeTaskId implements Serializable {
    //the names of the both fields should be same as the @Id fields in source class
    //the type of 'taskId' field is same as of the basic @Id field defined in the Task entity
    private long taskId;
    //the type of 'employee' field is same as of the @Id field defined in Employee entity.
    private long employee;

    public CompositeTaskId() {
    }

    public CompositeTaskId(long employeeId, long taskId) {
        this.employee = employeeId;
        this.taskId = taskId;
    }

    @Override
    public boolean equals(Object o) {
        if (this == o) return true;
        if (o == null || getClass() != o.getClass()) return false;
        CompositeTaskId taskId1 = (CompositeTaskId) o;
        if (employee != taskId1.employee) return false;
        return taskId == taskId1.taskId;
    }

    @Override
    public int hashCode() {
        return Objects.hash(employee, taskId);
    }
}