single table inheritance

msi_333 · **Posted:** Wed Apr 21, 2010 11:10 pm

We can map between the inheritance rational technology and the object oriented technology .In this example will show single table per hierarchy inheritance . In this case we have a single rational database table to represent the entire class hierarchy. There is a discriminator column is used to differ between subclasses.The discriminator column in database will specify the java type which represents.

Advantage of this :
It supports polymorphism.

Disadvantage :
Table must has column that represents all classes in hierarchy . And each filed maps to a subclass must be null-able .

Here is the JPA class hierarchy :

Code:

package com.codemiles.jpa;

import java.io.Serializable;

import javax.persistence.Column;
import javax.persistence.DiscriminatorColumn;
import javax.persistence.DiscriminatorType;
import javax.persistence.DiscriminatorValue;
import javax.persistence.Entity;
import javax.persistence.Id;
import javax.persistence.Inheritance;
import javax.persistence.InheritanceType;
import javax.persistence.Table;
 
@Entity
@Table(name="SHAPE")
@Inheritance(strategy=InheritanceType.SINGLE_TABLE)
@DiscriminatorColumn(name="TYPE",discriminatorType=DiscriminatorType.STRING)
@DiscriminatorValue("JustShape")
public class Shape implements Serializable{
    /**
     * 
     */
    private static final long serialVersionUID = 1L;
    @Id
    protected int id;
    protected int scale;
    protected float rotate;
    protected String colour;
    protected String points;
    protected int numOfPoints;
    
    public Shape(){
    id=(int)System.currentTimeMillis();
    }
    @Column(name="SHAPE_ID")
    public int getId() {
        return id;
    }

    public void setId(int id) {
        this.id = id;
    }
    @Column(name="SCALE")
    public int getScale() {
        return scale;
    }

    public void setScale(int scale) {
        this.scale = scale;
    }
    @Column(name="ROTATE")
    public float getRotate() {
        return rotate;
    }

    public void setRotate(float rotate) {
        this.rotate = rotate;
    }
 
    @Column(name="POINTS")
    public String getPoints() {
        return points;
    }

    public void setPoints(String points) {
        this.points = points;
    }
   
    public void setColour(String colour) {
    this.colour = colour;
    }

    @Column(name="COLOUR")
    public String getColour() {
    return colour;
    }
    public void setNumOfPoints(int numOfPoints) {
    this.numOfPoints = numOfPoints;
    }
    @Column(name="NUM_POINTS")
    public int getNumOfPoints() {
    return numOfPoints;
    }
    
}
 

Code:

package com.codemiles.jpa;

import java.io.Serializable;

import javax.persistence.Column;
import javax.persistence.DiscriminatorValue;
import javax.persistence.Entity;

@Entity
@DiscriminatorValue("Circle")
public class Circle extends Shape implements Serializable{
    /**
     * 
     */
    private static final long serialVersionUID = 1L;
    protected float radius; 
  

    
    public Circle(){
    super();
    numOfPoints =1 ;
    
    }


    public void setRadius(float radius) {
    this.radius = radius;
    }

    @Column(name="RADIUS")
    public float getRadius() {
    return radius;
    }
}

 

Code:

package com.codemiles.jpa;

import java.io.Serializable;

import javax.persistence.Column;
import javax.persistence.DiscriminatorValue;
import javax.persistence.Entity;
import javax.persistence.Transient;

@Entity
@DiscriminatorValue("Triangle")
public class Triangle extends Shape implements Serializable{
    /**
     * 
     */
    private static final long serialVersionUID = 1L;
    
    private String isosceles  ;
    
    public Triangle(){
    super();
    numOfPoints=3;
    
    }

    public void setIsosceles(String isosceles) {
    this.isosceles = isosceles;
    }
    
    @Column(name="ISOSCELES")
    public String getIsosceles() {
    return this.isosceles;
    }
    @Transient
    public boolean isIsosceles() {
    return isosceles.equals("Y");
    }

 
}
 

You can see a new annotations in the code snippets :
@Inheritance : specify the type of inheritance .
@DiscriminatorColumn : used to specify the columns which is used the mapping to java type .
@DiscriminatorValue : value of discriminator column for this class.

as you may notice , calling super() function in sub-classes to call the root classes where the simple id is generated.I am using simple id here by @id annotation.

single table inheritance

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