How to allow null to an object attribute?

[Danilo Dadonas]

Hi, all. There are 2 enitities: EntityOne and EntityTwo.

    
@Entity
@Table(name="X")
public class EntityOne{
  @Id
  private Long id;
 
  @ManyToOne
  @JoinColum(name="ENTITYTWO_ID", nullable=true)
  private EntityTwo entityTwo;
}

When the application loads and EntityOne object and the attribute entityTwo is not null, everything goes fine. But when I try to retrieve an EntityOne object and the attribute entityTwo is null, the application throws a NullPointerException.

If the attribute is allowed to be null, why an exception is occuring?

Thanks...

[Mark Spritzler]

Can you paste the code and the Exception trace.

Mark

[Danilo Dadonas]

    
@Entity
@Table(name="COMPANY")
public class Agent extends Company implements java.io.Serializable {
 
    @ManyToOne(fetch=FetchType.LAZY)
    @JoinColumn(name="TRADER")
    private Trader trader;
...
[setters/getters]

Code to load the object

    
   Agent age = (Agent) HibernateUtil.getSession().load(Agent.class,"123456789121212");
   out.println(rep.getTrader().getName());

In this case, the trader of this object is null. But intead of take an exception I'd like to take an empty String, without catch the exception manually.

The exception:

    
12/03/2008 13:22:13 org.apache.catalina.core.StandardWrapperValve invoke
SEVERE: Servlet.service() for servlet TesteLogon threw exception
java.lang.NullPointerException
        at TesteLogon.processRequest(TesteLogon.java:58)
        at TesteLogon.doGet(TesteLogon.java:70)
        at javax.servlet.http.HttpServlet.service(HttpServlet.java:690)
        at javax.servlet.http.HttpServlet.service(HttpServlet.java:803)
        at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:290)
        at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:206)
        at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:233)
        at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:175)
        at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:128)
        at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:102)
        at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:109)
        at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:286)
        at org.apache.coyote.http11.Http11Processor.process(Http11Processor.java:844)
        at org.apache.coyote.http11.Http11Protocol$Http11ConnectionHandler.process(Http11Protocol.java:583)
        at org.apache.tomcat.util.net.JIoEndpoint$Worker.run(JIoEndpoint.java:447)
        at java.lang.Thread.run(Thread.java:619)
 

[Paul Sturrock]

So the NullPointerException is caused by you trying to access an associated object when there is not associated object there? Could you not just get round this with a simple defensive check?

[Danilo Dadonas]

Hi, Paul. I could but this attribute can be null. I will need to acces this attribute in the future to show the trader's name in a form. Will I have to implement checks for all attributes I think can be null?
Why it not work?

    
@ManyToOne([B]optional = true[/B])
@JoinColum(name="ENTITYTWO_ID", [B]nullable=true[/B])
private EntityTwo entityTwo;

If I can insert null attributes easily, why can't I retrive it easily too?

[Paul Clapham]

You can retrieve a null value easily. But your question is about what happens after you retrieve the null value. You're asking for a change to the Java language such that when you call a method that returns a String on a null reference, it shouldn't crash, it should return an empty String. And that isn't going to happen. So yes, it's up to you to handle nulls appropriately for your application.

[Danilo Dadonas]

For each nullable attribute I'll have to check like the following code?:

    
Agent age = (Agent)   HibernateUtil.getSession().load(Agent.class,"123456789121212");
if (!age.getTrader == null){
    out.println(age.getTrader().getName());
} else{
    out.println("");
}

My intencion is when I retrieving the object from the database to show in a form, the null attributes be tabled as "".

[Mark Spritzler]

Yes, similar to that code, you are missing a "()" in the if part of the statement.

See if you call rep.getTrader() and it returns null, you can't call a method on null.

rep.getTrader().getName());

so the first part returns null, then you try to call getName() on null, that will always throw a Null pointer exception. Because getTrader() might return null, you will have to check for null.

Mark

[Danilo Dadonas]

Ok, Mark. Than, I'll return a new object when it's null. Like this:

    
@Entity
@Table(name="COMPANY")
public class Agent extends Company implements java.io.Serializable {
    ... 
    @ManyToOne(fetch=FetchType.LAZY)
    @JoinColumn(name="TRADER")
    private Trader trader;
    ...
    public Trader getTrader(){
        if (trader == null){
            return new Trader();
        } else{
            return trader;
        }
    }

Is it a good practice?

Thanks.

[Paul Sturrock]

I wouldn't do it. If there is no associated object then I would expect the association to be null. What happens when later on you try to persist this non-existant object? Best case scenario you create a relationship where one shouldn't exist.
[ March 13, 2008: Message edited by: Paul Sturrock ]

[Danilo Dadonas]

Ok, Paul, I agree. Won't I have problem if I use ActionForm to retrieve this object with null properties?

[Paul Sturrock]

That depends on how you implement your ActionForm.

[Danilo Dadonas]

I'll try. After, I'll post the code.

Thanks.