Hello,
In my application I have a class which has a field of type
Set<com.google.appengine.api.datastore.Key>. I want to store objects
of that class in datastore, but I can't make it work.
At first my class looked like this:
@PersistenceCapable(identityType=IdentityType.APPLICATION)
public class Kuku {
@PrimaryKey
@Persistent(valueStrategy=IdGeneratorStrategy.IDENTITY)
protected Key id;
public Key getId() {
return id;
}
@Persistent
protected Set<Key> mySet = new HashSet<Key>();
public int getMySetSize() {
return mySet.size();
}
public void addToSet(Key what) {
mySet.add(what);
}
}
In some JSP page I tried to create a new object of that class, then
store it in a datastore, then fetch it back from datastore and add
some elements to its mySet, the fetch it again and see how many
elements there are in mySet:
<%@ page contentType="text/html;charset=UTF-8" language="java" %>
<%@ page import="psobolewski.Kuku" %>
<%@ page import="psobolewski.PMF" %>
<%@ page import="javax.jdo.PersistenceManager" %>
<%@ page import="com.google.appengine.api.datastore.Key" %>
<html>
<body>
<%
Kuku kuku;
javax.jdo.PersistenceManager pm;
Key kukuId;
String result;
kuku = new Kuku();
pm = PMF.get().getPersistenceManager();
pm.makePersistent(kuku);
kukuId = kuku.getId();
pm.close();
%>
<%
pm = PMF.get().getPersistenceManager();
kuku = pm.getObjectById(Kuku.class, kukuId);
kuku.addToSet(kukuId.getChild(kukuId.getKind(), 0));
kuku.addToSet(kukuId.getChild(kukuId.getKind(), 1));
kuku.addToSet(kukuId.getChild(kukuId.getKind(), 2));
pm.close();
pm = PMF.get().getPersistenceManager();
kuku = pm.getObjectById(Kuku.class, kukuId);
result = "size: " + kuku.getMySetSize();
pm.close();
%>
<%= result %>
</body>
</html>
However, when I try to run it, I get java.lang.NullPointerException in
this place:
public void addToSet(Key what) {
mySet.add(what);
}
Then I decided to change my class this way:
@Persistent(defaultFetchGroup = "true")
protected Set<Key> mySet = new HashSet<Key>();
However, this didn't help - I still get this NullPointerException. So
I decided to change my class this way:
@Persistent(defaultFetchGroup = "true", serialized = "true")
protected Set<Key> mySet = new HashSet<Key>();
Now I don't get NullPointerException any more, but the reported size
of mySet is zero, as if I haven't added any elements to it.
What do I do wrong? How can I keep such field in a datastore?
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