Question

I have thhis few lines of code:

Set<Group> setofAllGroups;
setofAllGroups = new TreeSet<Group>();
Group[] allGroupsArray = (Group[]) setofAllGroups.toArray();

the last line causes a runtime error, in debug mode I get “Source not found”

the code for the class Group:

public class Group 
{
String groupName;
Set<Recipient> groupMembers;

public Group()
{
    groupName = "";
    groupMembers = new TreeSet<Recipient>();
}

public void setGroupName(String name)
{
    groupName = name;
    return;
}

public void addMember(Recipient toAdd)
{
    groupMembers.add(toAdd);
    return;
}
public void addMember(String name, String phoneNumber)
{
    Recipient toAdd = new Recipient(name, phoneNumber);
    groupMembers.add(toAdd);
    return;
}
public void removeMember(Recipient toRemove)
{
    groupMembers.remove(toRemove);
}
public void removeMember(String name, String phoneNumber)
{
    Recipient toRemove = new Recipient(name, phoneNumber);
    groupMembers.remove(toRemove);
}
public void removeAllGroupMembers()
{
    groupMembers.clear();
}

}

What is the reason for the runtime error ?

Answer 1

Quick answer is that you cannot cast like that:

Group[] allGroupsArray = (Group[]) setofAllGroups.toArray();

Instead you have to use this:

Group[] allGroupsArray = setofAllGroups.toArray(
     new Group[setofAllGroups.size()]);

That’s because toArray() returns Object[] and after that you try to cast Object[] to Group[]. This fails.