write game with java3d

I write this program but it has exception in line

group.addChild(tg);

but when i add

TransformGroup tg = new TransformGroup();

into the for loop block it runs with any problem please tell me it's reason.

Thanks.

this is my code

public BranchGroup Creat()
{
    BranchGroup group = new BranchGroup();
    TransformGroup tg = new TransformGroup();
    for(float x = 0.0f; x < 1.0f; x += 0.1f)
    {
         Transform3D td = new Transform3D();
  开发者_Python百科       Vector3f vector3f = new Vector3f(x, x, x);             
         td.setTranslation(vector3f);
         tg.setTransform(td);
         tg.addChild(new Cone(0.05f, 0.1f));
         group.addChild(tg);             
   }



    return group;
}

this is it's exception

Exception in thread "main" javax.media.j3d.MultipleParentException: Group.addChild: child already has a parent
    at javax.media.j3d.GroupRetained.checkValidChild(GroupRetained.java:478)
    at javax.media.j3d.GroupRetained.addChild(GroupRetained.java:487)
    at javax.media.j3d.Group.addChild(Group.java:290)
    at t39.Draw.Creat(Draw.java:68)
    at t39.Draw.<init>(Draw.java:50)
    at t39.Main.main(Main.java:22)

The same element can't exist more than once in the scene graph. When you create a new TransformGroup within the loop it doesn't break the rule, but if you don't create a new one for every addChild() you break this rule.

(There are exceptions to the "only once in the graph", via weaker references instead of parent/child, e.g. for attributes)