Something unknown about the celltype of Header in 2.4 api

in the api
http://google-web-toolkit.googlecode.com/svn/javadoc/2.4/com/google/gwt/user/cellview/client/Header.html#getValue%28%29
H - the Cell type

but in fact, why the following can work

ActionCell<String> addPCCell = new ActionCell<String>("add", new Delegate<String>(){
@Override
public void execute(String projectCd) {
// TODO Auto-generated method stub
System.out.println("addPCCell execute is triggered, projectCd = " + projectCd);
}
});

Header<String> footerAdd = new Header<String>(addPCCell){
@Override
public String getValue() {
// TODO Auto-generated method stub
System.out.println("footerAdd getValue() is called");
return projectCdModel.getSelectedObject();
}
};

the H is String (Header<String>), but not the Cell type (ActionCell<String>), why?
 

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