String filename, int offset) {
Assert.isNotNull(filename);
String key = filename + "*" + offset; //use * as it is not a valid filename character//$NON-NLS-1$
ImageDescriptor desc = getImageDescriptor(key);
if (desc == null) {
desc = new ProgramImageDescriptor(filename, offset);
getDescriptors().put(key, desc);
}
return desc;
}