Iterator<StructSpec> commonStructs() {
        return new Iterator<StructSpec>() {
            private final Iterator<String> iter = commonStructNames.iterator();

            @Override
            public boolean hasNext() {
                return iter.hasNext();
            }

            @Override
            public StructSpec next() {
                return structs.get(iter.next()).spec;
            }
        };
    }
Returns an iterator that will step through all the common structures.
generator/src/main/java/org/apache/kafka/message/StructRegistry.java