### Question:
DocumentSkinReference implements SkinReference { @Override public String serialize() { return String.format("SkinDocument[%s]", entityReferenceSerializer.serialize(skinDocument)); } DocumentSkinReference(DocumentReference skinDocument,
        EntityReferenceSerializer<String> entityReferenceSerializer); DocumentReference getSkinDocument(); @Override boolean equals(Object o); @Override int hashCode(); @Override String serialize(); @Override String toString();  }### Answer:
@Test public void serialize() throws Exception { DocumentReference documentReference = new DocumentReference("wiki", "space", "page"); DocumentSkinReference documentSkinReference = new DocumentSkinReference(documentReference, entityReferenceSerializer); when(entityReferenceSerializer.serialize(documentReference)).thenReturn("wiki:space.page"); assertEquals("SkinDocument[wiki:space.page]", documentSkinReference.serialize()); }