### Question:
DocumentReferenceConverter extends AbstractConverter<DocumentReference> { @Override protected String convertToString(DocumentReference value) { if (value == null) { return null; } return this.serializer.serialize(value); }    }### Answer:
@Test void convertToString() { when(this.mockSerialier.serialize(new DocumentReference("wiki", "space", "page"))).thenReturn( "wiki:space.page"); assertEquals("wiki:space.page", this.converterManager.convert(String.class, new DocumentReference("wiki", "space", "page"))); when(this.mockSerialier.serialize(new DocumentReference("wiki", "space", "page"))).thenReturn( "space.page"); assertEquals("space.page", this.converterManager.convert(String.class, new DocumentReference("wiki", "space", "page"))); }