### Question:
Expression extends DMNElement implements HasTypeRef,
                                                               HasComponentWidths { @Override public List<HasTypeRef> getHasTypeRefs() { return new ArrayList<>(singletonList(this)); } Expression();  Expression(final Id id,
                      final Description description,
                      final QName typeRef); abstract Expression copy(); @Override QName getTypeRef(); @Override void setTypeRef(final QName typeRef); @Override java.util.List<Double> getComponentWidths(); @Override DMNModelInstrumentedBase asDMNModelInstrumentedBase(); @Override List<HasTypeRef> getHasTypeRefs();  }### Answer:
@Test public void testGetHasTypeRefs() { final List<HasTypeRef> actualHasTypeRefs = expression.getHasTypeRefs(); final List<HasTypeRef> expectedHasTypeRefs = singletonList(expression); assertEquals(expectedHasTypeRefs, actualHasTypeRefs); }