### Question:
SelfSchemaMapper implements SchemaMapper { @Override public Collection<ColumnPair<?>> create(Schema schema) { List<Column<?>> columns = schema.getColumns(); return columns.stream() .map(SelfSchemaMapper::toPair) .collect(Collectors.toList()); }  @Override Collection<ColumnPair<?>> create(Schema schema); @Override Collection<ColumnPair<?>> create(Schema schema1, Schema schema2);  }### Answer:
@Test public void test() { SchemaMapper schemaMapper = new SelfSchemaMapper(); List<Column<?>> columns = Arrays.asList(A, B, C); Schema schema = Schema.of(columns); when(relation.getSchema()).thenReturn(schema); Collection<ColumnPair<?>> mapping = schemaMapper.create(relation); assertThat(mapping).hasSize(3); assertThat(mapping).contains(new ColumnPair<>(A, A)); assertThat(mapping).contains(new ColumnPair<>(B, B)); assertThat(mapping).contains(new ColumnPair<>(C, C)); }