Get List of Objects via RestTemplate

After compiling, List<T> aList,  generic type parameter will be erased and replaced by an Object type. That is a  common Java generic operation. However, for a deserialization process, an Object type instance is too general to re-build. You may solve this by using Array or ParameterizedTypeReference interface.

 using Array instance instead of Collection of Generics

@Test
@Sql("classpath:testdata.sql")
@Sql(value = "classpath:deleteTestData.sql", executionPhase = Sql.ExecutionPhase.AFTER_TEST_METHOD)
void testFindListCustomerByArray() {
    URI uri = builder.build().toUri();

    ResponseEntity<CustomerDto[]> response = this.template.getForEntity(uri, CustomerDto[].class);

    CustomerDto[] customerDtoList = response.getBody();
    CustomerDto[] emptyArray = {};

    List<String> firstNames = Arrays.asList("Mike", "Mia");
    boolean matched = Arrays.stream(customerDtoList != null ? customerDtoList : emptyArray)
            .map(c -> c.getFirstName()).collect(toList()).equals(firstNames);

    assertAll(
            () -> assertThat(customerDtoList).hasSize(2),
            () -> assertThat(matched).isTrue()
    );
}

 using ParamterizedTypeReference Interface

@Test
@Sql("classpath:testdata.sql")
@Sql(value = "classpath:deleteTestData.sql", executionPhase = Sql.ExecutionPhase.AFTER_TEST_METHOD)
void testFindAllCustomers() {
    URI uri = builder.build().toUri();

    //this.template.getForEntity(uri, CustomerDto[].class);
    ResponseEntity<List<CustomerDto>> response = this.template.exchange(uri, HttpMethod.GET, null,
            new ParameterizedTypeReference<List<CustomerDto>>() {
            });

    List<CustomerDto> customerDtoList = response.getBody();

    List<String> firstNames = Arrays.asList("Mike", "Mia");
    boolean matched = customerDtoList.stream().map(c -> c.getFirstName()).collect(toList()).equals(firstNames);

    assertAll(
            () -> assertThat(customerDtoList).hasSize(2),
            () -> assertThat(matched).isTrue()
    );
}