Apex MyServiceAPITest Unit Test using GenericRESTMock

gistfile1.txt

@isTest
private class MyServiceAPITest {
  @isTest static void getTransactionsTest() {
      Test.startTest();
      Map<String,String> headers = new Map<String, String>();
      headers.put('Accept','application/json');
      Test.setMock(HttpCalloutMock.class, new GenericRESTMock(200,'Success','{"Items":[{"Id":"string","Code":"string","EffectiveDate":"2019-07-15T22:03:31.145Z","Type":"APP"]}',headers));
      String str = MyServiceAPI.getTransactions('7/15/2019');
      Test.stopTest();
      system.assertEquals(null, str);
  }
}