DI

Class Service {

  // call some external API
  // the implementation can very without effecting the consumers
  // it can be mocked for testing
  
  string getData() { return string } 
  
}

Class Consumer 

   // 1. service is injected (I) not created internally or maintained by this class
   // 2. consumer is dependent on service to operate (D)
   // 3. service can be changed without letting the consumer know 

  Consumer(Service service) { // you should abstract an interface for service

   // you can save the service and use it .

  }
  
}

// when testing you can mock a test service
class MockService {
string getData { return "A"} // expected results are returned without a need to call the real service
}

// test the consumer with the mock service by injecting it instead of the real thing
new consumer( new mockService())