How to write a unit test for a method that calls an external system so that no real call occurs?

To write a unit test for a method that interacts with an external system and to avoid actual calls, mocking is used. This means that instead of calling the real external system, a mock is created that returns predefined data.

In Java, libraries like Mockito or EasyMock are often used for this purpose. Here's an example with Mockito:

import static org.mockito.Mockito.*;
import org.junit.jupiter.api.Test;

class ExternalService {
    public String fetchData() {
        // Real call to external system
        return "real data";
    }
}

class MyService {
    private ExternalService externalService;

    public MyService(ExternalService externalService) {
        this.externalService = externalService;
    }

    public String process() {
        String data = externalService.fetchData();
        return "Processed " + data;
    }
}

public class MyServiceTest {
    @Test
    void testProcess() {
        ExternalService mockService = mock(ExternalService.class);
        when(mockService.fetchData()).thenReturn("mocked data");

        MyService myService = new MyService(mockService);
        String result = myService.process();

        assert result.equals("Processed mocked data");
    }
}

This approach allows isolating the code under test from external dependencies and testing the logic without network delays and external system instability.