I am working on a Java project where I need to write unit tests with testng for a class that has a method similar to the following:

public boolean isValidCode(String code) {
    // Some logic here
    return service.exists(new CodeInput(code));
}

The isValidCode method calls a method exists from a service interface GenericService. The exists method in GenericService is responsible for checking the existence of a code. The issue I’m facing is that the exists method reaches out to an external service, and I do not have access to the endpoint in my development environment. Therefore, I’m looking for a way to mock the exists method without having the actual endpoint available.

Here’s the interface for GenericService:

@WebService(name = "GenericService", targetNamespace = "http://example.com/GenericService")
@SOAPBinding(parameterStyle = SOAPBinding.ParameterStyle.BARE)
@BindingType(javax.xml.ws.soap.SOAPBinding.SOAP12HTTP_BINDING)
@XmlSeeAlso({
    ObjectFactory.class
})
public interface GenericService {

    @WebMethod(operationName = "GenericExists", action = "http://example.com/GenericService/GenericExists")
    @WebResult(name = "GenericExistsResponse", targetNamespace = "http://example.com/GenericService", partName = "response")
    public GenericExists exists(
        @WebParam(name = "GenericExistsRequest", targetNamespace = "http://example.com/GenericService", partName = "CodeInput")
        CodeInput codeInput);
}

Is there a way I can use WireMock or any other mocking framework to mock the exists method without having access to the actual endpoint? Any suggestions or examples would be greatly appreciated.

I tried using Mockito to do the following:

CodeInput codeInput = mock(CodeInput.class);
GenericService serviceMock = mock(GenericService.class);
when(genericService.exists(any(CodeInput.class))).thenReturn(true);

But I get a null pointer exception when I do this.