VatChecker.java

public class VatChecker {
    static VatCheckResponse checkVat(String vat) {
        return null;
    }
}

class VatCheckResponse {
    private VatCheckResponse() {}

    boolean isValid() {
        return true;
    }
}

VatCheckerTest.java

import org.junit.jupiter.api.Test;
import org.mockito.MockedStatic;
import org.mockito.Mockito;

import static org.assertj.core.api.Assertions.assertThat;
import static org.mockito.ArgumentMatchers.any;

class VatCheckerTests {

    @Test
    void foo() {
        try (MockedStatic<VatChecker> vatChecker = Mockito.mockStatic(VatChecker.class, Mockito.RETURNS_DEEP_STUBS)) {
            vatChecker.when(() -> VatChecker.checkVat(any()).isValid()).thenReturn(false);
            assertThat(VatChecker.checkVat("asd").isValid()).isFalse();
        }
    }
}