add missing Validate.java + Test

src/main/java/net/hostsharing/hsadminng/errors/Validate.java

@@ -0,0 +1,23 @@
+package net.hostsharing.hsadminng.errors;
+
+import lombok.AllArgsConstructor;
+
+import jakarta.validation.ValidationException;
+
+@AllArgsConstructor
+public class Validate {
+
+    final String variableNames;
+
+    public static Validate validate(final String variableNames) {
+        return new Validate(variableNames);
+    }
+
+    public final void atMaxOneNonNull(final Object var1, final Object var2) {
+        if (var1 != null && var2 != null) {
+            throw new ValidationException(
+                    "Exactly one of (" + variableNames + ") must be non-null, " +
+                            "but are (" + var1 + ", " + var2 + ")");
+        }
+    }
+}

src/test/java/net/hostsharing/hsadminng/errors/ValidateUnitTest.java

@@ -0,0 +1,31 @@
+package net.hostsharing.hsadminng.errors;
+
+import org.junit.jupiter.api.Test;
+
+import jakarta.validation.ValidationException;
+
+import static org.assertj.core.api.Assertions.assertThat;
+import static org.assertj.core.api.Assertions.catchThrowable;
+
+class ValidateUnitTest {
+
+    @Test
+    void shouldFailValidationIfBothParametersAreNotNull() {
+        final var throwable = catchThrowable(() ->
+            Validate.validate("var1, var2").atMaxOneNonNull("val1", "val2")
+        );
+        assertThat(throwable).isInstanceOf(ValidationException.class)
+                .hasMessage("Exactly one of (var1, var2) must be non-null, but are (val1, val2)");
+    }
+
+    @Test
+    void shouldNotFailValidationIfBothParametersAreull() {
+        Validate.validate("var1, var2").atMaxOneNonNull(null, null);
+    }
+
+    @Test
+    void shouldNotFailValidationIfExactlyOneParameterIsNonNull() {
+        Validate.validate("var1, var2").atMaxOneNonNull("val1", null);
+        Validate.validate("var1, var2").atMaxOneNonNull(null, "val2");
+    }
+}