extract OptionalFromJson for JSON mapper bug

src/test/java/net/hostsharing/hsadminng/OptionalFromJsonUnitTest.java

@@ -0,0 +1,59 @@
+package net.hostsharing.hsadminng;
+
+import org.junit.jupiter.api.Test;
+import org.openapitools.jackson.nullable.JsonNullable;
+
+import static org.assertj.core.api.Assertions.assertThat;
+
+class OptionalFromJsonUnitTest {
+
+    private String value = "unchanged initial value";
+
+    @Test
+    void shouldHandleActualValue() {
+        // given
+        final JsonNullable<String> given = JsonNullable.of("actual new value");
+
+        // when
+        OptionalFromJson.of(given).ifPresent(valueFromJson -> value = valueFromJson);
+
+        // then
+        assertThat(value).isEqualTo("actual new value");
+    }
+
+    @Test
+    void shouldHandleNullValue() {
+        // given
+        final JsonNullable<String> given = JsonNullable.of(null);
+
+        // when
+        OptionalFromJson.of(given).ifPresent(valueFromJson -> value = valueFromJson);
+
+        // then
+        assertThat(value).isNull();
+    }
+
+    @Test
+    void shouldHandleUndefinedValue() {
+        // given - what should have happen in the JSON mapper
+        final JsonNullable<String> given = JsonNullable.undefined();
+
+        // when
+        OptionalFromJson.of(given).ifPresent(valueFromJson -> value = valueFromJson);
+
+        // then
+        assertThat(value).isEqualTo("unchanged initial value");
+    }
+
+    @Test
+    void shouldHandleInvalidNullJsonNullable() {
+        // given - what seems to happen in the JSON mapper
+        final JsonNullable<String> given = null;
+
+        // when
+        OptionalFromJson.of(given).ifPresent(valueFromJson -> value = valueFromJson);
+
+        // then
+        assertThat(value).isEqualTo("unchanged initial value");
+    }
+}