BAEL-7083: Gson class declares multiple fields (#15244)

* Add initial classes and tests for Gson multiple fields error

* Add test classes and exclusion strategy

* Add test cases

* Simplify example code

json-modules/gson-2/src/test/java/com/baeldung/gson/multiplefields/GsonMultipleFieldsUnitTest.java

@@ -0,0 +1,65 @@
+package com.baeldung.gson.multiplefields;
+
+import static org.assertj.core.api.Assertions.assertThat;
+import static org.assertj.core.api.Assertions.assertThatThrownBy;
+
+import org.junit.Test;
+
+import com.google.gson.Gson;
+import com.google.gson.GsonBuilder;
+
+public class GsonMultipleFieldsUnitTest {
+
+    @Test
+    public void givenBasicStudent_whenSerializingWithGson_thenTransientFieldNotSet() {
+        // Given a class with a transient field
+        BasicStudent student = new BasicStudent("Henry Winter", "Greek Studies", "Classical Greek Studies");
+
+        // When serializing using Gson
+        Gson gson = new Gson();
+        String json = gson.toJson(student);
+
+        // Then the deserialized instance doesn't contain the transient field value
+        BasicStudent deserialized = gson.fromJson(json, BasicStudent.class);
+        assertThat(deserialized.getMajor()).isNull();
+    }
+
+    @Test
+    public void givenStudentV2_whenSerializingWithGson_thenIllegalArgumentExceptionIsThrown() {
+        // Given a class with a class hierarchy that defines multiple fields with the same name
+        StudentV2 student = new StudentV2("Henry", "Winter", "Greek Studies");
+
+        // When serializing using Gson, then an IllegalArgumentException exception is thrown
+        Gson gson = new Gson();
+        assertThatThrownBy(() -> gson.toJson(student)).isInstanceOf(IllegalArgumentException.class)
+            .hasMessageContaining("declares multiple JSON fields named 'firstName'");
+    }
+
+    @Test
+    public void givenStudentV2_whenSerializingWithGsonExposeAnnotation_thenSerializes() {
+        // Given a class with a class hierarchy that defines multiple fields with the same name
+        StudentV2 student = new StudentV2("Henry", "Winter", "Greek Studies");
+
+        // When serializing using Gson exclude fields without @Expose
+        Gson gson = new GsonBuilder().excludeFieldsWithoutExposeAnnotation()
+            .create();
+
+        // Then ensure the student can be serialized, then deserialized back into an equal instance
+        String json = gson.toJson(student);
+        assertThat(gson.fromJson(json, StudentV2.class)).isEqualTo(student);
+    }
+
+    @Test
+    public void givenStudentV2_whenSerializingWithGsonExclusionStrategy_thenSerializes() {
+        // Given a class with a class hierarchy that defines multiple fields with the same name
+        StudentV2 student = new StudentV2("Henry", "Winter", "Greek Studies");
+
+        // When serializing using Gson add an ExclusionStrategy
+        Gson gson = new GsonBuilder().setExclusionStrategies(new StudentExclusionStrategy())
+            .create();
+
+        // Then ensure the student can be serialized, then deserialized back into an equal instance
+        assertThat(gson.fromJson(gson.toJson(student), StudentV2.class)).isEqualTo(student);
+    }
+
+}