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/main/java/com/baeldung/gson/multiplefields/BasicStudent.java

@@ -0,0 +1,64 @@
+package com.baeldung.gson.multiplefields;
+
+import java.util.Objects;
+
+import com.google.gson.annotations.SerializedName;
+
+public class BasicStudent {
+
+    private String name;
+
+    private transient String major;
+
+    @SerializedName("major")
+    private String concentration;
+
+    public BasicStudent() {
+
+    }
+
+    public BasicStudent(String name, String major, String concentration) {
+        this.name = name;
+        this.major = major;
+        this.concentration = concentration;
+    }
+
+    @Override
+    public boolean equals(Object o) {
+        if (this == o)
+            return true;
+        if (!(o instanceof BasicStudent))
+            return false;
+        BasicStudent student = (BasicStudent) o;
+        return Objects.equals(name, student.name) && Objects.equals(major, student.major) && Objects.equals(concentration, student.concentration);
+    }
+
+    @Override
+    public int hashCode() {
+        return Objects.hash(name, major, concentration);
+    }
+
+    public String getName() {
+        return name;
+    }
+
+    public void setName(String name) {
+        this.name = name;
+    }
+
+    public String getMajor() {
+        return major;
+    }
+
+    public void setMajor(String major) {
+        this.major = major;
+    }
+
+    public String getConcentration() {
+        return concentration;
+    }
+
+    public void setConcentration(String concentration) {
+        this.concentration = concentration;
+    }
+}

json-modules/gson-2/src/main/java/com/baeldung/gson/multiplefields/StudentExclusionStrategy.java

@@ -0,0 +1,18 @@
+package com.baeldung.gson.multiplefields;
+
+import com.google.gson.ExclusionStrategy;
+import com.google.gson.FieldAttributes;
+
+public class StudentExclusionStrategy implements ExclusionStrategy {
+
+    @Override
+    public boolean shouldSkipField(FieldAttributes field) {
+        // Ignore all field values from V1 type
+        return field.getDeclaringClass() == StudentV1.class;
+    }
+
+    @Override
+    public boolean shouldSkipClass(Class<?> aClass) {
+        return false;
+    }
+}

json-modules/gson-2/src/main/java/com/baeldung/gson/multiplefields/StudentV1.java

@@ -0,0 +1,34 @@
+package com.baeldung.gson.multiplefields;
+
+public class StudentV1 {
+
+    private String firstName;
+    private String lastName;
+
+    public String getFirstName() {
+        return firstName;
+    }
+
+    public void setFirstName(String firstName) {
+        this.firstName = firstName;
+    }
+
+    public String getLastName() {
+        return lastName;
+    }
+
+    public void setLastName(String lastName) {
+        this.lastName = lastName;
+    }
+
+    // Default constructor for Gson
+    public StudentV1() {
+
+    }
+
+    public StudentV1(String firstName, String lastName) {
+        this.firstName = firstName;
+        this.lastName = lastName;
+    }
+
+}

json-modules/gson-2/src/main/java/com/baeldung/gson/multiplefields/StudentV2.java

@@ -0,0 +1,71 @@
+package com.baeldung.gson.multiplefields;
+
+import java.util.Objects;
+
+import com.google.gson.annotations.Expose;
+
+public class StudentV2 extends StudentV1 {
+
+    @Expose
+    private String firstName;
+
+    @Expose
+    private String lastName;
+
+    @Expose
+    private String major;
+
+    @Override
+    public String getFirstName() {
+        return firstName;
+    }
+
+    @Override
+    public void setFirstName(String firstName) {
+        this.firstName = firstName;
+    }
+
+    @Override
+    public String getLastName() {
+        return lastName;
+    }
+
+    @Override
+    public void setLastName(String lastName) {
+        this.lastName = lastName;
+    }
+
+    public String getMajor() {
+        return major;
+    }
+
+    public void setMajor(String major) {
+        this.major = major;
+    }
+
+    // Default constructor for Gson
+    public StudentV2() {
+
+    }
+
+    public StudentV2(String firstName, String lastName, String major) {
+        this.firstName = firstName;
+        this.lastName = lastName;
+        this.major = major;
+    }
+
+    @Override
+    public boolean equals(Object o) {
+        if (this == o)
+            return true;
+        if (!(o instanceof StudentV2))
+            return false;
+        StudentV2 studentV2 = (StudentV2) o;
+        return Objects.equals(firstName, studentV2.firstName) && Objects.equals(lastName, studentV2.lastName) && Objects.equals(major, studentV2.major);
+    }
+
+    @Override
+    public int hashCode() {
+        return Objects.hash(firstName, lastName, major);
+    }
+}

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);
+    }
+
+}