gson advanced list examples

gson/src/main/java/org/baeldung/gson/entities/Animal.java

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+package org.baeldung.gson.entities;
+
+public abstract class Animal {
+    public String type = "Animal";
+}

gson/src/main/java/org/baeldung/gson/entities/Cow.java

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+package org.baeldung.gson.entities;
+
+public class Cow extends Animal {
+    private String breed;
+
+    public Cow() {
+        breed = "Jersey";
+        type = "Cow";
+    }
+
+    public String getBreed() {
+        return breed;
+    }
+
+    public void setBreed(String breed) {
+        this.breed = breed;
+    }
+}
+

gson/src/main/java/org/baeldung/gson/entities/Dog.java

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+package org.baeldung.gson.entities;
+
+public class Dog extends Animal {
+    private String petName;
+
+    public Dog() {
+        petName = "Milo";
+        type = "Dog";
+    }
+
+    public String getPetName() {
+        return petName;
+    }
+
+    public void setPetName(String petName) {
+        this.petName = petName;
+    }
+}

gson/src/main/java/org/baeldung/gson/entities/MyClass.java

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+package org.baeldung.gson.entities;
+
+public class MyClass {
+    private int id;
+    private String[] strings;
+
+    public MyClass() {
+        id = 1;
+        strings = new String[] { "a", "b" };
+    }
+
+
+    //  @Override
+    //  public String toString() {
+    //    return "{" +
+    //        "id=" + id +
+    //        ", name='" + name + '\'' +
+    //        ", strings=" + Arrays.toString(strings) +
+    //        '}';
+    //  }
+
+    public int getId() {
+        return id;
+    }
+
+    public void setId(int id) {
+        this.id = id;
+    }
+
+    public String[] getStrings() {
+        return strings;
+    }
+
+    public void setStrings(String[] strings) {
+        this.strings = strings;
+    }
+}

gson/src/main/java/org/baeldung/gson/serialization/AnimalDeserializer.java

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+package org.baeldung.gson.serialization;
+
+import com.google.gson.Gson;
+import com.google.gson.JsonDeserializationContext;
+import com.google.gson.JsonDeserializer;
+import com.google.gson.JsonElement;
+import com.google.gson.JsonObject;
+import java.lang.reflect.Type;
+import java.util.HashMap;
+import java.util.Map;
+import org.baeldung.gson.entities.Animal;
+
+public class AnimalDeserializer implements JsonDeserializer<Animal> {
+    private String animalTypeElementName;
+    private Gson gson;
+    private Map<String, Class<? extends Animal>> animalTypeRegistry;
+
+    public AnimalDeserializer(String animalTypeElementName) {
+        this.animalTypeElementName = animalTypeElementName;
+        this.gson = new Gson();
+        this.animalTypeRegistry = new HashMap<>();
+    }
+
+    public void registerBarnType(String animalTypeName, Class<? extends Animal> animalType) {
+        animalTypeRegistry.put(animalTypeName, animalType);
+    }
+
+    public Animal deserialize(JsonElement json, Type typeOfT, JsonDeserializationContext context) {
+        JsonObject animalObject = json.getAsJsonObject();
+        JsonElement animalTypeElement = animalObject.get(animalTypeElementName);
+
+        Class<? extends Animal> animalType = animalTypeRegistry.get(animalTypeElement.getAsString());
+        return gson.fromJson(animalObject, animalType);
+    }
+}

gson/src/test/java/org/baeldung/gson/advance/GsonAdvanceUnitTest.java

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+package org.baeldung.gson.advance;
+
+import static org.junit.Assert.assertEquals;
+import static org.junit.Assert.assertTrue;
+
+import com.google.gson.Gson;
+import com.google.gson.GsonBuilder;
+import com.google.gson.reflect.TypeToken;
+import java.lang.reflect.Type;
+import java.util.ArrayList;
+import java.util.List;
+import org.baeldung.gson.entities.Animal;
+import org.baeldung.gson.entities.Cow;
+import org.baeldung.gson.entities.Dog;
+import org.baeldung.gson.entities.MyClass;
+import org.baeldung.gson.serialization.AnimalDeserializer;
+import org.junit.Test;
+
+public class GsonAdvanceUnitTest {
+
+    @Test public void givenListOfMyClass_whenSerializing_thenCorrect() {
+        List<MyClass> list = new ArrayList<>();
+        list.add(new MyClass());
+        list.add(new MyClass());
+
+        Gson gson = new Gson();
+        String jsonString = gson.toJson(list);
+        String expectedString = "[{\"id\":1,\"strings\":[\"a\",\"b\"]},{\"id\":1,\"strings\":[\"a\",\"b\"]}]";
+
+        assertEquals(expectedString, jsonString);
+    }
+
+    @Test(expected = ClassCastException.class)
+    public void givenJsonString_whenIncorrectDeserializing_thenThrowClassCastException() {
+        String inputString = "[{\"id\":1,\"strings\":[\"a\",\"b\"]},{\"id\":1,\"strings\":[\"a\",\"b\"]}]";
+
+        Gson gson = new Gson();
+        List<MyClass> list = gson.fromJson(inputString, ArrayList.class);
+
+        assertEquals(2, list.size());
+        int id = list.get(0).getId();
+    }
+
+    @Test public void givenJsonString_whenDeserializing_thenReturnListOfMyClass() {
+        String inputString = "[{\"id\":1,\"strings\":[\"a\",\"b\"]},{\"id\":1,\"strings\":[\"a\",\"b\"]}]";
+        Type listOfMyClassObject = new TypeToken<ArrayList<MyClass>>() {}.getType();
+
+        Gson gson = new Gson();
+        List<MyClass> list = gson.fromJson(inputString, listOfMyClassObject);
+
+        assertEquals(2, list.size());
+        assertEquals(1, list.get(0).getId());
+    }
+
+    @Test public void givenPolymorphicList_whenSerializeWithTypeAdapter_thenCorrect() {
+        String expectedString = "[{\"petName\":\"Milo\",\"type\":\"Dog\"},{\"breed\":\"Jersey\",\"type\":\"Cow\"}]";
+
+        List<Animal> inList = new ArrayList<>();
+        inList.add(new Dog());
+        inList.add(new Cow());
+
+        String jsonString = new Gson().toJson(inList);
+
+        assertEquals(expectedString, jsonString);
+    }
+
+    @Test public void givenPolymorphicList_whenDeserializeWithTypeAdapter_thenCorrect() {
+        String inputString = "[{\"petName\":\"Milo\",\"type\":\"Dog\"},{\"breed\":\"Jersey\",\"type\":\"Cow\"}]";
+
+        AnimalDeserializer deserializer = new AnimalDeserializer("type");
+        deserializer.registerBarnType("Dog", Dog.class);
+        deserializer.registerBarnType("Cow", Cow.class);
+        Gson gson = new GsonBuilder()
+          .registerTypeAdapter(Animal.class, deserializer)
+          .create();
+
+        List<Animal> outList = gson.fromJson(inputString, new TypeToken<List<Animal>>(){}.getType());
+
+        assertEquals(2, outList.size());
+        assertTrue(outList.get(0) instanceof Dog);
+    }
+
+    @Test public void givenPolymorphicList_whenSerializeWithRuntimeTypeAdapter_thenCorrect() {
+        String expectedString = "[{\"petName\":\"Milo\",\"type\":\"Dog\"},{\"breed\":\"Jersey\",\"type\":\"Cow\"}]";
+
+        List<Animal> inList = new ArrayList<>();
+        inList.add(new Dog());
+        inList.add(new Cow());
+        String jsonString = new Gson().toJson(inList);
+
+        assertEquals(expectedString, jsonString);
+    }
+
+    @Test public void givenPolymorphicList_whenDeserializeWithRuntimeTypeAdapter_thenCorrect() {
+        String inputString = "[{\"petName\":\"Milo\",\"type\":\"Dog\"},{\"breed\":\"Jersey\",\"type\":\"Cow\"}]";
+
+        Type listOfAnimals = new TypeToken<ArrayList<Animal>>() {}.getType();
+
+        RuntimeTypeAdapterFactory<Animal> adapter = RuntimeTypeAdapterFactory.of(Animal.class)
+          .registerSubtype(Dog.class)
+          .registerSubtype(Cow.class);
+
+        Gson gson = new GsonBuilder().registerTypeAdapterFactory(adapter).create();
+
+        List<Animal> outList = gson.fromJson(inputString, listOfAnimals);
+
+        assertEquals(2, outList.size());
+        assertTrue(outList.get(0) instanceof Dog);
+    }
+}