How to deserialize custom Map with GSON

Question

I am using Apache Collection's BidiMap which offers DualHashBidiMap class. I have to use this class for the project.

There is no problem with serialization with it. But I have problem with deserializition!.

Here is a sample class :

package com.description;

import org.apache.commons.collections4.BidiMap;
import org.apache.commons.collections4.bidimap.DualHashBidiMap;

public class Sample {

    private String id;
    private String adress;

    BidiMap<Integer, String> items = new DualHashBidiMap<Integer, String>();


    public Sample() {
    }

    public String getId() {
        return id;
    }

    public void setId(String id) {
        this.id = id;
    }

    public String getAdress() {
        return adress;
    }

    public void setAdress(String adress) {
        this.adress = adress;
    }

    public BidiMap<Integer, String> getItems() {
        return items;
    }

    public void setItems(BidiMap<Integer, String> items) {
        this.items = items;
    }
}

And main method

Sample sample = new Sample();
        sample.setId("12312xoa01");
        sample.setAdress("Houston, 43.1");
        BidiMap<Integer, String> items = new DualHashBidiMap<Integer, String>();
        items.put(1,  "gloves");
        items.put(90, "boots");
        sample.setItems(items);

        try {
            String result =  gson.toJson(sample);
            System.out.println("result : "+result);
            Sample sample2 = gson.fromJson(result, Sample.class);
            System.out.println("address : "+sample2.getAdress());

        }catch (Exception e){
            e.printStackTrace();
        }

**

---

**

result : {"id":"12312xoa01","adress":"Houston, 43.1","items":{"1":"gloves","90":"boots"}}
java.lang.IllegalArgumentException: Can not set org.apache.commons.collections4.BidiMap field com.description.Sample.items to java.util.LinkedHashMap
    at sun.reflect.UnsafeFieldAccessorImpl.throwSetIllegalArgumentException(UnsafeFieldAccessorImpl.java:164)
    at sun.reflect.UnsafeFieldAccessorImpl.throwSetIllegalArgumentException(UnsafeFieldAccessorImpl.java:168)
    at sun.reflect.UnsafeObjectFieldAccessorImpl.set(UnsafeObjectFieldAccessorImpl.java:81)
    at java.lang.reflect.Field.set(Field.java:741)
    at com.google.gson.internal.bind.ReflectiveTypeAdapterFactory$1.read(ReflectiveTypeAdapterFactory.java:118)
    at com.google.gson.internal.bind.ReflectiveTypeAdapterFactory$Adapter.read(ReflectiveTypeAdapterFactory.java:216)
    at com.google.gson.Gson.fromJson(Gson.java:879)
    at com.google.gson.Gson.fromJson(Gson.java:844)
    at com.google.gson.Gson.fromJson(Gson.java:793)
    at com.google.gson.Gson.fromJson(Gson.java:765)
    at Main.main(Main.java:79)
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:57)
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
    at java.lang.reflect.Method.invoke(Method.java:606)
    at com.intellij.rt.execution.application.AppMain.main(AppMain.java:144)

Answer 1

That is not that straightforward, but can be done:

public class BidiMapTypeAdapterFactory implements TypeAdapterFactory {

    @Override
    public <T> TypeAdapter<T> create(Gson gson, TypeToken<T> type) {
        if (!BidiMap.class.isAssignableFrom(type.getRawType())) {
            return null;
        }
        final TypeAdapter<T> delegate = gson.getDelegateAdapter(this, type);
        return new TypeAdapter<T>() {
            @Override
            public void write(JsonWriter out, T value) throws IOException {
                delegate.write(out, value);
            }

            @Override
            public T read(JsonReader in) throws IOException {
                return (T) new DualHashBidiMap<>((Map) delegate.read(in));
            }
        };
    }
}

Register the TypeAdapterFactory:

GsonBuilder b = new GsonBuilder().registerTypeAdapterFactory(new BidiMapTypeAdapterFactory());
Gson gson = b.create();

Now you can run your sample and it should work.