How to Deserialize a List Using Gson or Another JSON Library in Java

How to deserialize a list using GSON or another JSON library in Java?

With Gson, you'd just need to do something like:

List<Video> videos = gson.fromJson(json, new TypeToken<List<Video>>(){}.getType());

You might also need to provide a no-arg constructor on the Video class you're deserializing to.

Deserialize a ListT object with Gson?

Method to deserialize generic collection:

import java.lang.reflect.Type;
import com.google.gson.reflect.TypeToken;

...

Type listType = new TypeToken<ArrayList<YourClass>>(){}.getType();
List<YourClass> yourClassList = new Gson().fromJson(jsonArray, listType);

Since several people in the comments have mentioned it, here's an explanation of how the TypeToken class is being used. The construction new TypeToken<...>() {}.getType() captures a compile-time type (between the < and >) into a runtime java.lang.reflect.Type object. Unlike a Class object, which can only represent a raw (erased) type, the Type object can represent any type in the Java language, including a parameterized instantiation of a generic type.

The TypeToken class itself does not have a public constructor, because you're not supposed to construct it directly. Instead, you always construct an anonymous subclass (hence the {}, which is a necessary part of this expression).

Due to type erasure, the TypeToken class is only able to capture types that are fully known at compile time. (That is, you can't do new TypeToken<List<T>>() {}.getType() for a type parameter T.)

For more information, see the documentation for the TypeToken class.

Deserializing list with GSON

Try this -

AllEntity.java

import com.google.gson.annotations.Expose;
import com.google.gson.annotations.SerializedName;

public class AllEntity {
    @SerializedName("kbIdentifier")
    @Expose
    private String kbIdentifier;
    @SerializedName("disambiguationScore")
    @Expose
    private String disambiguationScore;
    public String getKbIdentifier() {
        return kbIdentifier;
    }
    public void setKbIdentifier(String kbIdentifier) {
        this.kbIdentifier = kbIdentifier;
    }
    public String getDisambiguationScore() {
        return disambiguationScore;
    }
    public void setDisambiguationScore(String disambiguationScore) {
        this.disambiguationScore = disambiguationScore;
    }
    @Override
    public String toString() {
        return "AllEntity [kbIdentifier=" + kbIdentifier
                + ", disambiguationScore=" + disambiguationScore + "]";
    }
}

BestEntity.java

import com.google.gson.annotations.Expose;
import com.google.gson.annotations.SerializedName;

public class BestEntity {
    @SerializedName("kbIdentifier")
    @Expose
    private String kbIdentifier;
    @SerializedName("disambiguationScore")
    @Expose
    private String disambiguationScore;
    public String getKbIdentifier() {
        return kbIdentifier;
    }
    public void setKbIdentifier(String kbIdentifier) {
        this.kbIdentifier = kbIdentifier;
    }
    public String getDisambiguationScore() {
        return disambiguationScore;
    }
    public void setDisambiguationScore(String disambiguationScore) {
        this.disambiguationScore = disambiguationScore;
    }
    @Override
    public String toString() {
        return "BestEntity [kbIdentifier=" + kbIdentifier
                + ", disambiguationScore=" + disambiguationScore + "]";
    }
}

Mention.java

import java.util.ArrayList;
import java.util.List;

import com.google.gson.annotations.Expose;
import com.google.gson.annotations.SerializedName;

public class Mention {
    @SerializedName("allEntities")
    @Expose
    private List<AllEntity> allEntities = new ArrayList<AllEntity>();
    @SerializedName("name")
    @Expose
    private String name;
    @SerializedName("bestEntity")
    @Expose
    private BestEntity bestEntity;
    public List<AllEntity> getAllEntities() {
        return allEntities;
    }
    public void setAllEntities(List<AllEntity> allEntities) {
        this.allEntities = allEntities;
    }
    public String getName() {
        return name;
    }
    public void setName(String name) {
        this.name = name;
    }
    public BestEntity getBestEntity() {
        return bestEntity;
    }
    public void setBestEntity(BestEntity bestEntity) {
        this.bestEntity = bestEntity;
    }
    @Override
    public String toString() {
        return "Mention [allEntities=" + allEntities + ", name=" + name
                + ", bestEntity=" + bestEntity + "]";
    }
}

Main.java

import com.example.ElemntList;
import com.google.gson.Gson;
import com.google.gson.GsonBuilder;

public class Main {
    private static Gson gson;

    static {
        gson = new GsonBuilder().create();
    }

    /**
     * @param args
     */
    public static void main(String[] args) {
        String s = "{\"mentions\":[{\"allEntities\":[{\"kbIdentifier\":\"YAGO:Bob_Dylan\",\"disambiguationScore\":\"0.63692\"}],\"name\":\"Dylan\",\"bestEntity\":{\"kbIdentifier\":\"YAGO:Bob_Dylan\",\"disambiguationScore\":\"0.63692\"}},{\"name\":\"Duluth\",\"bestEntity\":{\"kbIdentifier\":\"YAGO:Duluth\\u002c_Minnesota\",\"disambiguationScore\":\"0.63149\"}}]}";
        ElemntList info = gson.fromJson(s, ElemntList.class);
        System.out.println(info);
   }
}

Result is -

ElemntList [mentions=[Mention [allEntities=[AllEntity [kbIdentifier=YAGO:Bob_Dylan, disambiguationScore=0.63692]], name=Dylan, bestEntity=BestEntity [kbIdentifier=YAGO:Bob_Dylan, disambiguationScore=0.63692]], Mention [allEntities=[], name=Duluth, bestEntity=BestEntity [kbIdentifier=YAGO:Duluth,_Minnesota, disambiguationScore=0.63149]]]]

Unable to deserialize the list of object data using GSON library

1. Why JsonProperty instead of SerializableName? Are you mixing Jackson and Gson?
2. What is the output of ser.getKeywordStats()?

Because I have tested your code by hardcoding that json-string instead of ser.getKeywordStats(), and it worked without any issue.

Deserialise a generic list in Gson

There is no way to do it without passing actual type of T (as Class<T>) to your method.

But if you pass it explicitly, you can create a TypeToken for List<T> as follows:

private <T> List<T> GetListFromFile(String filename, Class<T> elementType) {
    ...
    TypeToken<ArrayList<T>> token = new TypeToken<ArrayList<T>>() {};
    List<T> something = gson.fromJson(data, token.getType());
    ...
}

See also:

• TypeToken

How to deserialize JSON string between JSON.org library using GSON library

Just make keywords and countries a java List type. I've never seen org.json mixed with gson. Usually gson replaces org.json it's not meant to be used together.

EDIT:

Small example:

class Example {
   private String name;
   private Integer age;
   private List<String> keywords;
   private List<String> countries;

   public String toString() {
       return new Gson().toJson(this);
   }
}

Trouble deserializing JSON into Java objects with GSON

The JSON you are deserializing represents an object with a list of objects on it. The Java object you are trying to deserialize to needs to match that.

First, create a new class MovieList.

class MovieList {
     List<Movie> movies;
}

Update your Movies class to be called Movie, since it represents a single movie.

class Movie {
     String name;
     String url;
     String IMAX;
     String rating;
     List<Cast> cast;
}

Now try calling gson.fromJson(...) with the following

MovieList movieList = gson.fromJson(new FileReader("src/main/input.json"), MovieList.class);
System.out.println(movieList.getMovies().get(0));

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