GSON -- convert java object to json

Library:

• gson (2.2.4)

<dependency>
    <groupId>com.google.code.gson</groupId>
    <artifactId>gson</artifactId>
    <version>2.2.4</version>
</dependency>

( 1 ) Simple Java “POJO”/JSON

package com.hmkcode.vo;
 
import java.util.LinkedList;
import java.util.List;
 
public class Article {
 
    private String title;
    private String url;
    private List<String> categories;
    private List<String> tags;
     
//getters & setters...
     
}

 Convert Java to JSON & JSON to Java

package com.hmkcode;
 
import com.google.gson.Gson;
import com.hmkcode.vo.Article;
 
 
public class App 
{
    public static void main( String[] args )
    {
        Gson gson = new Gson();
        // Java --> JSON
        String json = gson.toJson(createArticle());
        System.out.println("toJson: "+json);
         
        // JSON --> Java
        Article article = gson.fromJson(json, Article.class);
        System.out.println("fromJson: "+article);
         
         
 
    }
    private static Article createArticle(){
         
        Article article = new Article();
         
        article.setTitle("GSON - Java JSON Library");
        article.setUrl("http://hmkcode.com/gson-json-java");
        article.addCategory("Java");
        article.addTag("Java");
        article.addTag("GSON");
        article.addTag("JSON");
         
        return article;
    }
}

( 2 ) Dealing with Collection<E>

Gson gson = new Gson();
         
List<Article> articles = new LinkedList<Article>();
         
articles.add(createArticle());
articles.add(createArticle());
         
// Java --> JSON
String json = gson.toJson(articles);

 However, when converting this list back “from JSON to Java” we need to be a little bit careful. Converting JSON to Java using the simple way, will work but it will not give us List<Article> it will give us List<Map>.

// JSON --> Java
List list = gson.fromJson(json, List.class);
System.out.println("Class Type: "+list.get(0).getClass());

 OutPut:

Class Type: class com.google.gson.internal.LinkedTreeMap

To get List<Article> we need to do one more step.

// JSON --> Java "Get the actual type"
Type type = new TypeToken<List<Article>>(){}.getType();
List list = gson.fromJson(json, type);
System.out.println("Class Type: "+list.get(0).getClass());

 Output:

Class Type: class com.hmkcode.vo.Article

complete code:

package com.hmkcode;
 
import java.lang.reflect.Type;
import java.util.LinkedList;
import java.util.List;
 
import com.google.gson.Gson;
import com.google.gson.reflect.TypeToken;
import com.hmkcode.vo.Article;
 
 
public class App 
{
    public static void main( String[] args )
    {
        Gson gson = new Gson();
         
        List<Article> articles = new LinkedList<Article>();
         
        articles.add(createArticle());
        articles.add(createArticle());
         
        // Java --> JSON
        String json = gson.toJson(articles);
        System.out.println("toJson: "+json);
         
         
        // JSON --> Java
        List list = gson.fromJson(json, List.class);
        System.out.println("fromJson: "+list);
        System.out.println("Class Type: "+list.get(0).getClass());
 
        // JSON --> Java "Get the actual type"
        Type type = new TypeToken<List<Article>>(){}.getType();
        list = gson.fromJson(json, type);
        System.out.println("fromJson: "+list);
        System.out.println("Class Type: "+list.get(0).getClass());
 
 
    }
    private static Article createArticle(){
         
        Article article = new Article();
         
        article.setTitle("GSON - Java JSON Library");
        article.setUrl("http://hmkcode.com/gson-json-java");
        article.addCategory("Java");
        article.addTag("Java");
        article.addTag("GSON");
        article.addTag("JSON");
         
        return article;
    }
}