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java – Gson turn an array of data objects into json – Android

Posted by: admin April 23, 2020 Leave a comment

Questions:

Currently I am working on a native android app with webView front end.

I have something like:

public class dataObject
{
  int a;
  String b;
}

and in activity,

I have made an array of dataObject, say dataObject x[5];

Now i want to pass these 5 dataObject to my javascript webView interface as JSON in a callback function.

I looked through the internet, seems like most tutorials talk about how to convert fromJson(). There are not a lot about toJson(). I found one that taught me that dataObject.toJson(), would work.

But how can I pass all 5 dataObjects?

How to&Answers:

Here’s a comprehensive example on how to use Gson with a list of objects. This should demonstrate exactly how to convert to/from Json, how to reference lists, etc.

Test.java:

import com.google.gson.Gson;
import java.util.List;
import java.util.ArrayList;
import com.google.gson.reflect.TypeToken;
import java.lang.reflect.Type;


public class Test {

  public static void main (String[] args) {

    // Initialize a list of type DataObject
    List<DataObject> objList = new ArrayList<DataObject>();
    objList.add(new DataObject(0, "zero"));
    objList.add(new DataObject(1, "one"));
    objList.add(new DataObject(2, "two"));

    // Convert the object to a JSON string
    String json = new Gson().toJson(objList);
    System.out.println(json);

    // Now convert the JSON string back to your java object
    Type type = new TypeToken<List<DataObject>>(){}.getType();
    List<DataObject> inpList = new Gson().fromJson(json, type);
    for (int i=0;i<inpList.size();i++) {
      DataObject x = inpList.get(i);
      System.out.println(x);
    }

  }


  private static class DataObject {
    private int a;
    private String b;

    public DataObject(int a, String b) {
      this.a = a;
      this.b = b;
    }

    public String toString() {
      return "a = " +a+ ", b = " +b;
    }
  }

}

To compile it:

javac -cp "gson-2.1.jar:." Test.java

And finally to run it:

java -cp "gson-2.1.jar:." Test

Note that if you’re using Windows, you’ll have to switch : with ; in the previous two commands.

After you run it, you should see the following output:

[{"a":0,"b":"zero"},{"a":1,"b":"one"},{"a":2,"b":"two"}]
a = 0, b = zero
a = 1, b = one
a = 2, b = two

Keep in mind that this is only a command line program to demonstrate how it works, but the same principles apply within the Android environment (referencing jar libs, etc.)

Answer:

My version of gson list deserialization using a helper class:

public List<E> getList(Class<E> type, JSONArray json) throws Exception {
    Gson gsonB = new GsonBuilder().setDateFormat("yyyy-MM-dd HH:mm:ss").create();

    return gsonB.fromJson(json.toString(), new JsonListHelper<E>(type));
}



public class JsonListHelper<T> implements ParameterizedType {

  private Class<?> wrapped;

  public JsonListHelper(Class<T> wrapped) {
    this.wrapped = wrapped;
  }

  public Type[] getActualTypeArguments() {
    return new Type[] {wrapped};
  }

  public Type getRawType() {
    return List.class;
  }

  public Type getOwnerType() {
    return null;
  }

}

Usage

List<Object> objects = getList(Object.class, myJsonArray);