Version: 1.2

The following code returns an invalid JSON string:

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

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

public class App {
    public static void main(String[] args) {
        Node root = new Node("root");
        root.createChild("Pete");

        Gson gson = new GsonBuilder()
                .setPrettyPrinting()
                .create();

        System.out.println(gson.toJson(root));
    }
}

class Node {
    private final String name;
    private final List<Node> children = new ArrayList<Node>();

    public Node(String name) {
        this.name = name;
    }

    public Node createChild(String childName) {
        Node child = new Node(childName);
        children.add(child);
        return child;
    }
}