Converter.java

import java.io.BufferedReader;
import java.io.FileReader;
import java.io.IOException;
import java.util.ArrayList;
import java.util.HashMap;
import java.util.List;
import java.util.Map;

import com.google.gson.GsonBuilder;

public class Converter {
    public static void main(String[] args) throws IOException {
        final Map<String, Object> holder = new HashMap<>();
        holder.put("hierarchy", fromCsvFile("mapping.csv"));

        System.out.println(new GsonBuilder().setPrettyPrinting().create().toJson(holder));
    }

    public static class Node {
        private String label;
        private List<Node> children;

        public String getLabel() {
            return label;
        }

        public void setLabel(String label) {
            this.label = label;
        }

        public List<Node> getChildren() {
            return children;
        }

        public void setChildren(List<Node> children) {
            this.children = children;
        }

        public void addChild(Node child) {
            if (children == null) {
                children = new ArrayList<>();
            }
            children.add(child);
        }
    }

    public static List<Node> fromCsvFile(String filename) throws IOException {
        Node root = new Node();
        Map<String, Node> existingNodes = new HashMap<>();

        try (BufferedReader input = new BufferedReader(new FileReader(filename))) {
            String parentName, childName;
            String[] entry;

            String line = input.readLine();
            while (line != null) {
                entry = line.split(",");
                parentName = entry[0].trim();
                childName = entry[1].trim();

                // find or create
                Node node = existingNodes.get(parentName);
                if (node == null) {
                    // new top level node
                    node = new Node();
                    node.setLabel(parentName);
                    root.addChild(node);
                    existingNodes.put(parentName, node);
                }

                // add child
                Node child = new Node();
                child.setLabel(childName);
                node.addChild(child);
                existingNodes.put(childName, child);

                // next line
                line = input.readLine();
            }
        }

        return root.getChildren();
    }
}