Flattener.java

import java.util.HashMap;
import java.util.List;
import java.util.Map;
import java.util.Vector;

/**
 * Flattens a Map
 */
public class Flattener {
    static public void process(Map<String, Object> target) {
        process(target, "");
    }

    static public void process(Map<String, Object> target, String prefix) {
        List<String> removeKeys = new Vector<>();
        Map<String, Object> flattened = new HashMap<>();
        target.forEach((k, v) -> {
            if (v instanceof Map) {
                Map<String, Object> m = (Map<String, Object>) v;
                process(m, prefix + k + "_");
                m.forEach((mk,mv) -> flattened.put(mk, mv));
                removeKeys.add(k);
            } else if (v instanceof List) {
                flattened.put(prefix + k + "_count", ((List) v).size());
                removeKeys.add(k);
            } else {
                flattened.put(prefix + k, v);
                if (prefix.length() > 0) removeKeys.add(k);
            }
        });
        removeKeys.forEach(k -> target.remove(k));
        target.putAll(flattened);
    }
}

Flattens a hierarchical object (usually from JSON) into a flat structure. Arrays are summarized by counting entries.

Useful when summarizing hierarchical objects as a single relational database table (assuming structure matches a consistent schema).

Unit Test here: https://gist.github.com/pbatey/c8381d3b1e529c5a611a84909c8733ee