Converting angular2-query-builder query to logical expression

I use angular2-query-builder in my project. Besides, the back-end project using Java, Spring boot and Rulebook as a rule engine in order to fire rules which are generated by angular2-query-builder. So I need to make a logical expression from the output of angular2-query-builder.

This is a type-script object generated by query-builder.

  query = {
    condition: 'and',
    rules: [
      {field: 'age', operator: '<=', value: '16'},
      {field: 'gender', operator: '=', value: 'm'}
    ]
  };

The mapped model class which I've made in the back-end :

RuleSet {
    String field;
    String value;
    String operator;
    String entity;
    String condition;
    RuleSet[] rules;
//...getters and setters}

I need to get (age <= 16) && (gender = 'm') from the object.

making a recursive method in order to traverse the rules

private void decode(RuleSet[] rules, List<String> expressions, String condition) {
        for (int i = 0; i < rules.length; i++) {
            RuleSet item = rules[i];
            if (item.rules == null) {
                expressions.add(merge("(", item.field, item.operator, item.value, ")"));
                if (i != rules.length - 1) {
                    expressions.add(condition);
                }
            } else {
                condition = item.getCondition();
                expressions.add("(");
                decode(item.rules, expressions, condition);
                expressions.add(")");
            }
        }
    }
public String merge(String... strings) {
    StringBuilder result = new StringBuilder();
    for (String item : strings) {
        if (item != null) {
            result.append(item);
        }
    }
    return result.toString();
}

then call it like this:

List<String> expressions = new ArrayList<>();
decode(querySchema.ruleSet.rules, expressions, querySchema.ruleSet.condition);

by adding all items of expressions into a string object. The logical expression will be generated.