How to Overload Groovy Operators for JSONPath Validation

The author explores using Groovy’s operator‑overloading capabilities to build a JSONPath‑style verification DSL for API validation and chaining.

First, they review the relevant operators (=, >, >=, classType) and re‑read “Groovy in Action” to recall how Groovy allows custom implementations of these symbols. They then examine the official Groovy API and implement most overloads, noting that the unary minus and plus operators are still missing because in Groovy they represent sign conversion rather than arithmetic addition/subtraction, and they currently work only for numeric values and numeric Lists.

Special attention is needed for the ++ and -- operators: Groovy does not distinguish pre‑ and post‑increment, and the generated code assigns the result back to the current object, which can cause a NullPointerException unless a return value is provided. A safe‑navigation operator (?.) can avoid the issue, and the dot (.) operator can also be overridden.

Finally, a complete demo class Verify is presented. The class extends a custom SourceCode base and implements Comparable. Each overloaded method logs the operator name and its arguments, for example:

def plus(int i) {
    logger.info("operator: + {}", i)
}

def minus(int i) {
    logger.info("operator: - {}", i)
}

def next() {
    logger.info("operator: i++ or ++i")
    this
}

def previous() {
    logger.info("operator: i-- or --i")
    this
}

def compareTo(Object o) {
    logger.info("operator: <=> >= > < etc.")
    return 0
}

The class also defines overloads for multiplication, division, modulus, power (**), logical operators (|, &, ^), bitwise negation (~), array access (getAt, putAt), shift operators (<<, >>, >>>), case testing (isCase), and type conversion (asType). Each method follows the same pattern: log the operator and return a suitable value when required.

The article ends with a brief note that the implementation is still a work‑in‑progress and invites readers to experiment further.