# Groovy Contracts – design by contract support for Groovy

version 4.0.18

This module provides contract annotations that support the specification of class-invariants, pre- and post-conditions on Groovy classes and interfaces. Special support is provided so that post-conditions may refer to the old value of variables or to the result value associated with calling a method.

## 1. Applying @Invariant, @Requires and @Ensures

With GContracts in your class-path, contracts can be applied on a Groovy class or interface by using one of the assertions found in package org.gcontracts.annotations.

    package acme

    import groovy.contracts.*

    @Invariant({ speed() >= 0 })
    class Rocket {
        int speed = 0
        boolean started = true

        @Requires({ isStarted() })
        @Ensures({ old.speed < speed })
        def accelerate(inc) { speed += inc }

        def isStarted() { started }

        def speed() { speed }
    }

    def r = new Rocket()
    r.accelerate(5)

## 2. More Features

GContracts supports the following feature set:

- definition of class invariants, pre- and post-conditions via @Invariant, @Requires and @Ensures

- inheritance of class invariants, pre- and post-conditions of concrete predecessor classes

- inheritance of class invariants, pre- and post-conditions in implemented interfaces

- usage of old and result variable in post-condition assertions

- assertion injection in Plain Old Groovy Objects (POGOs)

- human-readable assertion messages, based on Groovy power asserts

- enabling contracts at package- or class-level with @AssertionsEnabled

- enable or disable contract checking with Java’s -ea and -da VM parameters

- annotation contracts: a way to reuse reappearing contract elements in a project domain model

- detection of circular assertion method calls

## 3. The Stack Example

Currently, Groovy contracts supports 3 annotations: @Invariant, @Requires and @Ensures – all of them work as annotations with closures, where closures allow you to specify arbitrary code pieces as annotation parameters:

    @Grab(group='org.apache.groovy', module='groovy-contracts', version='4.0.0')
    import groovy.contracts.*

    @Invariant({ elements != null })
    class Stack<T> {

        List<T> elements

        @Ensures({ is_empty() })
        def Stack()  {
            elements = []
        }

        @Requires({ preElements?.size() > 0 })
        @Ensures({ !is_empty() })
        def Stack(List<T> preElements)  {
            elements = preElements
        }

        boolean is_empty()  {
            elements.isEmpty()
        }

        @Requires({ !is_empty() })
        T last_item()  {
            elements.get(count() - 1)
        }

        def count() {
            elements.size()
        }

        @Ensures({ result == true ? count() > 0 : count() >= 0  })
        boolean has(T item)  {
            elements.contains(item)
        }

        @Ensures({ last_item() == item })
        def push(T item)  {
           elements.add(item)
        }

        @Requires({ !is_empty() })
        @Ensures({ last_item() == item })
        def replace(T item)  {
            remove()
            elements.add(item)
        }

        @Requires({ !is_empty() })
        @Ensures({ result != null })
        T remove()  {
            elements.remove(count() - 1)
        }

        String toString() { elements.toString() }
    }

    def stack = new Stack<Integer>()

The example above specifies a class-invariant and methods with pre- and post-conditions. Note, that preconditions may reference method arguments and post-conditions have access to the method’s result with the result variable and old instance variables values with old.

Indeed, Groovy AST transformations change these assertion annotations into Java assertion statements (can be turned on and off with a JVM param) and inject them at appropriate places, e.g. class-invariants are used to check an object’s state before and after each method call.