Classes and invariant-preserving operations

The idea

A class can protect its representation with private properties and expose operations that make sense for its domain. A constructor establishes the initial invariant, such as a nonnegative remaining capacity. Public methods must preserve the same rule after every successful or rejected operation. Providing an unrestricted setter can undermine the constructor’s work by letting callers install invalid state later.

Choose failure behavior before changing fields. A take method can return false for a negative or oversized request and leave the object unchanged; a query reports the remaining amount without modifying it. Typed properties and parameters help express accepted kinds of values, but type declarations alone do not express every range or relationship. Validate those semantic constraints explicitly and test both returned outcomes and resulting state.

<?php
class Stock {private int $count;public function __construct(int $n){if($n<0)throw new InvalidArgumentException();$this->count=$n;}public function take(int $n):bool{if($n<0||$n>$this->count)return false;$this->count-=$n;return true;}public function count():int{return $this->count;}}
$s=new Stock(4);var_export($s->take(5));echo "\n",$s->count(),"\n";

false
4

The failed request is rejected before subtraction, so the query still reports four.