src/Composer/Autoload/ClassLoader.php
public function setPsr4($prefix, $paths)
    {
        if (!$prefix) {
            $this->fallbackDirsPsr4 = (array) $paths;
        } else {
            $length = strlen($prefix);
            if ('\\' !== $prefix[$length - 1]) {
                throw new \InvalidArgumentException("A non-empty PSR-4 prefix must end with a namespace separator.");
            }
            $this->prefixLengthsPsr4[$prefix[0]][$prefix] = $length;
            $this->prefixDirsPsr4[$prefix] = (array) $paths;
        }
    }
public function setPsr4($prefix, $paths)
    {
        if (!$prefix) {
            $this->fallbackDirsPsr4 = (array) $paths;
        } else {
            $length = strlen($prefix);
            if ('\\' !== $prefix[$length - 1]) {
                throw new \InvalidArgumentException("A non-empty PSR-4 prefix must end with a namespace separator.");
            }
            $this->prefixLengthsPsr4[$prefix[0]][$prefix] = $length;
            $this->prefixDirsPsr4[$prefix] = (array) $paths;
        }
    }
Registers a set of PSR-4 directories for a given namespace,
replacing any others previously set for this namespace.

@param string       $prefix The prefix/namespace, with trailing '\\'
@param array|string $paths  The PSR-4 base directories

@throws \InvalidArgumentException