Cannot extend helper of third party module

Question

My config.xml:

<?xml version="1.0"?>
<config>
    <modules>
        <Company_Mailchimpextend>
            <version>0.1.0</version>
        </Company_Mailchimpextend>
    </modules>
    <global>
        <helpers>
            <monkey><!-- tried MageMonkey, magemonkey etc. -->
                <rewrite>
                    <data>Company_Mailchimpextend_Helper_Data</data>
                </rewrite>
            </monkey>
        </helpers>
    </global>
</config>

etc/modules/Company_Mailchimpextend.xml:

<?xml version="1.0"?>
<config>
    <modules>
        <Company_Mailchimpextend>
            <active>true</active>
            <codePool>local</codePool>
                <depends>
                <Ebizmarts_MageMonkey/>
            </depends>
        </Company_Mailchimpextend>
    </modules>
</config>

Helper/Data.php:

<?php
    class Company_Mailchimpextend_Helper_Data extends Ebizmarts_MageMonkey_Helper_Data {

        private function _setMaps($maps,$customer,$merge_vars, $websiteId)
        {
            Mage::log("test",null,"test.log");

Third party config.xml:

<helpers>
        <monkey>
            <class>Ebizmarts_MageMonkey_Helper</class>
        </monkey>
    </helpers>

It doesn't seem to be extended somewhere else. Cleared cache, the module is showing in config > advance. It still uses the method _setMaps from the third party module.

Thanks!

Answer 1

Overriding private methods has no effect, it does not even raise any PHP errors. In order to alter the behaviour of private parent methods you need to override the public or protected method that invokes the private one and change the behaviour there:

<?php
class Base
{
    private function _output()
    {
        return 'foo';
    }

    public function printOutput()
    {
        print $this->_output();
    }

    public function printMyOutput()
    {
        print $this->_output();
    }

    public function printMyOutput2()
    {
        print $this->_output();
    }
}

class Override extends Base
{
    private function _output()
    {
        return 'bar';
    }

    public function printMyOutput()
    {
        print $this->_output();
    }

    public function printMyOutput2()
    {
        print 'bar';
    }
}

$override = new Override();
$override->printOutput();    // prints 'foo'
$override->printMyOutput();  // prints 'bar'
$override->printMyOutput2(); // prints 'bar'

Personally, I would not even re-declare the private method but integrate its logic either directly into the overriden method or, if more complex, move it to a separate model.