Save Error in Test Class for @InvocableMethod: Static method cannot be referenced from a non static context

Question

I'm trying to write a simple text class for another simple class that has an @InvocableMethod method, but receive the save error in the question's title. I can't figure out its cause or how to overcome it.

Class with @InvocableMethod Method:

global with sharing class InputOutput_Placeholder_Plugin {

    @InvocableMethod
    public static List<List<String>> placeholderText(List<String> input) {

        String s  = input.get(0);
        List<String> allString = new List<String>();
        List<List<String>> allStrings = new List<List<String>>();

        allString.add(s);   
        allStrings.add(allString);

        return allStrings;
    }
}

Test Class:

@isTest
private class InputOutput_Placeholder_Plugin_Test {

    static testmethod void InputOutput_Placeholder_Plugin_Tests() {

        InputOutput_Placeholder_Plugin plugin = new InputOutput_Placeholder_Plugin();
        String s = 'Searching';
        List<String> stringList = new List<String>();
        stringList.add(s);

        plugin.placeholderText(stringList);  
    }    
}

The error occurs at plugin.placeholderText(stringList);

Thanks in advance.

Answer 1

Change:

plugin.placeholderText(stringList);  

With:

InputOutput_Placeholder_Plugin.placeholderText(stringList);  

Static method or variables can be accessed by class name, only non-static ones require class instance for invocation.