Get current page query string parameters in apex

This would be a clean way to do it:

apexPages.currentPage().getParameters().get();

// For URL - http://test.salesforce.com/apex/helloworld?q=texas


//use the following
String queryString ='';
queryString = ApexPages.currentPage().getParameters().get('q');

Source

Parameters passed to the page through URL can be accessed in controller using below syntax,

String parameterValue = ApexPages.currentPage().getParameters().get('parameterName');

Usually it is used in constructor of controller to get value and then do operations based on the parameter value.