public with sharing class MultiselectController {
    public SelectOption[] leftOptions { get; set; }
    public SelectOption[] rightOptions { get; set; }
    
    @TestVisible private void setOptions(SelectOption[] options, String value) {
        options = new SelectOption[]{};
        system.debug('##--setOptions.options: '+ options);
        system.debug('##--setOptions.value: '+ value);
        options.clear();
        String[] parts = value.split('&');
        for (Integer i=0; i<parts.size()/2; i++) {
            options.add(new SelectOption(EncodingUtil.urlDecode(parts[i*2], 'UTF-8'), 
                                         EncodingUtil.urlDecode(parts[(i*2)+1], 'UTF-8')));
        }
    }
    
    public String leftOptionsHidden { get; set {
        leftOptionsHidden = value;
        setOptions(leftOptions, value);
    }
                                    }
    
    public String rightOptionsHidden { get; set {
        rightOptionsHidden = value;
        setOptions(rightOptions, value);
    }
                                     }
}

@isTest
public class MultiselectControllerTest {
    
    static testMethod void testOptions(){        
        MultiselectController msc=new MultiselectController();
        SelectOption[] selop = new SelectOption[]{};
            String str='UTF-8&UTF-1&ISO';
        msc.setOptions(selop,str);
        selop.clear();
    }
    
    static testMethod void testOptions1(){        
        MultiselectController msc=new MultiselectController();
        List<SelectOption> leftOptions = new List<SelectOption>{};        
            String str1='UTF-8&UTF-1&ISO';
        msc.leftOptionsHidden = 'PB';
        msc.setOptions(leftOptions, str1);
    }
    
    static testMethod void testOptions2(){        
        MultiselectController msc=new MultiselectController();
        List<SelectOption> rightOptions = new List<SelectOption>{};        
            String str2='UTF-8&UTF-1&ISO';
        msc.rightOptionsHidden = 'Pitney';
        msc.setOptions(rightOptions, str2);
    }
}