ContactSampleExtension.cls

public with sharing class ContactSampleExtension {

    private final Contact cont;
    public String selected {get; set;}

    public ContactSampleExtension(ApexPages.StandardController stdController) {
        this.cont = (Contact)stdController.getRecord();
    }

    public List<SelectOption> getItems(){
        List<SelectOption> items = new List<SelectOption>();
        items.add(new SelectOption('1', 'select1'));
        items.add(new SelectOption('2', 'select2'));
        items.add(new SelectOption('3', 'select3'));
        items.add(new SelectOption('4', 'select4'));
        items.add(new SelectOption('5', 'select5'));
        return items;
    }

    public void doButtonAction(){
        System.debug('選択肢の値を表示 >> ' + selected);
    }

}