public class SetIndustryChemical {

    public Mylist(String list, Integer Id)
    {
        List<Account> Acclist = New list<Account>();
AccList.add(Name = 'Rumi Bajaj', Industry = 'Chemical');
AccList.add(Name = 'Gautam Bajaj', Industry = 'Chemical');
AccList.add(Name = 'Parth', Industry = 'Chemical');
system.debug('Current List='+AccList);
Acclist=[SELECT id FROM Account WHERE Industry='Chemical'];
Set<id> AccSet = New Set<id>();
for (Account a:Acclist){
    AccSet.add(a.id);
}

    }
}
}

public class SetIndustryChemical {

    public void Mylist() {
        Set<id> AccSet = New Set<id>();
        for (Account a:[SELECT id FROM Account WHERE Industry='Chemical']){
            AccSet.add(a.id);
        }
    }
}