Missing Static Method In Non Instantiatable Class

public class AccountSelectionSubForm extends Form implements WidgetEventListener
{
    @SuppressWarnings("PMD.UselessOverridingMethod") // the method is protected in the base class
    public static class AccountSelectionSubFormObservable extends Observable
    {
        @Override
        public void setChanged()
        {
            super.setChanged();
        }
    }

    private BaseConfig baseConfig;
    private AccountManager accountManager;
    private WorkingAccountStore workingAccountStore;

    private AccountSelectionSubFormObservable observable;
    private AccountSelectionDetailsSubForm details;
    private Combobox<Account> accountSelection;
    private boolean supportAllAccountsSelection;
    private boolean supportAccountless;
    private boolean localSelect;
    private Button refreshButton;

    public static class Factory
    {
        private BaseConfig baseConfig;
        private AccountManager accountManager;
        private WorkingAccountStore workingAccountStore;

        @Inject
        public Factory( BaseConfig baseConfig, AccountManager accountManager, WorkingAccountStore workingAccountStore )
        {
            this.baseConfig = baseConfig;
            this.accountManager = accountManager;
            this.workingAccountStore = workingAccountStore;
        }

        public AccountSelectionSubForm create( Form parent, boolean supportAllAccountsSelection )
        {
            return create( parent, supportAllAccountsSelection, false );
        }

        public AccountSelectionSubForm create( Form parent, boolean supportAllAccountsSelection, boolean supportAccountless )
        {
            return new AccountSelectionSubForm( parent, baseConfig, accountManager, workingAccountStore, supportAllAccountsSelection, supportAccountless );
        }

        public AccountSelectionSubForm create( Form parent, RequestTypeIdentifier requestTypeIdentifier, boolean signAvailable, boolean supportAllAccountsSelection )
        {
            return create( parent, new RequestTypeIdentifier[]{requestTypeIdentifier}, signAvailable, supportAllAccountsSelection );
        }

        public AccountSelectionSubForm create( Form parent, RequestTypeIdentifier[] requestTypeIdentifiers, boolean signAvailable, boolean supportAllAccountsSelection )
        {
            return new AccountSelectionSubForm( parent, baseConfig, accountManager, workingAccountStore, requestTypeIdentifiers, signAvailable, supportAllAccountsSelection, false );
        }
    }

    private AccountSelectionSubForm( Form parent, BaseConfig baseConfig, AccountManager accountManager, WorkingAccountStore workingAccountStore, boolean supportAllAccountsSelection, boolean supportAccountless )
    {
        super( parent, null );
        this.baseConfig = baseConfig;
        this.accountManager = accountManager;
        this.workingAccountStore = workingAccountStore;

        this.supportAllAccountsSelection = supportAllAccountsSelection;
        this.supportAccountless = supportAccountless;

        init();
    }

    private AccountSelectionSubForm( Form parent, BaseConfig baseConfig, AccountManager accountManager, WorkingAccountStore workingAccountStore, RequestTypeIdentifier[] requestTypeIdentifiers, boolean signAvailable, boolean supportAllAccountsSelection, boolean supportAccountLess )
    {
        this( parent, baseConfig, accountManager, workingAccountStore, supportAllAccountsSelection, supportAccountLess );

        // now get the requested DataProvider (with or without all accounts option)
        changeRequestIds( requestTypeIdentifiers, signAvailable );

        refresh();
    }

    private void init()
    {
        // create combobox
        accountSelection = new Combobox<Account>( this );
        accountSelection.setSortComparator( new AccountComparator( baseConfig.getAccountSorting() ).getComboboxItemComparator() );

        // create details sub form
        details = new AccountSelectionDetailsSubForm( this );
        details.changeSelected( getSelectedAccount() );

        // create refresh Button
        refreshButton = new Button( this );
        refreshButton.setImageKey( "accountSelectionForm.image.refreshButton" );
        refreshButton.addEventListener( this );

        // create a observable proxy
        observable = new AccountSelectionSubFormObservable();
    }
...