@SuppressWarnings("unchecked")
public enum ModelConverters {
    INSTANCE;

    private final Map<Class<?>, ModelConverter<?, String>> modelConverters = new HashMap<>();
    {
        modelConverters.put(AlphaModel.class, new AlphaModelConverter());
        modelConverters.put(BetaModel.class, new BetaModelConverter());
    }

    public static <T> String convert(final T input) {
        ModelConverter<?, String> modelConverter = ModelConverters.INSTANCE.modelConverters.get(input.getClass());
        if (modelConverter == null) {

@SuppressWarnings("unchecked")
public enum ModelConverters {
    INSTANCE;

    private final ModelConverterBacking<String> modelConverterBacking = new ModelConverterBacking<>();
    {
        modelConverterBacking.put(AlphaModel.class, new AlphaModelConverter());
        modelConverterBacking.put(BetaModel.class, new BetaModelConverter());
    }

    public static <T> String convert(final T input) {
        ModelConverter<T, String> modelConverter = INSTANCE.modelConverterBacking.get((Class<T>)input.getClass());

Exception in thread "main" java.lang.NullPointerException
	at testproject8.vogel612.overengineering.cr57768.ModelConverters.convert(ModelConverters.java:19)

Object thing = (Object)null;
thing.toString();

Object thing = null;
thing.toString();

 AlphaController implements Serializable {
     private static final ModelConverter<T> converter = ModelConverterFactory.createConverterFor(AlhpaModel.class);

     private final Collection<AlphaModel> items;
     public AlphaController() {
           items = AlphaService.loadAll();
     }
     public Collection<String> getItemRepresentation() {
            return converter.convert(items);
     }
  }

return items.stream()
    .map(ModelConverters::convert)
    .collect(Collectors.toList());

public static <T> ModelConverter<T, String> getConverterFor(final Class<T> clazz) {
    return INSTANCE.modelConverterBacking.get(clazz);
}

 return items.stream().map(AlphaModel::getAttribute()).collect(Collectors.toList());

private final ModelConverterBacking<String> modelConverterBacking = new ModelConverterBacking<>();
{
    modelConverterBacking.put(AlphaModel.class, alphaModel -> "ALPHA");
    modelConverterBacking.put(BetaModel.class, betaModel -> "BETA");
}

modelConverterBacking.put(AlphaModel.class, alphaModel -> alphaModel.getAttribute());

public class OrganizationBuilder {
    private final static Map<OrganizationField, Predicate<String>> DEFAULT_VALIDATION_MAPPING = new HashMap<>();
    static {
        DEFAULT_VALIDATION_MAPPING.put(IBAN_NUMBER, IbanNumberValidator::validate);
        DEFAULT_VALIDATION_MAPPING.put(IS_PARENT, isParent -> (isParent.toUpperCase().equals("Y") || isParent.toUpperCase().equals("N")));
        DEFAULT_VALIDATION_MAPPING.put(VAT_NUMBER, VatNumberValidator::validate);
    }

public class IbanNumberValidator {
    private static final String IBAN_NUMBER_REGEX = "(?i)^[a-z]{2}\\d{2}[a-z0-9]{11,}$";
    private static final Pattern IBAN_NUMBER_PATTERN = Pattern.compile(IBAN_NUMBER_REGEX);

    private IbanNumberValidator() {
        throw new UnsupportedOperationException();
    }

    public static String getRegex() {
        return IBAN_NUMBER_REGEX;
    }

    public static boolean validate(final String ibanNumber) {
        Objects.requireNonNull(ibanNumber, "ibanNumber");

/**
* Write additional type-variable flags associated with methods and class symbols.
* This routine assumes that no more than (2^8 - 1) type-variables are declared by a given
* class/method. All flags are stored as a sequence of consecutive bytes in the
* TypeVariableMap attribute. If the input type-variable list is empty, no attribute is added.
*/