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Lyra grid - LGPL Java library
There is a newer version: 1.2.48
package ru.curs.lyra.kernel.grid;

import java.math.BigInteger;

/**
 * Numerator for a nullable field.
 */
public abstract class NullableFieldEnumerator extends KeyEnumerator {

    private final KeyEnumerator parent;
    private boolean valueIsNull = true;

    NullableFieldEnumerator(KeyEnumerator parent) {
        this.parent = parent;
    }

    @Override
    public BigInteger cardinality() {
        return getParent().cardinality().add(BigInteger.ONE);
    }

    @Override
    public void setValue(Object value) {
        if (value == null) {
            setValueIsNull(true);
        } else {
            setValueIsNull(false);
            getParent().setValue(value);
        }
    }

    @Override
    public Object getValue() {
        return isValueIsNull() ? null : getParent().getValue();
    }

    boolean isValueIsNull() {
        return valueIsNull;
    }

    void setValueIsNull(boolean valueIsNull) {
        this.valueIsNull = valueIsNull;
    }

    KeyEnumerator getParent() {
        return parent;
    }

    /**
     * Create NullableFieldEnumerator either for NULLS FIRST or NULLS LAST
     * database behaviour.
     *
     * @param nullsFirst true for NULLS FIRST.
     * @param parent     parent enumerator.
     */
    public static NullableFieldEnumerator create(boolean nullsFirst, KeyEnumerator parent) {
        return nullsFirst ? new NullsFirst(parent) : new NullsLast(parent);
    }

}

/**
 * NullableFieldEnumerator for NULLS FIRST.
 */
class NullsFirst extends NullableFieldEnumerator {
    NullsFirst(KeyEnumerator parent) {
        super(parent);
    }

    @Override
    public BigInteger getOrderValue() {
        return isValueIsNull() ? BigInteger.ZERO : getParent().getOrderValue().add(BigInteger.ONE);
    }

    @Override
    public void setOrderValue(BigInteger value) {
        if (BigInteger.ZERO.equals(value)) {
            setValueIsNull(true);
        } else {
            setValueIsNull(false);
            getParent().setOrderValue(value.subtract(BigInteger.ONE));
        }
    }
}

/**
 * NullableFieldEnumerator for NULLS LAST.
 */
class NullsLast extends NullableFieldEnumerator {
    NullsLast(KeyEnumerator parent) {
        super(parent);
    }

    @Override
    public BigInteger getOrderValue() {
        return isValueIsNull() ? getParent().cardinality() : getParent().getOrderValue();
    }

    @Override
    public void setOrderValue(BigInteger value) {
        if (getParent().cardinality().equals(value)) {
            setValueIsNull(true);
        } else {
            setValueIsNull(false);
            getParent().setOrderValue(value);
        }
    }

}