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// Burton Computer Corporation
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package org.javimmutable.collections;

import javax.annotation.Nonnull;
import javax.annotation.Nullable;

/**
 * Provides static utility methods for constructing Holder instances.
 *
 * @param 
 */
public abstract class Holders
        implements Holder
{
    private static final Empty EMPTY = new Empty();

    /**
     * Creates an empty Holder for the specified type.
     * Shares a single instance for all empty Holders to save memory.
     *
     * @param 
     * @return the Holder
     */
    @SuppressWarnings("unchecked")
    @Nonnull
    public static  Holders of()
    {
        return (Holders)EMPTY;
    }

    /**
     * Creates a filled Holder for the specified type and (possibly null) value.
     *
     * @param value
     * @param 
     * @return the Holder
     */
    @Nonnull
    public static  Holders of(@Nullable V value)
    {
        return new Filled(value);
    }

    /**
     * Creates an empty Holder if value is null or a filled Holder if value is non-null.
     *
     * @param value
     * @param 
     * @return the Holder
     */
    @Nonnull
    public static  Holders fromNullable(@Nullable V value)
    {
        if (value == null) {
            return of();
        } else {
            return of(value);
        }
    }

    private Holders()
    {
        // prevent unrelated derived classes
    }

    @Override
    public int hashCode()
    {
        return Holders.hashCode(this);
    }

    @Override
    @SuppressWarnings("unchecked")
    public boolean equals(Object o)
    {
        return (o instanceof Holder) && Holders.areEqual(this, (Holder)o);
    }

    private static class Empty
            extends Holders
    {

        public boolean isEmpty()
        {
            return true;
        }

        public boolean isFilled()
        {
            return false;
        }

        public V getValue()
        {
            throw new UnsupportedOperationException("cannot get empty value");
        }

        @Override
        public V getValueOrNull()
        {
            return null;
        }

        @Override
        public V getValueOr(V defaultValue)
        {
            return defaultValue;
        }

        @Override
        public String toString()
        {
            return "[]";
        }
    }

    private static class Filled
            extends Holders
    {
        private final V value;

        private Filled(V value)
        {
            this.value = value;
        }

        public boolean isEmpty()
        {
            return false;
        }

        public boolean isFilled()
        {
            return true;
        }

        public V getValue()
        {
            return value;
        }

        @Override
        public V getValueOrNull()
        {
            return value;
        }

        @Override
        public V getValueOr(V defaultValue)
        {
            return value;
        }

        @Override
        public String toString()
        {
            return String.format("[%s]", value);
        }
    }

    public static  boolean areEqual(Holder a,
                                       Holder b)
    {
        if ((a == null) || (b == null)) {
            return (a == null) && (b == null);
        } else if (a.isEmpty()) {
            return b.isEmpty();
        } else if (b.isEmpty()) {
            return false;
        } else {
            T v1 = a.getValue();
            T v2 = b.getValue();
            if ((v1 == null) || (v2 == null)) {
                return (v1 == null) && (v2 == null);
            } else {
                return v1.equals(v2);
            }
        }
    }

    public static  int hashCode(Holder a)
    {
        if (a.isEmpty()) {
            return -1;
        }
        if (a.getValue() == null) {
            return 1;
        }
        return a.getValue().hashCode();
    }
}