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Functional structures and utilities for Java 8 and lambdas
There is a newer version: 0.27
package no.finn.lambdacompanion;

import java.util.Objects;
import java.util.Optional;
import java.util.function.Consumer;
import java.util.function.Function;
import java.util.function.Predicate;
import java.util.function.Supplier;
import java.util.stream.Stream;

/**
 * Wrapper around an usual {@link java.util.Optional} that provides the very missed {@link #stream()} method useful for
 * flatMapping a {@link java.util.stream.Stream} of {@link java.util.Optional} (i.e. only keep present values).
 * 
 * example: the good
 * 
 * {@code
 * collection.stream().flatMap(item -> StreamableOptional.ofNullable(item.getNullableProperty()).stream())
 * }
 * 
 * would replace the bad:
 *  * {@code
 * collection.stream().map(item -> Optional.ofNullable(item.getNullableProperty())).filter(Optional::isPresent).map(Optional::get)
 * }
 * 
 * or the ugly:
 *  * {@code
 * collection.stream().flatMap(item -> Optional.ofNullable(item.getNullableProperty()).map(Stream::of).orElseGet(Stream::empty))
 * }
 * 
 */
public class StreamableOptional {

    private static final StreamableOptional EMPTY = new StreamableOptional(Optional.empty());

    private final Optional optional;

    private StreamableOptional(final Optional optional) {
        this.optional = optional;
    }

    /**
     * @see java.util.Optional#ifPresent(java.util.function.Consumer)
     * @param consumer consumer
     */
    public void ifPresent(Consumer consumer) {
        optional.ifPresent(consumer);
    }

    /**
     * @see java.util.Optional#filter(java.util.function.Predicate)
     * @param predicate predicate
     * @return StreamableOptional
     */
    public StreamableOptional filter(Predicate predicate) {
        return optional.filter(predicate).map(present -> this).orElse(empty());
    }

    /**
     * @see java.util.Optional#map(java.util.function.Function)
     * @param  u
     * @param mapper mapper
     * @return U
     */
    public  StreamableOptional map(Function mapper) {
        return optional.map(mapper).map(StreamableOptional::of).orElse(empty());
    }

    /**
     * @see java.util.Optional#flatMap(java.util.function.Function)
     * @param  u
     * @param mapper mapper
     * @return U
     */
    public  StreamableOptional flatMap(Function> mapper) {
        return optional.map(present -> Objects.requireNonNull(mapper.apply(present))).orElse(empty());
    }

    /**
     * @see java.util.Optional#orElse(Object)
     * @param other other
     * @return T t
     */
    public T orElse(T other) {
        return optional.orElse(other);
    }

    /**
     * @see java.util.Optional#orElseGet(java.util.function.Supplier)
     * @param other other
     * @return T t
     */
    public T orElseGet(Supplier other) {
        return optional.orElseGet(other);
    }

    /**
     * @see java.util.Optional#orElseThrow(java.util.function.Supplier)
     * @param exceptionSupplier exceptionSupplier
     * @param  x
     * @throws X x
     * @return T t
     */
    public  T orElseThrow(Supplier exceptionSupplier) throws X {
        return optional.orElseThrow(exceptionSupplier);
    }

    /**
     * Turns this optional into a {@link java.util.stream.Stream}
     * @return a {@link java.util.stream.Stream} of the one value contained within this optional if it is present, an empty stream else
     */
    public Stream stream() {
        return optional.map(Stream::of).orElseGet(Stream::empty);
    }

    /**
     * @return the {@link java.util.Optional} within
     */
    public Optional toOptional() {
        return optional;
    }

    /**
     * @see java.util.Optional#of(Object)
     * @param  t
     * @param value value
     * @return T t
     */
    public static  StreamableOptional of(final T value) {
        return ofOptional(Optional.of(value));
    }

    /**
     * @see java.util.Optional#ofNullable(Object)
     * @param  t
     * @param value value
     * @return T t
     */
    public static  StreamableOptional ofNullable(final T value) {
        return ofOptional(Optional.ofNullable(value));
    }

    /**
     * Builds a StreamableOptional around the given {@link java.util.Optional}
     *
     * @param  t
     * @param optional a regular {@link java.util.Optional}
     * @return a StreamableOptional
     */
    public static  StreamableOptional ofOptional(final Optional optional) {
        Objects.requireNonNull(optional);
        if (!optional.isPresent()) {
            return empty();
        }
        return new StreamableOptional<>(optional);
    }

    /**
     * @see java.util.Optional#empty()
     * @param  t
     * @return T t
     */
    public static  StreamableOptional empty() {
        return (StreamableOptional) EMPTY;
    }

    @Override
    public boolean equals(final Object o) {
        if (this == o) {
            return true;
        }
        if (o == null || getClass() != o.getClass()) {
            return false;
        }

        final StreamableOptional that = (StreamableOptional) o;

        if (!optional.equals(that.optional)) {
            return false;
        }

        return true;
    }

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

}