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Solid is an Android library, which provides lightweight data streams and immutable+parcelable collections.

There is a newer version: 3.0.1

package solid.collectors;

import android.util.SparseArray;

import solid.functions.Func1;

public class ToSparseArray {

    /**
     * Returns a method that can be used with {@link solid.stream.Stream#collect(Func1)}
     * to convert a stream into a {@link SparseArray}.
     *
     * @param        a type of stream items.
     * @param itemToKey a method that should return a key for an item.
     * @return a method that converts an iterable into a {@link SparseArray}.
     */
    public static  Func1, SparseArray> toSparseArray(Func1 itemToKey) {
        return toSparseArray(itemToKey, 10);
    }

    /**
     * Returns a method that can be used with {@link solid.stream.Stream#collect(Func1)}
     * to convert a stream into a {@link SparseArray}.
     * 
     * Use this method instead of {@link #toSparseArray(Func1)}} for better performance on
     * streams that can have more than 10 items.
     *
     * @param              a type of stream items.
     * @param itemToKey       a method that should return a key for an item.
     * @param initialCapacity initial capacity on the sparse array.
     * @return a method that converts an iterable into a {@link SparseArray}.
     */
    public static  Func1, SparseArray> toSparseArray(final Func1 itemToKey, final int initialCapacity) {
        return new Func1, SparseArray>() {
            @Override
            public SparseArray call(Iterable iterable) {
                SparseArray array = new SparseArray<>(initialCapacity);
                for (T value : iterable)
                    array.put(itemToKey.call(value), value);
                return array;
            }
        };
    }
}