src/main/java/org/apache/commons/collections4/collection/CompositeCollection.java [133:221]:
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            if (item.contains(obj)) {
                return true;
            }
        }
        return false;
    }

    /**
     * Gets an iterator over all the collections in this composite.
     * <p>
     * This implementation uses an {@code IteratorChain}.
     * </p>
     * @return an {@code IteratorChain} instance which supports
     *  {@code remove()}. Iteration occurs over contained collections in
     *  the order they were added, but this behavior should not be relied upon.
     * @see IteratorChain
     */
    @Override
    public Iterator<E> iterator() {
        if (all.isEmpty()) {
            return EmptyIterator.<E>emptyIterator();
        }
        final IteratorChain<E> chain = new IteratorChain<>();
        all.forEach(item -> chain.addIterator(item.iterator()));
        return chain;
    }

    /**
     * Returns an array containing all of the elements in this composite.
     *
     * @return an object array of all the elements in the collection
     */
    @Override
    public Object[] toArray() {
        final Object[] result = new Object[size()];
        int i = 0;
        for (final Iterator<E> it = iterator(); it.hasNext(); i++) {
            result[i] = it.next();
        }
        return result;
    }

    /**
     * Returns an object array, populating the supplied array if possible.
     * See {@code Collection} interface for full details.
     *
     * @param <T>  the type of the elements in the collection
     * @param array  the array to use, populating if possible
     * @return an array of all the elements in the collection
     */
    @Override
    @SuppressWarnings("unchecked")
    public <T> T[] toArray(final T[] array) {
        final int size = size();
        Object[] result = null;
        if (array.length >= size) {
            result = array;
        } else {
            result = (Object[]) Array.newInstance(array.getClass().getComponentType(), size);
        }

        int offset = 0;
        for (final Collection<E> item : all) {
            for (final E e : item) {
                result[offset++] = e;
            }
        }
        if (result.length > size) {
            result[size] = null;
        }
        return (T[]) result;
    }

    /**
     * Adds an object to the collection, throwing UnsupportedOperationException
     * unless a CollectionMutator strategy is specified.
     *
     * @param obj  the object to add
     * @return {@code true} if the collection was modified
     * @throws UnsupportedOperationException if CollectionMutator hasn't been set
     * @throws UnsupportedOperationException if add is unsupported
     * @throws ClassCastException if the object cannot be added due to its type
     * @throws NullPointerException if the object cannot be added because its null
     * @throws IllegalArgumentException if the object cannot be added
     */
    @Override
    public boolean add(final E obj) {
        if (mutator == null) {
            throw new UnsupportedOperationException(
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src/main/java/org/apache/commons/collections4/set/CompositeSet.java [132:219]:
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            if (item.contains(obj)) {
                return true;
            }
        }
        return false;
    }

    /**
     * Gets an iterator over all the sets in this composite.
     * <p>
     * This implementation uses an {@code IteratorChain}.
     *
     * @return an {@code IteratorChain} instance which supports
     *  {@code remove()}. Iteration occurs over contained collections in
     *  the order they were added, but this behavior should not be relied upon.
     * @see IteratorChain
     */
    @Override
    public Iterator<E> iterator() {
        if (all.isEmpty()) {
            return EmptyIterator.<E>emptyIterator();
        }
        final IteratorChain<E> chain = new IteratorChain<>();
        all.forEach(item -> chain.addIterator(item.iterator()));
        return chain;
    }

    /**
     * Returns an array containing all of the elements in this composite.
     *
     * @return an object array of all the elements in the collection
     */
    @Override
    public Object[] toArray() {
        final Object[] result = new Object[size()];
        int i = 0;
        for (final Iterator<E> it = iterator(); it.hasNext(); i++) {
            result[i] = it.next();
        }
        return result;
    }

    /**
     * Returns an object array, populating the supplied array if possible.
     * See {@code Collection} interface for full details.
     *
     * @param <T>  the type of the elements in the collection
     * @param array  the array to use, populating if possible
     * @return an array of all the elements in the collection
     */
    @Override
    @SuppressWarnings("unchecked")
    public <T> T[] toArray(final T[] array) {
        final int size = size();
        Object[] result = null;
        if (array.length >= size) {
            result = array;
        } else {
            result = (Object[]) Array.newInstance(array.getClass().getComponentType(), size);
        }

        int offset = 0;
        for (final Collection<E> item : all) {
            for (final E e : item) {
                result[offset++] = e;
            }
        }
        if (result.length > size) {
            result[size] = null;
        }
        return (T[]) result;
    }

    /**
     * Adds an object to the collection, throwing UnsupportedOperationException
     * unless a SetMutator strategy is specified.
     *
     * @param obj  the object to add
     * @return {@code true} if the collection was modified
     * @throws UnsupportedOperationException if SetMutator hasn't been set or add is unsupported
     * @throws ClassCastException if the object cannot be added due to its type
     * @throws NullPointerException if the object cannot be added because its null
     * @throws IllegalArgumentException if the object cannot be added
     */
    @Override
    public boolean add(final E obj) {
        if (mutator == null) {
            throw new UnsupportedOperationException(
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