Java – Transforms the contents of an optional or stream

Instead of writing my own method or class, I want to check the optional and convert it to another class, or have an empty object.

For this sub-problem in my application, I want to convert the custom user object of a TreeNode instance to a CustomUserObject, provided that the TreeNode is an instance of DefaultMutableTreeNode.

private Optional<CustomUserObject> getCustomUserObject(TreeNode node) {
    Optional<DefaultMutableTreeNode> optDefaultMutableTreeNode = OptionalUtil.cast(Optional.ofNullable(node), DefaultMutableTreeNode.class);
    Optional<Object> optUserObject = optDefaultMutableTreeNode.map(DefaultMutableTreeNode::getUserObject); //
    return OptionalUtil.cast(optUserObject, CustomUserObject.class);
}

/**
 * Maps the given optional, if the containing element is an instance of the given class.
 * Returns empty if the containing object is not an instance of the given class.
 * 
 * @param orgOptional
 *        given optional
 * @param clazz
 *        given class.
 * @return the resulting {@link Optional}.
 */
public static <T, X> Optional<T> cast(Optional<X> orgOptional, Class<T> clazz) {
    return orgOptional //
        .filter(clazz::isInstance) // check instance
        .map(clazz::cast);  cast
}

/**
 * Maps the given stream, if the containing element is an instance of the given class.
 * Returns empty if the containing object is not an instance of the given class.
 * 
 * @param orgStream
 *        given optional
 * @param clazz
 *        given class.
 * @return the resulting {@link Optional}.
 */
public static <T, X> Stream<T> cast(Stream<X> orgStream, Class<T> clazz) {
    return orgStream //
        .filter(clazz::isInstance) // check instance
        .map(clazz::cast);  cast
}

I

remember that I often needed to convert optional values or streams in this way. It is not fluent. I actually want java Optional or Stream to have a cast method that performs the above steps. I don’t want to write CustomOptional fluently myself. Am I missing something? Is there an easier way to do this?

You can easily make it smoother by relying on the map()/flatMap() and cast methods that return the function.

For Optional, this is easy because map() can act as a filter by returning null. So just define:

public static <U> Function<Object, U> filterAndCast(Class<? extends U> clazz) {
    return t -> clazz.isInstance(t) ? clazz.cast(t) : null;
}

And use it as:

Optional<Number> number = Optional.of(42L);
System.out.println(number.map(filterAndCast(Integer.class)));
System.out.println(number.map(filterAndCast(Long.class)));

Output:

Optional.empty
Optional[42]

For streams, you can apply more or less the same trick by relying on flatMap() and functions that return an empty Stream:

public static <U> Function<Object, Stream<U>> streamFilterAndCast(Class<? extends U> clazz) {
    return t -> clazz.isInstance(t) ? Stream.of(clazz.cast(t)) : Stream.empty();
     or alternatively
    return t -> Stream.of(t).filter(clazz::isInstance).map(clazz::cast);
}

And use it as:

Stream.of(42L, "Hello world", 1024, 3.14)
        .flatMap(streamFilterAndCast(Number.class))
        .forEach(System.out::println);

Output:

42
1024
3.14