Java 8 Streams and try with resources

Question

I thought that the stream API was here to make the code easier to read. I found something quite annoying. The Stream interface extends the java.lang.AutoC

Answer 1

It is possible to create a utility method that reliably closes streams with a try-with-resource-statement.

It is a bit like a try-finally that is an expression (something that is the case in e.g. Scala).

/**
 * Applies a function to a resource and closes it afterwards.
 * @param sup Supplier of the resource that should be closed
 * @param op operation that should be performed on the resource before it is closed
 * @return The result of calling op.apply on the resource 
 */
private static  B applyAndClose(Callable sup, Function op) {
    try (A res = sup.call()) {
        return op.apply(res);
    } catch (RuntimeException exc) {
        throw exc;
    } catch (Exception exc) {
        throw new RuntimeException("Wrapped in applyAndClose", exc);
    }
}

^{(Since resources that need to be closed often also throw exceptions when they are allocated non-runtime exceptions are wrapped in runtime exceptions, avoiding the need for a separate method that does that.)}

With this method the example from the question looks like this:

Set photos = new HashSet(Arrays.asList(1, 2, 3));

List collect = applyAndClose(photos::stream, s -> s
    .map(photo -> new ImageView(new Image(String.valueOf(photo))))
    .collect(Collectors.toList()));

This is useful in situations when closing the stream is required, such as when using Files.lines. It also helps when you have to do a "double close", as in your example in Listing 3.

^{This answer is an adaptation of an old answer to a similar question.}