如何在java 8流上使用foreach循环进行迭代

Question

Suppose we try to apply to java 8 stream a lambda that could throw checked exception:

Stream<String> stream = Stream.of("1", "2", "3");
Writer writer = new FileWriter("example.txt");

stream.forEach(s -> writer.append(s)); // Unhandled exception: java.io.IOException

This won't compile.

One workaround is to nest checked exception in RuntimeException but it complicates later exception handling and it's just ugly:

stream.forEach(s -> {
    try {
        writer.append(s);
    } catch (IOException e) {
        throw new RuntimeException(e);
    }
});

Alternative workaround could be to convert limited functional forEach to plain old foreach loop that is more friendly to checked exceptions.

But naive approaches fail:

for (String s : stream) { // for-each not applicable to expression type 'java.util.stream.Stream<java.lang.String>'
    writer.append(s);
}

for (String s : stream.iterator()) { // foreach not applicable to type 'java.util.Iterator<java.lang.String>'
    writer.append(s);
}

Update

A trick that answers this question was previosly posted at Why does Stream<T> not implement Iterable<T>? as side answer that doesn't really answer that question itself. I think this is not enough to qualify this question as duplicate of that one because they ask different things.

Answer 1

By definition foreach loop requires an Iterable to be passed in.

It can be achieved with anonymous class:

    for (String s : new Iterable<String>() {
        @Override
        public Iterator<String> iterator() {
            return stream.iterator();
        }
    }) {
        writer.append(s);
    }

This can be simplified with lambda because Iterable is a functional interface :

    for (String s : (Iterable<String>) () -> stream.iterator()) {
        writer.append(s);
    }

This can be converted to method reference :

    for (String s : (Iterable<String>) stream::iterator) {
        writer.append(s);
    }

Explicit cast can be avoided with intermediate variable or method param:

    Iterable<String> iterable = stream::iterator;
    for (String s : iterable) {
        writer.append(s);
    }

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There is also StreamEx library in maven central that features iterable streams and other perks out of the box.

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Here are some most popular questions and approaches that provide workarounds on checked exception handling within lambdas and streams:

Java 8 Lambda function that throws exception?

Java 8: Lambda-Streams, Filter by Method with Exception

How can I throw CHECKED exceptions from inside Java 8 streams?

Java 8: Mandatory checked exceptions handling in lambda expressions. Why mandatory, not optional?

jOOλ Unchecked

Lombok @SneakyThrows

Kotlin ;)

Answer 2

Stream does not implement Iterable, nor is it an array, so it is not eligible for use in an enhanced for loop. The reason it does not implement Iterable is because it is a single use data structure. Each time Iterable.iterator is called, a new Iterator should be returned which covers all of the elements. The iterator method on Stream just reflects the current state of the Stream. It is effectively a different view of the Stream.

You could make a class which implements Iterable by wrapping a stream for use in the enhanced for loop. But this is a questionable idea as you cannot truly implement Iterable (as described above).

Answer 3

I wrote an extension to the Stream API which allows for checked exceptions to be thrown.

Stream<String> stream = Stream.of("1", "2", "3");
Writer writer = new FileWriter("example.txt");

ThrowingStream.of(stream, IOException.class).forEach(writer::append);

Answer 4

You can also do it like this:

Path inputPath = Paths.get("...");
Stream<Path> stream = Files.list(inputPath);

for (Iterator<Path> iterator = stream.iterator(); iterator.hasNext(); )
{
    Path path = iterator.next();
    // ...
}