Checked exceptions

Why it matters

IntStream.range(0, n).forEach(...) takes an IntConsumer, which cannot throw checked exceptions. The usual workaround wraps and unwraps.

IntStream.range(0, 3).forEach(i -> {
    try {
        Files.createFile(path(i));
    } catch (IOException e) {
        throw new UncheckedIOException(e);
    }
});

repeat(3, i -> Files.createFile(path(i)));
// throws IOException

How the type is inferred

repeat is declared <E extends Throwable> ... throws E. The compiler picks E from what the lambda throws:

• Throws nothing checked: E is RuntimeException; nothing to declare.
• Throws one checked type: E is that type, for example IOException.
• Throws several: E is their common supertype, often Exception. Catch inside the action if you need them apart.

A failure stops the loop

repeat(5, i -> {
    ran.add(i);
    if (i == 1) throw new IOException("ERROR!");
});
// ran == [0, 1]; the IOException reaches the caller

Keep going after a failure

To run every iteration regardless, catch inside the action and collect what failed.

List<IOException> failures = new ArrayList<>();
repeat(5, i -> {
    try {
        Files.createFile(path(i));
    } catch (IOException e) {
        failures.add(e);
    }
});