david-message / throwing-lambdas

Wrappers for methods/interfaces/lambdas throwing checked exceptions for use in functional interfaces

Read me first

This project is licensed under both LGPLv3 and ASL 2.0. See file LICENSE for more details.

Requires Java 8.

No further dependencies than the JRE itself is required.

You might also be interested in ThrowingStream.

What this is

This package allows you to use lambdas, methods or interfaces whose only impediment to their usages as functional interfaces is the fact that they throw one or more exception(s).

All functional interfaces used in Streams are covered, along with Runnable.

The current version is 0.5.0. It is available on Maven central:

    compile(group: "com.github.fge", name: "throwing-lambdas", version: "0.5.0");

Sample usage

For instance, let's suppose you want to sum the size of all regular files in a file tree.

The JDK provides Files.walk() to walk a file tree; you can therefore filter that stream to list only regular files and obtain the size using Files.size()...

Except no you cannot; Files.size() throws an IOException... Therefore you have to write this:

public static long totalSize(final Path baseDir)
    throws IOException
{
    try (
        final Stream<Path> stream = Files.walk(baseDir);
    ) {
        return stream.filter(Files::isRegularFile)
            .mapToLong(path -> {
                try {
                    return Files.size(path);
                } catch (IOException e) {
                    throw new RuntimeException e;
                }
            })
            .sum();
    }
}

You cannot throw checked exceptions from stream and this means writing a lot of boilerplate code like this, which can quickly become unmaintainable. This package, instead, allows you to write this:

public static long totalSize(final Path baseDir)
    throws IOException
{
    try (
        final Stream<Path> stream = Files.walk(baseDir);
    ) {
        return stream.filter(Files::isRegularFile)
            .mapToLong(Throwing.toLongFunction(Files::size))
            .sum();
    }
}

You can do more; for instance:

// Throw a custom RuntimeException instead of the default ThrownByLambdaException:
Throwing.function(Statement::executeQuery).orThrow(MyException.class);

// Try with another throwing lambda if the first throws an exception, and if both fail launch a
// custom exception. Both methods below "are" BinaryOperator<Path>s:
Throwing.binaryOperator(Files::createLink).orTryWith(Files::copy)
    .orThrow(UncheckedIOException.class);

And you can do even more than that. Read this page for more information.

Further reading

If you want to see how this works, see this page.

There is also a FAQ available.