How would you convert a checked-exception-throwing method into a lambda usable with a standard functional interface (which doesn't declare any checked exceptions), given standard interfaces like Function don't permit checked exceptions in their abstract method signature?

Since none of the standard java.util.function interfaces declare a checked exception in their functional method's signature, a lambda body that calls a method throwing a checked exception won't compile directly against one of these interfaces -- common workarounds include wrapping the checked exception in an unchecked one inside the lambda body (catching and rethrowing as a RuntimeException), or defining a custom functional interface whose method DOES declare the checked exception (usable in contexts where you control what accepts it), or using a utility 'sneaky throw' technique (an unusual, somewhat controversial approach exploiting generics type erasure to throw a checked exception without a throws declaration).

// Standard Function doesn't allow this to compile directly:
// Function<String, byte[]> reader = Files::readAllBytes;  // COMPILE ERROR: IOException is checked

// Workaround: wrap the checked exception as unchecked inside the lambda
Function<String, byte[]> reader = path -> {
    try {
        return Files.readAllBytes(Path.of(path));
    } catch (IOException e) {
        throw new UncheckedIOException(e);  // wraps checked as unchecked, now compiles fine
    }
};