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

Question

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

In other words, I want to make code like this compile:

public List

        

Answer 1

Probably, a better and more functional way is to wrap exceptions and propagate them further in the stream. Take a look at the Try type of Vavr for example.

Example:

interface CheckedFunction {
    O apply(I i) throws Exception; }

static  Function unchecked(CheckedFunction f) {
    return i -> {
        try {
            return f.apply(i);
        } catch(Exception ex) {

            throw new RuntimeException(ex);
        }
    } }

fileNamesToRead.map(unchecked(file -> Files.readAllLines(file)))

OR

@SuppressWarnings("unchecked")
private static  T throwUnchecked(Exception e) throws E {
    throw (E) e;
}

static  Function unchecked(CheckedFunction f) {
    return arg -> {
        try {
            return f.apply(arg);
        } catch(Exception ex) {
            return throwUnchecked(ex);
        }
    };
}

2nd implementation avoids wrapping the exception in a RuntimeException. throwUnchecked works because almost always all generic exceptions are treated as unchecked in java.