[FIXED] Compilation error while wrapping method throwing exception

Issue

Checked exceptions wrapped to throw Runtime exception in streams is failing with compilation error in JDK 11 and 12.

package com.mmk.test;

import java.net.URL;
import java.util.ArrayList;
import java.util.function.Function;

public class App {
    public static void main(String[] args) {
        var list = new ArrayList<String>();
        list.add("http://foo.com");
        list.stream().map(wrap(url -> new URL(url)));
    }

    static <T, R, E extends Throwable> Function<T, R>
    wrap(FunException<T, R, E> fn) {
        return t -> {
            try {
            return fn.apply(t);
            } catch (Throwable throwable) {
                throw new RuntimeException(throwable);
            }
        };
    }

    interface FunException<T, R, E extends Throwable> {
        R apply(T t);
    }
}

Expected: No compilation error and no output. Actual: Compilation error, Unhandled Exception.

Solution

The FunException::apply method does not declare any exceptions to be thrown, so when you create a lambda that can throw an exception

url -> new URL(url)

The compiler complains because you are not handling this exception, and FunException::apply does not declare it in it's throws clause either.

You can make it work by adding a throws clause, i.e.:

interface FunException<T, R, E extends Throwable> {
    R apply(T t) throws E;
}

Answered By - Jorn Vernee
Answer Checked By - Senaida (JavaFixing Volunteer)