Java 8 Lambda function that throws exception

javajava-8lambda

I know how to create a reference to a method that has a String parameter and returns an int, it's:

Function<String, Integer>

However, this doesn't work if the function throws an exception, say it's defined as:

Integer myMethod(String s) throws IOException

How would I define this reference?

Best Answer

You'll need to do one of the following.

  • If it's your code, then define your own functional interface that declares the checked exception:

    @FunctionalInterface
public interface CheckedFunction<T, R> {
   R apply(T t) throws IOException;
}

    and use it:

    void foo (CheckedFunction f) { ... }

  • Otherwise, wrap Integer myMethod(String s) in a method that doesn't declare a checked exception:

    public Integer myWrappedMethod(String s) {
    try {
        return myMethod(s);
    }
    catch(IOException e) {
        throw new UncheckedIOException(e);
    }
}

    and then:

    Function<String, Integer> f = (String t) -> myWrappedMethod(t);

    or:

    Function<String, Integer> f =
    (String t) -> {
        try {
           return myMethod(t);
        }
        catch(IOException e) {
            throw new UncheckedIOException(e);
        }
    };
Related Topic