Java 8 Lambda function that throws exception?

Rocky Pulley picture Rocky Pulley · Aug 13, 2013 · Viewed 334k times · Source

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?

Answer

jason picture jason · Aug 13, 2013

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);
            }
        };