Java 8 Lambda function that throws exception?

Question

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 1

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);
          }
      };