How to throw an exception inside a Lambda?

How do you throw an exception inside a lambda e.g in ifPresentOrElse of a Optional?

try {
   foo.bar(baz).ifPresentOrElse(theBar -> { 
      // A code inside here needs to throw a custom exception so the outside can handle it
      // It can't throw atm, and it is an unhandled exception here
   }, () -> { response.set(notFound()); }
} catch(CustomException e) {
  somethingImportantWhenExceptionIsThrown();
}

Altough not explicitly mentioned in your question, i am going to assume that you intent to throw a checked exception from ifPresentOrElse.

First, you should try to understand why you cannot throw an exception. The Consumer interface has, as Runnable, only a single abstract method, which makes it a @FunctionalInterface. From the documentation:

> Note that instances of functional interfaces can be created with
> lambda expressions, method references, or constructor references.

This is a reduced version of the Consumer interface:

public interface Consumer<T> {
    void accept(T t); // <-- NO throws Exception
}

Alternatively, you can still use the old anonymous inner class:

Consumer<Object> consumer = new Consumer<>() {
    @Override
    public void accept(Object o) { // <-- NO throws Exception
       // consume
    }
};

Therefore, you cannot throw a checked exception from the accept method. It is not possible by design. You can, however, chose one of the following options:

• Use a custom ThrowingConsumer and ThrowingOptional class, which can throw exceptions (since you declare them in their method signatures), not recommended.
• Wrap the checked exception in a RuntimeException, which does not need to be declared in the method signature, and can therefore be thrown.
• Rewrite your code to not use the lambda expression.

Example, for the last point:

Optional<Bar> barOpt = foo.bar(baz);
if(bar.isPresent()) {
    Bar bar = barOpt.get();
    try {
        // code that throws checked exception
    } catch(CustomException e) {
        somethingImportantWhenExceptionIsThrown();
    }
} else {
    response.set(notFound());
} 

The solution for this is to use Project Lombok:

And have the method wrapping this function annotated with SneakyThrows, as such:

@SneakyThrows
void method() {
   try {
       foo.bar(baz).ifPresentOrElse(theBar -> { 
          throw()
       }, () -> { response.set(notFound()); }
   } catch(CustomException e) {
      somethingImportantWhenExceptionIsThrown();
   }
}

You can do it with apache commons-lang3 library.

https://commons.apache.org/proper/commons-lang/javadocs/api-release/org/apache/commons/lang3/function/Failable.html

Optional.of(obj).ifPresentOrElse(Failable.asConsumer(theBar -> {
    throw new Exception();
}), () -> {});