Why can't I throw an exception in a Java 8 lambda expression?

Question

(This question already has answers here): question from:https://stackoverflow.com/questions/37726874/why-cant-i-throw-an-exception-in-a-java-8-lambda-expression

Answer 1

You are not allowed to throw checked exceptions because the accept(T t, U u) method in the java.util.function.BiConsumer<T, U> interface doesn't declare any exceptions in its throws clause. And, as you know, Map#forEach takes such a type.

public interface Map<K, V> {

    default void forEach(BiConsumer<? super K, ? super V> action) { ... }

}                        |
                         |
                         V
@FunctionalInterface
public interface BiConsumer<T, U> {

    void accept(T t, U u); // <-- does throw nothing

}

That is true when we are talking about checked exceptions. But you still can throw an unchecked exception (e.g. a java.lang.IllegalArgumentException):

new HashMap<String, String>()
    .forEach((a, b) -> { throw new IllegalArgumentException(); });