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

Question

I upgraded to Java 8 and tried to replace a simple iteration through a Map with a new lamdba expression. The loop searches for null values and throws an exception if one is found. The old Java 7 code looks like this:

for (Map.Entry<String, String> entry : myMap.entrySet()) {     if(entry.getValue() == null) {         throw new MyException("Key '" + entry.getKey() + "' not found!");     } } 

And my attempt to convert this to Java 8 looks like this:

myMap.forEach((k,v) -> {     if(v == null) {         // OK         System.out.println("Key '" + k+ "' not found!");          // NOK! Unhandled exception type!         throw new MyException("Key '" + k + "' not found!");     } }); 

Can anyone explain why the throw statement not allowed here and how this could be corrected?

Eclipse's quick-fix suggestion does not look right to me... it simply surrounds the throw statement with a try-catch block:

myMap.forEach((k,v) -> {     if(v == null) {         try {             throw new MyException("Key '" + k + "' not found!");         }         catch (Exception e) {             e.printStackTrace();         }     } }); 

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(); });