forEach loop Java 8 for Map entry set

Question

I'm trying to convert old conventional for each loop till java7 to java8's for each loop for a map entry set but I'm getting an error. Here's the code I'm trying to convert:

for (Map.Entry<String, String> entry : map.entrySet()) {         System.out.println("Key : " + entry.getKey() + " Value : " + entry.getValue());     } 

Here's the changes I have done:

map.forEach( Map.Entry<String, String> entry -> {        System.out.println("Key : " + entry.getKey() + " Value : " + entry.getValue());     });  

I tried doing this as well :

Map.Entry<String, String> entry;    map.forEach(entry -> {        System.out.println("Key : " + entry.getKey() + " Value : " + entry.getValue());     }); 

But still facing error. The error I'm getting for this is : Lambda expression's signature does not match the signature of the functional interface method accept(String, String)

Answer 1

Read the javadoc: Map<K, V>.forEach() expects a BiConsumer<? super K,? super V> as argument, and the signature of the BiConsumer<T, U> abstract method is accept(T t, U u).

So you should pass it a lambda expression that takes two inputs as argument: the key and the value:

map.forEach((key, value) -> {     System.out.println("Key : " + key + " Value : " + value); }); 

Your code would work if you called forEach() on the entry set of the map, not on the map itself:

map.entrySet().forEach(entry -> {     System.out.println("Key : " + entry.getKey() + " Value : " + entry.getValue()); });  

Answer 2

Maybe the best way to answer the questions like "which version is faster and which one shall I use?" is to look to the source code:

map.forEach() - from Map.java

default void forEach(BiConsumer<? super K, ? super V> action) {     Objects.requireNonNull(action);     for (Map.Entry<K, V> entry : entrySet()) {         K k;         V v;         try {             k = entry.getKey();             v = entry.getValue();         } catch(IllegalStateException ise) {             // this usually means the entry is no longer in the map.             throw new ConcurrentModificationException(ise);         }         action.accept(k, v);     } } 

javadoc

map.entrySet().forEach() - from Iterable.java

default void forEach(Consumer<? super T> action) {     Objects.requireNonNull(action);     for (T t : this) {         action.accept(t);     } } 

javadoc

This immediately reveals that map.forEach() is also using Map.Entry internally. So I would not expect any performance benefit in using map.forEach() over the map.entrySet().forEach(). So in your case the answer really depends on your personal taste :)

For the complete list of differences please refer to the provided javadoc links. Happy coding!