How to print multiple parameters using Method reference in java8

Question

I'm trying to print out basic hashmap with twoin java.

Map<Integer, String> mp = new HashMap<Integer, String>();
mp.put(10, "apple");
mp.put(20, "orange");
mp.put(30, "banana");

But I can't figure out how to print multiple parameters, when it comes to method reference in java8.

I tried something like this. But it's giving me compile errors.

mp.forEach(System.out::println(i+" "+s););

Please help me to figure out this. Thank you.

Answer 1

Might be contradictory to other answers, yet I really don't see a need of you using a method reference here. IMHO,

mp.forEach((i, s) -> System.out.println(i + " " + s));

is far better than method reference for such a use case.

Answer 2

You can't. The language does not allow that, there is no implicit i and s there that can be passed to a method reference that way. What u can do, no idea why, but you could:

private static <K, V> void consumeBoth(K k, V v) {
     //Log how u want this
}

And use it with:

map.forEach(Yourclass::consumeBoth)

But this can be done in place with a lambda expression, I really see no benefit for this small example

Answer 3

You can write a separate method, for example:

public static <K, V> void printEntry(Map.Entry<K, V> e) {
    System.out.println(e.getKey() + " " + e.getValue());
}

map.entrySet().forEach(Demo::printEntry);

Or, if the Map.Entry<K, V>.toString() meets your requirements:

map.entrySet().forEach(System.out::println);

// 20=orange
// 10=apple
// 30=banana

Edit: Also, following @Holger's advice, you can safely omit the type parameters as long as the code inside the method doesn't depend on them:

public static void printEntry(Object k, Object v) {
    System.out.println(k + " " + v);
}

map.forEach(Demo::printEntry);

Answer 4

You cannot specify the whitespace by using the method reference System.out::println .
The argument passed to System.out::println is inferred by the BiConsumer parameter of Map.forEach(BiConsumer) .

But you could format the expected String with map() , in this way, the argument inferred in System.out::println would be the formatted string, what you need :

mp.entrySet()
  .stream()
  .map(e-> e.getKey() + " " + e.getValue())
  .forEach(System.out::println);

Answer 5

您还可以使用 entrySet 打印

 mp.entrySet().forEach(e->System.out.println(e.getKey()+"="+e.getValue()));

Answer 6

I finally found a solution, but using Apache API Pair class (ImmutablePair) in Java 8.

Stream.of(ImmutablePair.of("A", "1"), ImmutablePair.of("B", "0")) .collect(Collectors.toMap(Pair::getLeft, Pair::getRight));

Hope it helps.