Add peek methods (#127)

changelog/@unreleased/pr-127.v2.yml

@@ -0,0 +1,5 @@
+type: feature
+feature:
+  description: Add peek methods to KeyedStream
+  links:
+  - https://github.com/palantir/streams/pull/127

streams/src/main/java/com/palantir/common/streams/KeyedStream.java

@@ -46,6 +46,14 @@
  */
 public interface KeyedStream<K, V> {
 
+    /**
+     * Returns a keyed stream consisting of elements in this stream,
+     * performing the provided action on each element of this stream.
+     */
+    default KeyedStream<K, V> peek(BiConsumer<K, V> consumer) {
+        return new KeyedStreamImpl<>(entries().peek(entry -> consumer.accept(entry.getKey(), entry.getValue())));
+    }
+
     /**
      * Returns a keyed stream consisting of the entries of this stream whose values match
      * the given predicate.

streams/src/main/java/com/palantir/common/streams/KeyedStreamImpl.java

@@ -53,16 +53,14 @@ public <M extends Multimap<K, V>> M collectToMultimap(Supplier<M> supplier) {
     @Override
     public <K2, V2> KeyedStream<K2, V2> mapEntries(
             BiFunction<? super K, ? super V, ? extends Entry<? extends K2, ? extends V2>> entryMapper) {
-        return new KeyedStreamImpl<K2, V2>(entries.<Map.Entry<? extends K2, ? extends V2>>map(
-                entry -> entryMapper.apply(entry.getKey(), entry.getValue())));
+        return new KeyedStreamImpl<>(entries.map(entry -> entryMapper.apply(entry.getKey(), entry.getValue())));
     }
 
     @Override
     public <K2, V2> KeyedStream<K2, V2> flatMapEntries(
             BiFunction<? super K, ? super V, ? extends Stream<? extends Entry<? extends K2, ? extends V2>>>
                     entryMapper) {
-        return new KeyedStreamImpl<K2, V2>(
-                entries.flatMap(entry -> entryMapper.apply(entry.getKey(), entry.getValue())));
+        return new KeyedStreamImpl<>(entries.flatMap(entry -> entryMapper.apply(entry.getKey(), entry.getValue())));
     }
 
     @Override