list-ops: add to track

config.json

@@ -57,6 +57,7 @@
     "twelve-days",
     "sublist",
     "rectangles",
+    "matrix",
     "list-ops"
   ],
   "exercises": [
@@ -325,6 +326,11 @@
       "difficulty": 1,
       "topics": []
     },
+    {
+      "slug": "matrix",
+      "difficulty": 1,
+      "topics": []
+    },
     {
       "slug": "list-ops",
       "difficulty": 1,

exercises/list-ops/src/example/java/ListOps.java

@@ -3,10 +3,13 @@
 import java.util.Collection;
 import java.util.Collections;
 import java.util.List;
+import java.util.function.BiConsumer;
 import java.util.function.BiFunction;
 import java.util.function.BinaryOperator;
 import java.util.function.Predicate;
+import java.util.function.Supplier;
 import java.util.function.UnaryOperator;
+import java.util.stream.Collector;
 import java.util.stream.Collectors;
 import java.util.stream.Stream;
 
@@ -48,4 +51,16 @@ public static <T> List<T> concat(final List<T>... lists) {
                 .collect(Collectors.toList());
     }
 
+    public static <R,T> R collect(final List<T> list,
+            Supplier<R> supplier,
+            BiConsumer<R, ? super T> accumulator,
+            BiConsumer<R, R> combiner) {
+        return list.stream().collect(supplier, accumulator, combiner);
+    }
+
+    public static <R, T, A> R collect(final List<T> list,
+            Collector<T, A, R> collector) {
+        return list.stream().collect(collector);
+    }
+
 }