JAVA-1470 Move 10 articles to libraries-4 module

libraries-4/src/main/java/com/baeldung/distinct/DistinctWithJavaFunction.java

@@ -0,0 +1,15 @@
+package com.baeldung.distinct;
+
+import java.util.Map;
+import java.util.concurrent.ConcurrentHashMap;
+import java.util.function.Function;
+import java.util.function.Predicate;
+
+public class DistinctWithJavaFunction {
+
+    public static <T> Predicate<T> distinctByKey(Function<? super T, ?> keyExtractor) {
+        Map<Object, Boolean> seen = new ConcurrentHashMap<>();
+        return t -> seen.putIfAbsent(keyExtractor.apply(t), Boolean.TRUE) == null;
+    }
+
+}