Merge pull request #13184 from edizor/master

Update README

core-java-modules/core-java-collections-4/README.md

@@ -15,3 +15,4 @@
 - [Guide to Java PriorityQueue](https://www.baeldung.com/java-priorityqueue)
 - [Java Generics PECS – Producer Extends Consumer Super](https://www.baeldung.com/java-generics-pecs)
 - [Reversing a Stack in Java](https://www.baeldung.com/java-reversing-a-stack)
+- [Sorting a HashSet in Java](https://www.baeldung.com/java-sort-hashset)

core-java-modules/core-java-concurrency-collections-2/README.md

@@ -3,4 +3,5 @@
 - [Introduction to Lock Striping](https://www.baeldung.com/java-lock-stripping)
 - [Guide to the Java TransferQueue](http://www.baeldung.com/java-transfer-queue)
 - [Java Concurrent HashSet Equivalent to ConcurrentHashMap](https://www.baeldung.com/java-concurrent-hashset-concurrenthashmap)
+- [Reading and Writing With a ConcurrentHashMap](https://www.baeldung.com/concurrenthashmap-reading-and-writing)
 - [[<-- Prev]](/core-java-modules/core-java-concurrency-collections)

core-java-modules/core-java-lambdas/README.md

@@ -9,3 +9,4 @@
 - [The Double Colon Operator in Java 8](https://www.baeldung.com/java-8-double-colon-operator)
 - [Serialize a Lambda in Java](https://www.baeldung.com/java-serialize-lambda)
 - [Convert Anonymous Class into Lambda in Java](https://www.baeldung.com/java-from-anonymous-class-to-lambda)
+- [When to Use Callable and Supplier in Java](https://www.baeldung.com/java-callable-vs-supplier)

core-java-modules/core-java-lang-5/README.md

@@ -11,3 +11,4 @@ This module contains articles about core features in the Java language
 - [Variable Instantiation on Declaration vs. on Constructor in Java](https://www.baeldung.com/java-variable-instantiation-declaration-vs-constructor)
 - [Infinity in Java](https://www.baeldung.com/java-infinity)
 - [Type Parameter vs Wildcard in Java Generics](https://www.baeldung.com/java-generics-type-parameter-vs-wildcard)
+- [Convert Between int and char in Java](https://www.baeldung.com/java-convert-int-char)

core-java-modules/core-java-string-operations-5/README.md

@@ -7,3 +7,4 @@
 - [Convert String to char in Java](https://www.baeldung.com/java-convert-string-to-char)
 - [Convert String to String Array](https://www.baeldung.com/java-convert-string-to-string-array)
 - [String Interpolation in Java](https://www.baeldung.com/java-string-interpolation)
+- [Guide to Splitting a String by Whitespace in Java](https://www.baeldung.com/java-splitting-a-string-by-whitespace)