Feature/bael 5609 static method (#12370)

* BAEL-5609: init

* BAEL-5609: update examples

* BAEL-5609: refactor

* BAEL-5609: delete car example

core-java-modules/core-java-function/src/main/java/com/baeldung/staticmethods/CustomStringUtils.java

@@ -0,0 +1,9 @@
+package com.baeldung.staticmethods;
+
+public final class CustomStringUtils {
+
+    private CustomStringUtils() {}
+
+    public static boolean isEmpty(CharSequence cs) { return cs == null || cs.length() == 0; }
+
+}

core-java-modules/core-java-function/src/main/java/com/baeldung/staticmethods/StaticCounter.java

@@ -0,0 +1,15 @@
+package com.baeldung.staticmethods;
+
+public class StaticCounter {
+
+    private static int counter = 0;
+
+    public static int incrementCounter() {
+        return ++counter;
+    }
+
+    public static int getCounterValue() {
+        return counter;
+    }
+
+}