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;
+    }
+
+}

core-java-modules/core-java-function/src/test/java/com/baeldung/staticmethods/CollectionUtilsUnitTest.java

@@ -0,0 +1,20 @@
+package com.baeldung.staticmethods;
+
+import org.junit.jupiter.api.Test;
+
+import java.util.Arrays;
+import java.util.Collections;
+import java.util.List;
+
+import static org.assertj.core.api.Assertions.assertThat;
+
+class CollectionUtilsUnitTest {
+
+    @Test
+    void givenListOfNumbers_whenReverseStaticMethodIsCalled_thenNumbersInReversedOrderAreReturned() {
+        List<String> list = Arrays.asList("1", "2", "3");
+        Collections.reverse(list);
+        assertThat(list).containsExactly("3", "2", "1");
+    }
+
+}

core-java-modules/core-java-function/src/test/java/com/baeldung/staticmethods/CustomStringUtilsUnitTest.java

@@ -0,0 +1,21 @@
+package com.baeldung.staticmethods;
+
+import org.junit.jupiter.api.Test;
+
+import static org.assertj.core.api.Assertions.assertThat;
+
+class CustomStringUtilsUnitTest {
+
+    @Test
+    void givenNonEmptyString_whenIsEmptyMethodIsCalled_thenFalseIsReturned() {
+        boolean empty = CustomStringUtils.isEmpty("baeldung");
+        assertThat(empty).isFalse();
+    }
+
+    @Test
+    void givenEmptyString_whenIsEmptyMethodIsCalled_thenTrueIsReturned() {
+        boolean empty = CustomStringUtils.isEmpty("");
+        assertThat(empty).isTrue();
+    }
+
+}

core-java-modules/core-java-function/src/test/java/com/baeldung/staticmethods/StaticCounterUnitTest.java

@@ -0,0 +1,16 @@
+package com.baeldung.staticmethods;
+
+import org.junit.jupiter.api.Test;
+
+import static org.assertj.core.api.Assertions.assertThat;
+
+class StaticCounterUnitTest {
+
+    @Test
+    void givenStaticCounter_whenIncrementCounterIsCalled_thenValueIsIncresedByOne() {
+        int oldValue = StaticCounter.getCounterValue();
+        int newValue = StaticCounter.incrementCounter();
+        assertThat(newValue).isEqualTo(oldValue + 1);
+    }
+
+}

core-java-modules/core-java-function/src/test/java/com/baeldung/staticmethods/StringUtilsUnitTest.java

@@ -0,0 +1,16 @@
+package com.baeldung.staticmethods;
+
+import org.apache.commons.lang3.StringUtils;
+import org.junit.jupiter.api.Test;
+
+import static org.assertj.core.api.Assertions.assertThat;
+
+class StringUtilsUnitTest {
+
+    @Test
+    void givenSimpleString_whenCapitalizeStaticMethodIsCalled_thenCapitalizedStringIsReturned() {
+        String str = StringUtils.capitalize("baeldung");
+        assertThat(str).isEqualTo("Baeldung");
+    }
+
+}