Static ref non static (#15571)

* [rm-char-in-other-str] BAEL-7355 Remove characters from a string that are present in a second string

* [rm-char-in-other-str] shorten the package name

* [static-ref-non-static] static context ref non-static memebers

core-java-modules/core-java-lang-6/src/test/java/com/baeldung/staticnonstatic/StaticReferenceNonStaticMethodUnitTest.java

@@ -0,0 +1,60 @@
+package com.baeldung.staticnonstatic;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+
+import org.junit.jupiter.api.Test;
+
+class ToolBox {
+
+    private String concat(String str1, String str2) {
+        return str1 + str2;
+    }
+
+    private static String concatStatic(String str1, String str2) {
+        return str1 + str2;
+    }
+
+    static String joinTwoStrings(String str1, String str2) {
+        // return concat(str1, str2); //<-- compilation error
+        return concatStatic(str1, str2);
+    }
+
+    static String creatingInstanceJoinTwoStrings(String str1, String str2) {
+        ToolBox toolBox = new ToolBox();
+        return toolBox.concat(str1, str2);
+    }
+
+    String instanceJoinTwoStrings(String str1, String str2) {
+        return concat(str1, str2);
+    }
+
+    String instanceCallStaticJoinTwoStrings(String str1, String str2) {
+        return concatStatic(str1, str2);
+    }
+}
+
+public class StaticReferenceNonStaticMethodUnitTest {
+
+    @Test
+    void whenCallingStaticMethodFromStaticContext_thenGetExpectedResult() {
+        assertEquals("ab", ToolBox.joinTwoStrings("a", "b"));
+    }
+
+    @Test
+    void whenCallingInstanceMethodByCreatingAnInstance_thenGetExpectedResult() {
+        assertEquals("ab", ToolBox.creatingInstanceJoinTwoStrings("a", "b"));
+    }
+
+    @Test
+    void whenCallingInstanceMethodFromAnInstance_thenGetExpectedResult() {
+        ToolBox toolBox = new ToolBox();
+        assertEquals("ab", toolBox.instanceJoinTwoStrings("a", "b"));
+    }
+
+    @Test
+    void whenCallingStaticMethodFromAnInstance_thenGetExpectedResult() {
+        ToolBox toolBox = new ToolBox();
+        assertEquals("ab", toolBox.instanceCallStaticJoinTwoStrings("a", "b"));
+    }
+
+}