[BAEL-7048] - Passing Strings By Reference in Java (#15047)

* feat: pass by reference java string

* fix: review

core-java-modules/core-java-strings/src/test/java/com/baeldung/passstringbyreference/Dummy.java

@@ -0,0 +1,14 @@
+package com.baeldung.passstringbyreference;
+
+public class Dummy {
+
+    String dummyString;
+
+    public String getDummyString() {
+        return dummyString;
+    }
+
+    public void setDummyString(String dummyString) {
+        this.dummyString = dummyString;
+    }
+}

core-java-modules/core-java-strings/src/test/java/com/baeldung/passstringbyreference/PassStringUnitTest.java

@@ -0,0 +1,85 @@
+package com.baeldung.passstringbyreference;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+import static org.junit.jupiter.api.Assertions.assertNull;
+
+import org.junit.jupiter.api.Test;
+
+class PassStringUnitTest {
+
+    @Test
+    void givenAString_whenPassedToVoidMethod_thenStringIsNotModified() {
+        String s = "hello";
+        concatStringWithNoReturn(s);
+        assertEquals("hello", s);
+    }
+
+    void concatStringWithNoReturn(String input) {
+        input += " world";
+        assertEquals("hello world", input);
+    }
+
+    @Test
+    void givenAString_whenPassedToMethodAndReturnNewString_thenStringIsModified() {
+        String s = "hello";
+        assertEquals("hello world", concatStringWithReturn(s));
+    }
+
+    String concatStringWithReturn(String input) {
+        return input + " world";
+    }
+
+    @Test
+    void givenAString_whenPassStringBuilderToVoidMethod_thenConcatNewStringOk() {
+        StringBuilder builder = new StringBuilder("hello");
+        concatWithStringBuilder(builder);
+
+        assertEquals("hello world", builder.toString());
+    }
+
+    void concatWithStringBuilder(StringBuilder input) {
+        input.append(" world");
+    }
+
+    @Test
+    void givenAString_whenPassStringBufferToVoidMethod_thenConcatNewStringOk() {
+        StringBuffer builder = new StringBuffer("hello");
+        concatWithStringBuffer(builder);
+
+        assertEquals("hello world", builder.toString());
+    }
+
+    void concatWithStringBuffer(StringBuffer input) {
+        input.append(" world");
+    }
+
+    @Test
+    void givenObjectWithStringField_whenSetDifferentValue_thenObjectIsModified() {
+        Dummy dummy = new Dummy();
+        assertNull(dummy.getDummyString());
+        modifyStringValueInInputObject(dummy, "hello world");
+        assertEquals("hello world", dummy.getDummyString());
+    }
+
+    void modifyStringValueInInputObject(Dummy dummy, String dummyString) {
+        dummy.setDummyString(dummyString);
+    }
+
+    @Test
+    void givenObjectWithStringField_whenSetDifferentValueWithStringBuilder_thenSetStringInNewObject() {
+        assertEquals("hello world", getDummy("hello", "world").getDummyString());
+    }
+
+    Dummy getDummy(String hello, String world) {
+        StringBuilder builder = new StringBuilder();
+
+        builder.append(hello)
+          .append(" ")
+          .append(world);
+
+        Dummy dummy = new Dummy();
+        dummy.setDummyString(builder.toString());
+
+        return dummy;
+    }
+}