BAEL-5852: What does the Holder<T> do in Java? (#14477)

Co-authored-by: Vladyslav Chernov <vlad.chrv@gmail.com>

core-java-modules/core-java-lang-oop-generics/src/main/java/com/baeldung/holder/Holder.java

@@ -0,0 +1,9 @@
+package com.baeldung.holder;
+
+public class Holder<T> {
+    public T value;
+
+    public Holder(T value) {
+        this.value = value;
+    }
+}

core-java-modules/core-java-lang-oop-generics/src/main/java/com/baeldung/holder/SupplierService.java

@@ -0,0 +1,13 @@
+package com.baeldung.holder;
+
+public class SupplierService {
+    public void getSupplierByZipCode(String zip, Holder<Boolean> resultHolder) {
+        // Let's pretend we did some work here to get the supplier
+        // And let's say all zip codes starting with "9" are valid, just for this example
+        if (zip.startsWith("9")) {
+            resultHolder.value = true;
+        } else {
+            resultHolder.value = false;
+        }
+    }
+}

core-java-modules/core-java-lang-oop-generics/src/test/java/com/baeldung/holder/SupplierServiceUnitTest.java

@@ -0,0 +1,31 @@
+package com.baeldung.holder;
+
+import static org.junit.jupiter.api.Assertions.assertFalse;
+import static org.junit.jupiter.api.Assertions.assertTrue;
+
+import org.junit.jupiter.api.Test;
+
+public class SupplierServiceUnitTest {
+
+    @Test
+    public void givenValidZipCode_whenGetSupplierByZipCode_thenTrue() {
+        SupplierService service = new SupplierService();
+        Holder<Boolean> resultHolder = new Holder<>(false);
+        String zipCode = "98682";
+
+        service.getSupplierByZipCode(zipCode, resultHolder);
+
+        assertTrue(resultHolder.value);
+    }
+
+    @Test
+    public void givenInvalidZipCode_whenGetSupplierByZipCode_thenFalse() {
+        SupplierService service = new SupplierService();
+        Holder<Boolean> resultHolder = new Holder<>(true);
+        String zipCode = "12345";
+
+        service.getSupplierByZipCode(zipCode, resultHolder);
+
+        assertFalse(resultHolder.value);
+    }
+}