BAEL-4991 added in code for Java 9 private method introduction (#10884)

* BAEL-4991 added in code for Java 9 private method introduction

* BAEL-4991 added unit tests for java 9 private methods in interface article

Co-authored-by: Liam Garvie <liamgarvie@Liams-MacBook-Pro.local>

core-java-modules/core-java-9/src/main/java/com/baeldung/java9/interfaces/CustomFoo.java

@@ -0,0 +1,10 @@
+package com.baeldung.java9.interfaces;
+
+public class CustomFoo implements Foo {
+
+    public static void main(String... args) {
+        Foo customFoo = new CustomFoo();
+        customFoo.bar(); // 'Hello world!'
+        Foo.buzz(); // 'Hello static world!'
+    }
+}

core-java-modules/core-java-9/src/main/java/com/baeldung/java9/interfaces/Foo.java

@@ -0,0 +1,22 @@
+package com.baeldung.java9.interfaces;
+
+public interface Foo {
+
+    public default void bar() {
+        System.out.print("Hello");
+        baz();
+    }
+
+    public static void buzz() {
+        System.out.print("Hello");
+        staticBaz();
+    }
+
+    private void baz() {
+        System.out.print(" world!");
+    }
+
+    private static void staticBaz() {
+        System.out.print(" static world!");
+    }
+}

core-java-modules/core-java-9/src/test/java/com/baeldung/java9/interfaces/CustomFooUnitTest.java

@@ -0,0 +1,38 @@
+package com.baeldung.java9.interfaces;
+
+import org.junit.jupiter.api.AfterEach;
+import org.junit.jupiter.api.BeforeEach;
+import org.junit.jupiter.api.Test;
+
+import java.io.ByteArrayOutputStream;
+import java.io.PrintStream;
+
+import static org.assertj.core.api.Assertions.assertThat;
+
+class CustomFooUnitTest {
+    private ByteArrayOutputStream outContent = new ByteArrayOutputStream();
+    private PrintStream originalOut = System.out;
+
+    @BeforeEach
+    void setup() {
+        System.setOut(new PrintStream(outContent));
+    }
+
+    @AfterEach
+    void tearDown() {
+        System.setOut(originalOut);
+    }
+
+    @Test
+    void givenACustomFooObject_whenCallingDefaultMethodBar_thenExpectedStringIsWrittenToSystemOut() {
+        CustomFoo customFoo = new CustomFoo();
+        customFoo.bar();
+        assertThat(outContent.toString()).isEqualTo("Hello world!");
+    }
+
+    @Test
+    void givenAFooInterface_whenCallingStaticMethodBuzz_thenExpectedStringIsWrittenToSystemOut() {
+        Foo.buzz();
+        assertThat(outContent.toString()).isEqualTo("Hello static world!");
+    }
+}