diff --git a/core-java-modules/core-java-12/src/test/java/com/baeldung/newfeatures/CompactNumbersUnitTest.java b/core-java-modules/core-java-12/src/test/java/com/baeldung/newfeatures/CompactNumbersUnitTest.java
new file mode 100644
index 0000000000..08a6d58d72
--- /dev/null
+++ b/core-java-modules/core-java-12/src/test/java/com/baeldung/newfeatures/CompactNumbersUnitTest.java
@@ -0,0 +1,21 @@
+package java.com.baeldung.newfeatures;
+
+import org.junit.Test;
+
+import java.text.NumberFormat;
+import java.util.Locale;
+
+import static org.junit.Assert.assertEquals;
+
+public class CompactNumbersUnitTest {
+
+    @Test
+    public void givenNumber_thenCompactValues() {
+        NumberFormat likesShort = NumberFormat.getCompactNumberInstance(new Locale("en", "US"), NumberFormat.Style.SHORT);
+        likesShort.setMaximumFractionDigits(2);
+        assertEquals("2.59K", likesShort.format(2592));
+        NumberFormat likesLong = NumberFormat.getCompactNumberInstance(new Locale("en", "US"), NumberFormat.Style.LONG);
+        likesLong.setMaximumFractionDigits(2);
+        assertEquals("2.59 thousand", likesShort.format(2592));
+    }
+}
diff --git a/core-java-modules/core-java-12/src/test/java/com/baeldung/newfeatures/FileMismatchUnitTest.java b/core-java-modules/core-java-12/src/test/java/com/baeldung/newfeatures/FileMismatchUnitTest.java
new file mode 100644
index 0000000000..7f081fe399
--- /dev/null
+++ b/core-java-modules/core-java-12/src/test/java/com/baeldung/newfeatures/FileMismatchUnitTest.java
@@ -0,0 +1,32 @@
+package java.com.baeldung.newfeatures;
+
+import org.junit.Test;
+
+import java.io.IOException;
+import java.nio.file.Files;
+import java.nio.file.Path;
+
+import static org.junit.Assert.assertEquals;
+
+public class FileMismatchUnitTest {
+
+    @Test
+    public void givenIdenticalFiles_thenShouldNotFindMismatch() throws IOException {
+        Path filePath1 = Files.createTempFile("file1", ".txt");
+        Path filePath2 = Files.createTempFile("file2", ".txt");
+        Files.writeString(filePath1, "Java 12 Article");
+        Files.writeString(filePath2, "Java 12 Article");
+        long mismatch = Files.mismatch(filePath1, filePath2);
+        assertEquals(-1, mismatch);
+    }
+
+    @Test
+    public void givenDifferentFiles_thenShouldFindMismatch() throws IOException {
+        Path filePath3 = Files.createTempFile("file3", ".txt");
+        Path filePath4 = Files.createTempFile("file4", ".txt");
+        Files.writeString(filePath3, "Java 12 Article");
+        Files.writeString(filePath4, "Java 12 Tutorial");
+        long mismatch = Files.mismatch(filePath3, filePath4);
+        assertEquals(8, mismatch);
+    }
+}
diff --git a/core-java-modules/core-java-12/src/test/java/com/baeldung/newfeatures/StringUnitTest.java b/core-java-modules/core-java-12/src/test/java/com/baeldung/newfeatures/StringUnitTest.java
new file mode 100644
index 0000000000..5ae51bd960
--- /dev/null
+++ b/core-java-modules/core-java-12/src/test/java/com/baeldung/newfeatures/StringUnitTest.java
@@ -0,0 +1,15 @@
+package java.com.baeldung.newfeatures;
+
+import org.junit.Test;
+
+import static org.junit.Assert.assertEquals;
+
+public class StringUnitTest {
+
+    @Test
+    public void givenString_thenRevertValue() {
+        String text = "Baeldung";
+        String transformed = text.transform(value -> new StringBuilder(value).reverse().toString());
+        assertEquals("gnudleaB", transformed);
+    }
+}
diff --git a/core-java-modules/core-java-12/src/test/java/com/baeldung/newfeatures/TeeingCollectorUnitTest.java b/core-java-modules/core-java-12/src/test/java/com/baeldung/newfeatures/TeeingCollectorUnitTest.java
new file mode 100644
index 0000000000..30a5cb40a7
--- /dev/null
+++ b/core-java-modules/core-java-12/src/test/java/com/baeldung/newfeatures/TeeingCollectorUnitTest.java
@@ -0,0 +1,18 @@
+package java.com.baeldung.newfeatures;
+
+import org.junit.Test;
+
+import java.util.stream.Collectors;
+import java.util.stream.Stream;
+
+import static org.junit.Assert.assertEquals;
+
+public class TeeingCollectorUnitTest {
+
+    @Test
+    public void givenSetOfNumbers_thenCalculateAverage() {
+        double mean = Stream.of(1, 2, 3, 4, 5)
+          .collect(Collectors.teeing(Collectors.summingDouble(i -> i), Collectors.counting(), (sum, count) -> sum / count));
+        assertEquals(3.0, mean);
+    }
+}