diff --git a/core-java-concurrency/src/main/java/com/baeldung/concurrent/daemon/NewThread.java b/core-java-concurrency/src/main/java/com/baeldung/concurrent/daemon/NewThread.java
index d742d3a55f..4d87978070 100644
--- a/core-java-concurrency/src/main/java/com/baeldung/concurrent/daemon/NewThread.java
+++ b/core-java-concurrency/src/main/java/com/baeldung/concurrent/daemon/NewThread.java
@@ -3,7 +3,6 @@ package com.baeldung.concurrent.daemon;
 public class NewThread extends Thread {
 
     public void run() {
-    	
     	long startTime = System.currentTimeMillis();
         while (true) {
             for (int i = 0; i < 10; i++) {
diff --git a/core-java/src/main/java/com/baeldung/nestedclass/Enclosing.java b/core-java/src/main/java/com/baeldung/nestedclass/Enclosing.java
new file mode 100644
index 0000000000..a9911538b0
--- /dev/null
+++ b/core-java/src/main/java/com/baeldung/nestedclass/Enclosing.java
@@ -0,0 +1,11 @@
+package com.baeldung.nestedclass;
+
+public class Enclosing {
+
+    public static class Nested {
+
+        public void test() {
+            System.out.println("Calling test...");
+        }
+    }
+}
diff --git a/core-java/src/main/java/com/baeldung/nestedclass/NewEnclosing.java b/core-java/src/main/java/com/baeldung/nestedclass/NewEnclosing.java
new file mode 100644
index 0000000000..c7e04e8600
--- /dev/null
+++ b/core-java/src/main/java/com/baeldung/nestedclass/NewEnclosing.java
@@ -0,0 +1,15 @@
+package com.baeldung.nestedclass;
+
+public class NewEnclosing {
+
+    void run() {
+        class Local {
+
+            void run() {
+                System.out.println("Welcome to Baeldung!");
+            }
+        }
+        Local local = new Local();
+        local.run();
+    }
+}
\ No newline at end of file
diff --git a/core-java/src/main/java/com/baeldung/nestedclass/Outer.java b/core-java/src/main/java/com/baeldung/nestedclass/Outer.java
new file mode 100644
index 0000000000..ebd6d27293
--- /dev/null
+++ b/core-java/src/main/java/com/baeldung/nestedclass/Outer.java
@@ -0,0 +1,11 @@
+package com.baeldung.nestedclass;
+
+public class Outer {
+
+    public class Inner {
+
+        public void test() {
+            System.out.println("Calling test...");
+        }
+    }
+}
diff --git a/core-java/src/main/java/com/baeldung/nestedclass/SimpleAbstractClass.java b/core-java/src/main/java/com/baeldung/nestedclass/SimpleAbstractClass.java
new file mode 100644
index 0000000000..586e2d12b4
--- /dev/null
+++ b/core-java/src/main/java/com/baeldung/nestedclass/SimpleAbstractClass.java
@@ -0,0 +1,5 @@
+package com.baeldung.nestedclass;
+
+abstract class SimpleAbstractClass {
+    abstract void run();
+}
\ No newline at end of file
diff --git a/core-java/src/main/java/com/baeldung/util/StreamUtils.java b/core-java/src/main/java/com/baeldung/util/StreamUtils.java
new file mode 100644
index 0000000000..42f438732f
--- /dev/null
+++ b/core-java/src/main/java/com/baeldung/util/StreamUtils.java
@@ -0,0 +1,16 @@
+package com.baeldung.util;
+
+import java.io.IOException;
+import java.io.InputStream;
+import java.io.StringWriter;
+
+import org.apache.commons.io.IOUtils;
+
+public class StreamUtils {
+
+    public static String getStringFromInputStream(InputStream input) throws IOException {
+        StringWriter writer = new StringWriter();
+        IOUtils.copy(input, writer, "UTF-8");
+        return writer.toString();
+    }
+}
diff --git a/core-java/src/main/resources/countries.properties b/core-java/src/main/resources/countries.properties
new file mode 100644
index 0000000000..3c1f53aded
--- /dev/null
+++ b/core-java/src/main/resources/countries.properties
@@ -0,0 +1,3 @@
+UK
+US
+Germany
diff --git a/core-java/src/test/java/com/baeldung/file/FileOutputStreamTest.java b/core-java/src/test/java/com/baeldung/file/FileOutputStreamTest.java
new file mode 100644
index 0000000000..451c1b4c4d
--- /dev/null
+++ b/core-java/src/test/java/com/baeldung/file/FileOutputStreamTest.java
@@ -0,0 +1,36 @@
+package com.baeldung.file;
+
+import static org.assertj.core.api.Assertions.assertThat;
+
+import java.io.FileInputStream;
+import java.io.FileOutputStream;
+import java.io.IOException;
+import java.io.PrintWriter;
+
+import org.junit.After;
+import org.junit.Test;
+
+import com.baeldung.util.StreamUtils;
+
+public class FileOutputStreamTest {
+
+    public static final String fileName = "src/main/resources/countries.properties";
+
+    @Test
+    public void whenAppendToFileUsingFileOutputStream_thenCorrect() throws Exception {
+        FileOutputStream fos = new FileOutputStream(fileName, true);
+        fos.write("Spain\r\n".getBytes());
+        fos.close();
+
+        assertThat(StreamUtils.getStringFromInputStream(
+          new FileInputStream(fileName)))
+          .isEqualTo("UK\r\n" + "US\r\n" + "Germany\r\n" + "Spain\r\n");
+    }
+
+    @After
+    public void revertFile() throws IOException {
+        PrintWriter writer = new PrintWriter(fileName);
+        writer.print("UK\r\n" + "US\r\n" + "Germany\r\n");
+        writer.close();
+    }
+}
diff --git a/core-java/src/test/java/com/baeldung/file/FileUtilsTest.java b/core-java/src/test/java/com/baeldung/file/FileUtilsTest.java
new file mode 100644
index 0000000000..9ee8726575
--- /dev/null
+++ b/core-java/src/test/java/com/baeldung/file/FileUtilsTest.java
@@ -0,0 +1,37 @@
+package com.baeldung.file;
+
+import static org.assertj.core.api.Assertions.assertThat;
+
+import java.io.File;
+import java.io.FileInputStream;
+import java.io.IOException;
+import java.io.PrintWriter;
+import java.nio.charset.StandardCharsets;
+
+import org.apache.commons.io.FileUtils;
+import org.junit.After;
+import org.junit.Test;
+
+import com.baeldung.util.StreamUtils;
+
+public class FileUtilsTest {
+
+    public static final String fileName = "src/main/resources/countries.properties";
+
+    @Test
+    public void whenAppendToFileUsingFiles_thenCorrect() throws IOException {
+        File file = new File(fileName);
+        FileUtils.writeStringToFile(file, "Spain\r\n", StandardCharsets.UTF_8, true);
+        
+        assertThat(StreamUtils.getStringFromInputStream(
+          new FileInputStream(fileName)))
+          .isEqualTo("UK\r\n" + "US\r\n" + "Germany\r\n" + "Spain\r\n");
+    }
+    
+    @After
+    public void revertFile() throws IOException {
+        PrintWriter writer = new PrintWriter(fileName);
+        writer.print("UK\r\n" + "US\r\n" + "Germany\r\n");
+        writer.close();
+    }
+}
diff --git a/core-java/src/test/java/com/baeldung/file/FileWriterTest.java b/core-java/src/test/java/com/baeldung/file/FileWriterTest.java
new file mode 100644
index 0000000000..8d2ce4310e
--- /dev/null
+++ b/core-java/src/test/java/com/baeldung/file/FileWriterTest.java
@@ -0,0 +1,40 @@
+package com.baeldung.file;
+
+import static org.assertj.core.api.Assertions.assertThat;
+
+import java.io.BufferedWriter;
+import java.io.FileInputStream;
+import java.io.FileWriter;
+import java.io.IOException;
+import java.io.PrintWriter;
+
+import org.junit.After;
+import org.junit.Test;
+
+import com.baeldung.util.StreamUtils;
+
+public class FileWriterTest {
+    
+    public static final String fileName = "src/main/resources/countries.properties";
+
+    @Test
+    public void whenAppendToFileUsingFileWriter_thenCorrect() throws IOException {
+        FileWriter fw = new FileWriter(fileName, true);
+        BufferedWriter bw = new BufferedWriter(fw);
+        bw.write("Spain");
+        bw.newLine();
+        bw.close();
+
+        assertThat(
+          StreamUtils.getStringFromInputStream(
+          new FileInputStream(fileName)))
+          .isEqualTo("UK\r\n" + "US\r\n" + "Germany\r\n" + "Spain\n");
+    }
+
+    @After
+    public void revertFile() throws IOException {
+        PrintWriter writer = new PrintWriter(fileName);
+        writer.print("UK\r\n" + "US\r\n" + "Germany\r\n");
+        writer.close();
+    }
+}
diff --git a/core-java/src/test/java/com/baeldung/file/FilesTest.java b/core-java/src/test/java/com/baeldung/file/FilesTest.java
new file mode 100644
index 0000000000..bd39d004d3
--- /dev/null
+++ b/core-java/src/test/java/com/baeldung/file/FilesTest.java
@@ -0,0 +1,36 @@
+package com.baeldung.file;
+
+import static org.assertj.core.api.Assertions.assertThat;
+
+import java.io.FileInputStream;
+import java.io.IOException;
+import java.io.PrintWriter;
+import java.nio.file.Files;
+import java.nio.file.Paths;
+import java.nio.file.StandardOpenOption;
+
+import org.junit.After;
+import org.junit.Test;
+
+import com.baeldung.util.StreamUtils;
+
+public class FilesTest {
+
+    public static final String fileName = "src/main/resources/countries.properties";
+
+    @Test
+    public void whenAppendToFileUsingFiles_thenCorrect() throws IOException {
+        Files.write(Paths.get(fileName), "Spain\r\n".getBytes(), StandardOpenOption.APPEND);
+
+        assertThat(StreamUtils.getStringFromInputStream(
+          new FileInputStream(fileName)))
+          .isEqualTo("UK\r\n" + "US\r\n" + "Germany\r\n" + "Spain\r\n");
+    }
+
+    @After
+    public void revertFile() throws IOException {
+        PrintWriter writer = new PrintWriter(fileName);
+        writer.print("UK\r\n" + "US\r\n" + "Germany\r\n");
+        writer.close();
+    }
+}
diff --git a/core-java/src/test/java/com/baeldung/file/GuavaTest.java b/core-java/src/test/java/com/baeldung/file/GuavaTest.java
new file mode 100644
index 0000000000..5a7ec6c4a8
--- /dev/null
+++ b/core-java/src/test/java/com/baeldung/file/GuavaTest.java
@@ -0,0 +1,48 @@
+package com.baeldung.file;
+
+import static org.assertj.core.api.Assertions.assertThat;
+
+import java.io.File;
+import java.io.FileInputStream;
+import java.io.IOException;
+import java.io.PrintWriter;
+
+import org.junit.After;
+import org.junit.Before;
+import org.junit.Test;
+
+import com.baeldung.util.StreamUtils;
+import com.google.common.base.Charsets;
+import com.google.common.io.CharSink;
+import com.google.common.io.FileWriteMode;
+import com.google.common.io.Files;
+
+public class GuavaTest {
+
+    public static final String fileName = "src/main/resources/countries.properties";
+    
+    @Before
+    public void setup() throws Exception {
+        PrintWriter writer = new PrintWriter(fileName);
+        writer.print("UK\r\n" + "US\r\n" + "Germany\r\n");
+        writer.close();
+    }
+
+    @Test
+    public void whenAppendToFileUsingGuava_thenCorrect() throws IOException {
+        File file = new File(fileName);
+        CharSink chs = Files.asCharSink(file, Charsets.UTF_8, FileWriteMode.APPEND);
+        chs.write("Spain\r\n");
+        
+        assertThat(StreamUtils.getStringFromInputStream(
+          new FileInputStream(fileName)))
+          .isEqualTo("UK\r\n" + "US\r\n" + "Germany\r\n" + "Spain\r\n");
+    }
+    
+    @After
+    public void revertFile() throws IOException {
+        PrintWriter writer = new PrintWriter(fileName);
+        writer.print("UK\r\n" + "US\r\n" + "Germany\r\n");
+        writer.close();
+    }
+}
diff --git a/core-java/src/test/java/com/baeldung/nestedclass/AnonymousInnerTest.java b/core-java/src/test/java/com/baeldung/nestedclass/AnonymousInnerTest.java
new file mode 100644
index 0000000000..394c0bb57a
--- /dev/null
+++ b/core-java/src/test/java/com/baeldung/nestedclass/AnonymousInnerTest.java
@@ -0,0 +1,16 @@
+package com.baeldung.nestedclass;
+
+import org.junit.Test;
+
+public class AnonymousInnerTest {
+
+    @Test
+    public void whenRunAnonymousClass_thenCorrect() {
+        SimpleAbstractClass simpleAbstractClass = new SimpleAbstractClass() {
+            void run() {
+                System.out.println("Running Anonymous Class...");
+            }
+        };
+        simpleAbstractClass.run();
+    }
+}
\ No newline at end of file
diff --git a/core-java/src/test/java/com/baeldung/nestedclass/InnerClassTest.java b/core-java/src/test/java/com/baeldung/nestedclass/InnerClassTest.java
new file mode 100644
index 0000000000..e9cb119ac2
--- /dev/null
+++ b/core-java/src/test/java/com/baeldung/nestedclass/InnerClassTest.java
@@ -0,0 +1,13 @@
+package com.baeldung.nestedclass;
+
+import org.junit.Test;
+
+public class InnerClassTest {
+
+    @Test
+    public void givenInnerClassWhenInstantiating_thenCorrect() {
+        Outer outer = new Outer();
+        Outer.Inner inner = outer.new Inner();
+        inner.test();
+    }
+}
diff --git a/core-java/src/test/java/com/baeldung/nestedclass/LocalClassTest.java b/core-java/src/test/java/com/baeldung/nestedclass/LocalClassTest.java
new file mode 100644
index 0000000000..dad19161ad
--- /dev/null
+++ b/core-java/src/test/java/com/baeldung/nestedclass/LocalClassTest.java
@@ -0,0 +1,12 @@
+package com.baeldung.nestedclass;
+
+import org.junit.Test;
+
+public class LocalClassTest {
+
+    @Test
+    public void whenTestingLocalClass_thenCorrect() {
+        NewEnclosing newEnclosing = new NewEnclosing();
+        newEnclosing.run();
+    }
+}
diff --git a/core-java/src/test/java/com/baeldung/nestedclass/NestedClassTest.java b/core-java/src/test/java/com/baeldung/nestedclass/NestedClassTest.java
new file mode 100644
index 0000000000..16c883689a
--- /dev/null
+++ b/core-java/src/test/java/com/baeldung/nestedclass/NestedClassTest.java
@@ -0,0 +1,12 @@
+package com.baeldung.nestedclass;
+
+import org.junit.Test;
+
+public class NestedClassTest {
+
+    @Test
+    public void whenInstantiatingStaticNestedClass_thenCorrect() {
+        Enclosing.Nested nested = new Enclosing.Nested();
+        nested.test();
+    }
+}
diff --git a/core-java/src/test/java/com/baeldung/nestedclass/NewOuterTest.java b/core-java/src/test/java/com/baeldung/nestedclass/NewOuterTest.java
new file mode 100644
index 0000000000..e883687d33
--- /dev/null
+++ b/core-java/src/test/java/com/baeldung/nestedclass/NewOuterTest.java
@@ -0,0 +1,32 @@
+package com.baeldung.nestedclass;
+
+import static org.junit.Assert.assertEquals;
+import org.junit.Test;
+
+public class NewOuterTest {
+
+    int a = 1;
+    static int b = 2;
+
+    public class InnerClass {
+        int a = 3;
+        static final int b = 4;
+
+        @Test
+        public void whenShadowing_thenCorrect() {
+            assertEquals(3, a);
+            assertEquals(4, b);
+            assertEquals(1, NewOuterTest.this.a);
+            assertEquals(2, NewOuterTest.b);
+            assertEquals(2, NewOuterTest.this.b);
+        }
+    }
+
+    @Test
+    public void shadowingTest() {
+        NewOuterTest outer = new NewOuterTest();
+        NewOuterTest.InnerClass inner = outer.new InnerClass();
+        inner.whenShadowing_thenCorrect();
+
+    }
+}
\ No newline at end of file
diff --git a/core-java/src/test/java/com/baeldung/string/StringTest.java b/core-java/src/test/java/com/baeldung/string/StringTest.java
index fe1a69aa23..e88b2d7c2c 100644
--- a/core-java/src/test/java/com/baeldung/string/StringTest.java
+++ b/core-java/src/test/java/com/baeldung/string/StringTest.java
@@ -223,4 +223,4 @@ public class StringTest {
 
         assertEquals("200", String.valueOf(l));
     }
-}
+}
\ No newline at end of file