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/file/FileOutputStreamTest.java b/core-java/src/main/java/com/baeldung/file/FileOutputStreamTest.java
new file mode 100644
index 0000000000..d85c028dae
--- /dev/null
+++ b/core-java/src/main/java/com/baeldung/file/FileOutputStreamTest.java
@@ -0,0 +1,37 @@
+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.txt";
+
+    @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("");
+        writer.print("UK\r\n" + "US\r\n" + "Germany\r\n");
+        writer.close();
+    }
+}
diff --git a/core-java/src/main/java/com/baeldung/file/FileUtilsTest.java b/core-java/src/main/java/com/baeldung/file/FileUtilsTest.java
new file mode 100644
index 0000000000..6147653976
--- /dev/null
+++ b/core-java/src/main/java/com/baeldung/file/FileUtilsTest.java
@@ -0,0 +1,38 @@
+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.txt";
+
+    @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("");
+        writer.print("UK\r\n" + "US\r\n" + "Germany\r\n");
+        writer.close();
+    }
+}
diff --git a/core-java/src/main/java/com/baeldung/file/FileWriterTest.java b/core-java/src/main/java/com/baeldung/file/FileWriterTest.java
new file mode 100644
index 0000000000..67e4dfaf20
--- /dev/null
+++ b/core-java/src/main/java/com/baeldung/file/FileWriterTest.java
@@ -0,0 +1,42 @@
+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.txt";
+
+    @Test
+    public void whenAppendToFileUsingFileWriter_thenCorrect() throws IOException {
+        String fileName = "src/main/resources/countries.txt";
+        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\r\n");
+    }
+
+    @After
+    public void revertFile() throws IOException {
+        PrintWriter writer = new PrintWriter(fileName);
+        writer.print("");
+        writer.print("UK\r\n" + "US\r\n" + "Germany\r\n");
+        writer.close();
+    }
+}
diff --git a/core-java/src/main/java/com/baeldung/file/FilesTest.java b/core-java/src/main/java/com/baeldung/file/FilesTest.java
new file mode 100644
index 0000000000..0ca69db8ab
--- /dev/null
+++ b/core-java/src/main/java/com/baeldung/file/FilesTest.java
@@ -0,0 +1,37 @@
+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.txt";
+
+    @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("");
+        writer.print("UK\r\n" + "US\r\n" + "Germany\r\n");
+        writer.close();
+    }
+}
diff --git a/core-java/src/main/java/com/baeldung/file/GuavaTest.java b/core-java/src/main/java/com/baeldung/file/GuavaTest.java
new file mode 100644
index 0000000000..06379451f9
--- /dev/null
+++ b/core-java/src/main/java/com/baeldung/file/GuavaTest.java
@@ -0,0 +1,41 @@
+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.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.txt";
+
+    @Test
+    public void whenAppendToFileUsingFileWriter_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("");
+        writer.print("UK\r\n" + "US\r\n" + "Germany\r\n");
+        writer.close();
+    }
+}
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/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