BAEL-1309 Append Data to a File with Java

core-java/src/main/java/com/baeldung/file/FileOutputStreamTest.java

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+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();
+    }
+}

core-java/src/main/java/com/baeldung/file/FileUtilsTest.java

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+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();
+    }
+}

core-java/src/main/java/com/baeldung/file/FileWriterTest.java

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+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();
+    }
+}

core-java/src/main/java/com/baeldung/file/FilesTest.java

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+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();
+    }
+}

core-java/src/main/java/com/baeldung/file/GuavaTest.java

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+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();
+    }
+}

core-java/src/main/java/com/baeldung/util/StreamUtils.java

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+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();
+    }
+}