Merge pull request #16872 from Tirth007/master

BAEL-8072 read last n lines from file

Author: Maiklins

core-java-modules/core-java-io-6/pom.xml

@@ -14,7 +14,11 @@
     </parent>
 
     <dependencies>
-      
+        <dependency>
+            <groupId>commons-io</groupId>
+            <artifactId>commons-io</artifactId>
+            <version>${commons-io.version}</version>
+        </dependency>
     </dependencies>
 
     <properties>

core-java-modules/core-java-io-6/src/test/java/com/baeldung/readlastlines/ReadLastLinesUnitTest.java

@@ -0,0 +1,94 @@
+package com.baeldung.readlastlines;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+
+import java.io.BufferedReader;
+import java.io.File;
+import java.io.IOException;
+import java.nio.charset.StandardCharsets;
+import java.nio.file.Files;
+import java.nio.file.Paths;
+import java.util.Collections;
+import java.util.List;
+import java.util.Scanner;
+import java.util.stream.Collectors;
+import java.util.stream.Stream;
+
+import org.apache.commons.io.FileUtils;
+import org.apache.commons.io.input.ReversedLinesFileReader;
+import org.junit.jupiter.api.Test;
+
+public class ReadLastLinesUnitTest {
+
+    private static final String FILE_PATH = "src/test/resources/data.txt";
+    private static final int TOTAL_LINES = 10;
+    private static final int LAST_LINES_TO_READ = 3;
+    private static final String OUTPUT_TO_VERIFY = "line 8\nline 9\nline 10";
+
+    @Test
+    public void givenFile_whenUsingBufferedReader_thenExtractedLastLinesCorrect() throws IOException {
+        try (BufferedReader br = Files.newBufferedReader(Paths.get(FILE_PATH))) {
+            for (int i = 0; i < (TOTAL_LINES - LAST_LINES_TO_READ); i++) {
+                br.readLine();
+            }
+            StringBuilder stringBuilder = new StringBuilder();
+            for (int i = (TOTAL_LINES - LAST_LINES_TO_READ); i < TOTAL_LINES; i++) {
+                stringBuilder.append(br.readLine()).append("\n");
+            }
+
+            assertEquals(stringBuilder.toString().trim(), OUTPUT_TO_VERIFY);
+        }
+    }
+
+    @Test
+    public void givenFile_whenUsingScanner_thenExtractedLastLinesCorrect() throws IOException {
+        try (Scanner scanner = new Scanner(new File(FILE_PATH))) {
+            for (int i = 0; i < (TOTAL_LINES - LAST_LINES_TO_READ); i++) {
+                scanner.nextLine();
+            }
+            StringBuilder stringBuilder = new StringBuilder();
+            for (int i = (TOTAL_LINES - LAST_LINES_TO_READ); i < TOTAL_LINES; i++) {
+                stringBuilder.append(scanner.nextLine()).append("\n");
+            }
+
+            assertEquals(stringBuilder.toString().trim(), OUTPUT_TO_VERIFY);
+        }
+    }
+
+    @Test
+    public void givenLargeFile_whenUsingFilesAPI_thenExtractedLastLinesCorrect() throws IOException{
+        try (Stream<String> lines = Files.lines(Paths.get(FILE_PATH))) {
+            Stream<String> remainingLines = lines.skip(TOTAL_LINES - LAST_LINES_TO_READ);
+
+            assertEquals(OUTPUT_TO_VERIFY, remainingLines.collect(Collectors.joining("\n")));
+        }
+    }
+
+    @Test
+    public void givenFile_whenUsingFileUtils_thenExtractedLastLinesCorrect() throws IOException{
+        File file = new File(FILE_PATH);
+        List<String> lines = FileUtils.readLines(file, "UTF-8");
+        StringBuilder stringBuilder = new StringBuilder();
+        for (int i = (TOTAL_LINES - LAST_LINES_TO_READ); i < TOTAL_LINES; i++) {
+            stringBuilder.append(lines.get(i)).append("\n");
+        }
+
+        assertEquals(OUTPUT_TO_VERIFY, stringBuilder.toString().trim());
+    }
+
+    @Test
+    public void givenFile_whenUsingReverseFileReader_thenExtractedLastLinesCorrect() throws IOException{
+        File file = new File(FILE_PATH);
+        try (ReversedLinesFileReader rlfReader = new ReversedLinesFileReader(file, StandardCharsets.UTF_8)) {
+            List<String> lastLines = rlfReader.readLines(LAST_LINES_TO_READ);
+            StringBuilder stringBuilder = new StringBuilder();
+            Collections.reverse(lastLines);
+            lastLines.forEach(
+              line -> stringBuilder.append(line).append("\n")
+            );
+
+            assertEquals(OUTPUT_TO_VERIFY, stringBuilder.toString().trim());
+        }
+    }
+
+}

core-java-modules/core-java-io-6/src/test/resources/data.txt

@@ -0,0 +1,10 @@
+line 1
+line 2
+line 3
+line 4
+line 5
+line 6
+line 7
+line 8
+line 9
+line 10