fixes JabRef#6068

fixed downloading file with same name

src/main/java/org/jabref/gui/fieldeditors/LinkedFileViewModel.java

@@ -38,6 +38,7 @@
 import org.jabref.logic.externalfiles.LinkedFileHandler;
 import org.jabref.logic.l10n.Localization;
 import org.jabref.logic.net.URLDownload;
+import org.jabref.logic.util.io.FileNameHandler;
 import org.jabref.logic.xmp.XmpPreferences;
 import org.jabref.logic.xmp.XmpUtilWriter;
 import org.jabref.model.database.BibDatabaseContext;
@@ -419,6 +420,7 @@ public BackgroundTask<Path> prepareDownloadTask(Path targetDirectory, URLDownloa
                     String suggestedTypeName = externalFileType.getName();
                     linkedFile.setFileType(suggestedTypeName);
                     String suggestedName = linkedFileHandler.getSuggestedFileName(externalFileType.getExtension());
+                    suggestedName = FileNameHandler.getNonOverWritingFileName(targetDirectory, suggestedName);
                     return targetDirectory.resolve(suggestedName);
                 })
                 .then(destination -> new FileDownloadTask(urlDownload.getSource(), destination))

src/main/java/org/jabref/logic/util/io/FileNameHandler.java

@@ -1,6 +1,10 @@
 package org.jabref.logic.util.io;
 
+import java.nio.file.Files;
+import java.nio.file.Path;
+import java.nio.file.Paths;
 import java.util.Arrays;
+import java.util.Optional;
 
 /**
  * This class is based on http://stackoverflow.com/a/5626340/873282
@@ -67,6 +71,38 @@ public static String cleanDirectoryName(String badFileName) {
         return cleanName.toString().trim();
     }
 
+    /**
+     * Returns an absolute path to a file which does not match with any existing file names
+     *
+     * @param targetDirectory The directory in which file name should be unique
+     * @param fileName Suggested name for the file
+     * @return a file-name such that it does not match any existing files in targetDirectory.
+     * */
+    public static String getNonOverWritingFileName(Path targetDirectory, String fileName) {
+        String absoluteName = targetDirectory.resolve(fileName)
+                                            .toString();
+
+        Optional<String> extensionOptional = FileUtil.getFileExtension(fileName);
+
+        // the suffix include the '.' , if extension is present Eg: ".pdf"
+        String extensionSuffix = "";
+        String fileNameWithoutExtension = fileName;
+
+        if(extensionOptional.isPresent()) {
+            extensionSuffix = '.'+extensionOptional.get();
+            fileNameWithoutExtension = fileName.substring(0, fileName.lastIndexOf('.'));
+        }
+
+        Path absolutePath = Paths.get(absoluteName);
+        String newFileName = fileName;
+
+        for(int counter = 1; Files.exists(absolutePath); ++counter) {
+            newFileName = fileNameWithoutExtension+" ("+counter+")"+extensionSuffix;
+            absolutePath = targetDirectory.resolve(newFileName);
+        }
+        return newFileName;
+    }
+
     private static boolean isCharLegal(char c) {
         return Arrays.binarySearch(FileNameHandler.ILLEGAL_CHARS, c) < 0;
     }

src/test/java/org/jabref/logic/util/io/FileNameHandlerTest.java

@@ -1,6 +1,11 @@
 package org.jabref.logic.util.io;
 
 import org.junit.jupiter.api.Test;
+import org.junit.jupiter.api.io.TempDir;
+
+import java.io.IOException;
+import java.nio.file.Files;
+import java.nio.file.Path;
 
 import static org.junit.jupiter.api.Assertions.assertEquals;
 
@@ -30,4 +35,20 @@ public void testCleanDirectoryNameForWindows() {
     public void testCleanCurlyBracesAsWell() {
         assertEquals("The Evolution of Sentiment_ Analysis_A Review of Research Topics, Venues, and Top Cited Papers.PDF", FileNameHandler.cleanFileName("The Evolution of Sentiment} Analysis}A Review of Research Topics, Venues, and Top Cited Papers.PDF"));
     }
+
+    @Test
+    public void test_getNonOverWritingFileName_returnsUniqueName(@TempDir Path tempDirectory) throws IOException {
+        String dummyFile1 = "default.txt";
+        String dummyFile2 = "default (1).txt";
+        String expectedFileName = "default (2).txt";
+
+        Files.createFile(tempDirectory.resolve(dummyFile1));
+        Files.createFile(tempDirectory.resolve(dummyFile2));
+
+        String outputFileName = FileNameHandler.getNonOverWritingFileName(tempDirectory, "default.txt");
+        assertEquals(expectedFileName, outputFileName);
+
+        Files.delete(tempDirectory.resolve(dummyFile1));
+        Files.delete(tempDirectory.resolve(dummyFile2));
+    }
 }