First issue

.gitignore

@@ -1,4 +1,8 @@
-# ---> Java
+# Java
+bin/
+*.classpath
+*.project
+
 # Compiled class file
 *.class
 
@@ -24,3 +28,5 @@
 hs_err_pid*
 replay_pid*
 
+# do NOT ignores
+!FileMerger.jar

.settings/org.eclipse.core.resources.prefs

@@ -0,0 +1,6 @@
+eclipse.preferences.version=1
+encoding//src/main/java=UTF-8
+encoding//src/main/resources=UTF-8
+encoding//src/test/java=UTF-8
+encoding//src/test/resources=UTF-8
+encoding/<project>=UTF-8

.settings/org.eclipse.jdt.apt.core.prefs

@@ -0,0 +1,4 @@
+eclipse.preferences.version=1
+org.eclipse.jdt.apt.aptEnabled=true
+org.eclipse.jdt.apt.genSrcDir=target/generated-sources/annotations
+org.eclipse.jdt.apt.genTestSrcDir=target/generated-test-sources/test-annotations

.settings/org.eclipse.jdt.core.prefs

@@ -0,0 +1,10 @@
+eclipse.preferences.version=1
+org.eclipse.jdt.core.compiler.codegen.methodParameters=generate
+org.eclipse.jdt.core.compiler.codegen.targetPlatform=21
+org.eclipse.jdt.core.compiler.compliance=21
+org.eclipse.jdt.core.compiler.problem.enablePreviewFeatures=disabled
+org.eclipse.jdt.core.compiler.problem.forbiddenReference=warning
+org.eclipse.jdt.core.compiler.problem.reportPreviewFeatures=ignore
+org.eclipse.jdt.core.compiler.processAnnotations=enabled
+org.eclipse.jdt.core.compiler.release=enabled
+org.eclipse.jdt.core.compiler.source=21

.settings/org.eclipse.m2e.core.prefs

@@ -0,0 +1,4 @@
+activeProfiles=
+eclipse.preferences.version=1
+resolveWorkspaceProjects=true
+version=1

.settings/org.springframework.ide.eclipse.prefs

@@ -0,0 +1,2 @@
+boot.validation.initialized=true
+eclipse.preferences.version=1

src/FileMergerApp.java

@@ -0,0 +1,133 @@
+import javax.swing.*;
+import java.awt.*;
+import java.io.*;
+import java.nio.charset.StandardCharsets;
+import java.nio.file.Files;
+import java.text.SimpleDateFormat;
+import java.util.ArrayList;
+import java.util.Date;
+import java.util.List;
+
+public class FileMergerApp extends JFrame {
+	
+	private static final long serialVersionUID = 6160781397779652462L;
+	private final JTextArea textArea;
+    private final List<File> selectedFiles;
+    private File lastDirectory;
+
+    public FileMergerApp() {
+        setTitle("Multi File Merger");
+        setSize(600, 700);
+        setDefaultCloseOperation(EXIT_ON_CLOSE);
+        setLocationRelativeTo(null);
+
+        selectedFiles = new ArrayList<>();
+        textArea = new JTextArea();
+        textArea.setEditable(false);
+
+        JButton selectButton = new JButton("Select");
+        JButton mergeButton = new JButton("Merge");
+
+        selectButton.addActionListener(e -> selectFiles());
+        mergeButton.addActionListener(e -> mergeFiles());
+
+        JPanel buttonPanel = new JPanel();
+        buttonPanel.add(selectButton);
+        buttonPanel.add(mergeButton);
+
+        add(buttonPanel, BorderLayout.NORTH);
+        add(new JScrollPane(textArea), BorderLayout.CENTER);
+    }
+
+    private void selectFiles() {
+        JFileChooser chooser = new JFileChooser();
+
+        if (lastDirectory != null) {
+            chooser.setCurrentDirectory(lastDirectory);
+        }
+
+        chooser.setMultiSelectionEnabled(true);
+        chooser.setFileSelectionMode(JFileChooser.FILES_ONLY);
+
+        int result = chooser.showOpenDialog(this);
+        if (result != JFileChooser.APPROVE_OPTION) {
+            return;
+        }
+
+        File[] files = chooser.getSelectedFiles();
+        if (files.length == 0) {
+            return;
+        }
+
+        lastDirectory = chooser.getCurrentDirectory();
+
+        for (File file : files) {
+            selectedFiles.add(file);
+            textArea.append(file.getAbsolutePath());
+            textArea.append(System.lineSeparator());
+        }
+    }
+
+    private void mergeFiles() {
+        if (selectedFiles.isEmpty()) {
+            JOptionPane.showMessageDialog(
+                    this,
+                    "No files selected.",
+                    "Warning",
+                    JOptionPane.WARNING_MESSAGE
+            );
+            return;
+        }
+
+        String timestamp = new SimpleDateFormat("yyyyMMdd_HHmmss").format(new Date());
+        File outputFile = new File(
+                System.getProperty("user.dir"),
+                "SingleFile_" + timestamp + ".txt"
+        );
+
+        try (BufferedWriter writer = Files.newBufferedWriter(
+                outputFile.toPath(),
+                StandardCharsets.UTF_8
+        )) {
+            for (File file : selectedFiles) {
+                writer.write(file.getName());
+                writer.newLine();
+
+                Files.lines(file.toPath(), StandardCharsets.UTF_8)
+                        .forEach(line -> {
+                            try {
+                                writer.write(line);
+                                writer.newLine();
+                            } catch (IOException e) {
+                                throw new UncheckedIOException(e);
+                            }
+                        });
+
+                writer.newLine();
+            }
+
+            JOptionPane.showMessageDialog(
+                    this,
+                    "Merged file created:\n" + outputFile.getAbsolutePath(),
+                    "Success",
+                    JOptionPane.INFORMATION_MESSAGE
+            );
+
+            // cleanup after merge
+            selectedFiles.clear();
+            textArea.setText("");
+
+        } catch (IOException | UncheckedIOException ex) {
+            JOptionPane.showMessageDialog(
+                    this,
+                    "Error during merge:\n" + ex.getMessage(),
+                    "Error",
+                    JOptionPane.ERROR_MESSAGE
+            );
+        }
+    }
+
+    public static void main(String[] args) {
+        SwingUtilities.invokeLater(() -> new FileMergerApp().setVisible(true));
+    }
+}