Methods have been added to be able to write a CSV file or a list of o…

…bjects to an Excel file that already exists

src/main/java/samples/convertCsvFileToExcelFile/Main.java

@@ -1,19 +1,30 @@
 package samples.convertCsvFileToExcelFile;
 
 import enums.Extension;
+import org.apache.poi.ss.usermodel.Workbook;
 import tools.Converter;
+import tools.WorkbookUtility;
 
 import java.io.File;
+import java.io.FileOutputStream;
 
 public class Main {
 
     public static void main(String[] args) {
 
         File csvFile = new File("./src/main/resources/employee.csv");
+        File csvFile2 = new File("./src/main/resources/employee_2.csv");
 
         try {
             System.out.println("Start the conversion...");
             File excelFile = Converter.csvToExcel(csvFile, "./src/main/resources/", "employee_2", Extension.XLSX);
+            System.out.println("First conversion completed...");
+
+            Workbook workbook = WorkbookUtility.open(excelFile);
+            Converter.csvToExistingExcel(workbook, csvFile2);
+            FileOutputStream fileOutputStream = new FileOutputStream(excelFile);
+            workbook.write(fileOutputStream);
+            WorkbookUtility.close(workbook, fileOutputStream);
             System.out.println("The file is ready. Path: " + excelFile.getAbsolutePath());
         } catch (Exception e) {
             System.err.println("There was an error. Check the console");

src/main/java/samples/convertObjectsToExcelFileSample/Main.java

@@ -1,13 +1,17 @@
 package samples.convertObjectsToExcelFileSample;
 
 import enums.Extension;
+import org.apache.poi.ss.usermodel.Workbook;
 import tools.Converter;
+import tools.WorkbookUtility;
 
 import java.io.File;
+import java.io.FileOutputStream;
 import java.time.LocalDate;
 import java.time.LocalDateTime;
 import java.util.ArrayList;
 import java.util.Date;
+import java.util.LinkedList;
 import java.util.List;
 
 public class Main {
@@ -18,9 +22,20 @@ public static void main(String[] args) {
         employees.add(new Employee("Rossi", "Mario", 25, LocalDate.of(1987, 5, 22), new Date(), 28000.00, LocalDateTime.now(), true));
         employees.add(new Employee("Verdi", "Giuseppe", 22, LocalDate.of(1991, 2, 23), new Date(), 23670.89, LocalDateTime.now(), false));
 
+        List<Office> offices = new LinkedList<>();
+        offices.add(new Office("Nocera Inferiore", "Salerno", 40));
+        offices.add(new Office("Pero", "Milano", 73));
+
         try {
             System.out.println("Start the conversion...");
             File report = Converter.objectsToExcel(employees, Employee.class, "./src/main/resources/", "employee", Extension.XLSX, true);
+            System.out.println("First conversion completed...");
+
+            Workbook workbook = WorkbookUtility.open(report);
+            Converter.objectsToExistingExcel(workbook, offices, Office.class, true);
+            FileOutputStream fileOutputStream = new FileOutputStream(report);
+            workbook.write(fileOutputStream);
+            WorkbookUtility.close(workbook, fileOutputStream);
             System.out.println("The file is ready. Path: " + report.getAbsolutePath());
         } catch (Exception e) {
             System.err.println("There was an error. Check the console");

src/main/java/samples/convertObjectsToExcelFileSample/Office.java

@@ -0,0 +1,22 @@
+package samples.convertObjectsToExcelFileSample;
+
+import annotations.ExcelBodyStyle;
+import annotations.ExcelField;
+import annotations.ExcelHeaderStyle;
+import lombok.AllArgsConstructor;
+import lombok.NoArgsConstructor;
+import lombok.ToString;
+
+@AllArgsConstructor
+@NoArgsConstructor
+@ToString
+@ExcelHeaderStyle(autoSize = true)
+@ExcelBodyStyle
+public class Office {
+    @ExcelField(name = "CITY")
+    private String city;
+    @ExcelField(name = "PROVINCE")
+    private String province;
+    @ExcelField(name = "NUMBER OF STATIONS")
+    private Integer numStations;
+}

src/main/java/tools/Converter.java

@@ -252,6 +252,42 @@ public static File objectsToExcel(List<?> objects, Class<?> clazz, String path,
 
         /* Create workbook and sheet */
         Workbook workbook = WorkbookUtility.create(extension);
+        objectsToExistingExcel(workbook, objects, clazz, writeHeader);
+
+        /* Write file */
+        FileOutputStream fileOutputStream = new FileOutputStream(file);
+        workbook.write(fileOutputStream);
+
+        /* Close file */
+        WorkbookUtility.close(workbook, fileOutputStream);
+
+        return file;
+    }
+
+    /**
+     * This method allows you to convert objects into a Sheet of a Workbook that already exists.<p>
+     * Note: This method does not call the "write" method of the workbook.<p>
+     * By default, the header is added if not specified
+     * @param workbook The {@code Workbook} to update
+     * @param objects The list of objects that will be converted into an Excel file
+     * @param clazz The class of the list elements
+     * @throws IllegalAccessException If a field or fields of the {@code clazz} could not be accessed
+     */
+    public static void objectsToExistingExcel(Workbook workbook, List<?> objects, Class<?> clazz) throws IllegalAccessException {
+        objectsToExistingExcel(workbook, objects, clazz, true);
+    }
+
+    /**
+     * This method allows you to convert objects into a Sheet of a Workbook that already exists.<p>
+     * Note: This method does not call the "write" method of the workbook.
+     * @param workbook The {@code Workbook} to update
+     * @param objects The list of objects that will be converted into an Excel file
+     * @param clazz The class of the list elements
+     * @param writeHeader If {@code true} it will write the header to the first line
+     * @throws IllegalAccessException If a field or fields of the {@code clazz} could not be accessed
+     */
+    public static void objectsToExistingExcel(Workbook workbook, List<?> objects, Class<?> clazz, Boolean writeHeader) throws IllegalAccessException {
+        /* Create sheet */
         Sheet sheet = SheetUtility.create(workbook, clazz.getSimpleName());
 
         Field[] fields = clazz.getDeclaredFields();
@@ -269,15 +305,6 @@ public static File objectsToExcel(List<?> objects, Class<?> clazz, String path,
             CellStyle bodyCellStyle = createBodyStyle(workbook, clazz);
             writeExcelBody(workbook, sheet, fields, object, cRow++, bodyCellStyle, clazz);
         }
-
-        /* Write file */
-        FileOutputStream fileOutputStream = new FileOutputStream(file);
-        workbook.write(fileOutputStream);
-
-        /* Close file */
-        WorkbookUtility.close(workbook, fileOutputStream);
-
-        return file;
     }
 
     /**
@@ -518,6 +545,50 @@ public static File csvToExcel(File fileInput, String path, String filename, Exte
 
         /* Create workbook and sheet */
         Workbook workbook = WorkbookUtility.create(extension);
+        csvToExistingExcel(workbook, csvReader);
+
+        /* Write file */
+        FileOutputStream fileOutputStream = new FileOutputStream(outputFile);
+        workbook.write(fileOutputStream);
+
+        /* Close file */
+        WorkbookUtility.close(workbook, fileOutputStream, csvReader);
+
+        return outputFile;
+    }
+
+    /**
+     * Convert the CSV file into a new sheet of an existing Workbook.<p>
+     * Note: This method does not call the "write" method of the workbook.
+     * @param workbook The {@code Workbook} to update
+     * @param fileInput The input CSV file that will be converted into an Excel file
+     * @throws IOException If an I/O error has occurred
+     * @throws CsvValidationException If the CSV file has invalid formatting
+     * @throws ExtensionNotValidException If the input file extension does not belong to a CSV file
+     */
+    public static void csvToExistingExcel(Workbook workbook, File fileInput) throws IOException, CsvValidationException, ExtensionNotValidException {
+        /* Check exension */
+        String csvExt = FilenameUtils.getExtension(fileInput.getName());
+        isValidCsvExtension(csvExt);
+
+        /* Open CSV file */
+        FileReader fileReader = new FileReader(fileInput);
+        CSVReader csvReader = new CSVReader(fileReader);
+        csvToExistingExcel(workbook, csvReader);
+
+        /* Close CSV reader */
+        csvReader.close();
+    }
+
+    /**
+     * Writes the data present in the CSVReader to a new sheet of an existing Workbook.<p>
+     * Note: This method does not call the "write" method of the workbook.
+     * @param workbook The {@code Workbook} to update
+     * @param csvReader The {@code CSVReader} of the CSV input file
+     * @throws CsvValidationException If the CSV file has invalid formatting
+     * @throws IOException If an I/O error has occurred
+     */
+    public static void csvToExistingExcel(Workbook workbook, CSVReader csvReader) throws CsvValidationException, IOException {
         Sheet sheet = SheetUtility.create(workbook);
 
         /* Read CSV file */
@@ -532,15 +603,6 @@ public static File csvToExcel(File fileInput, String path, String filename, Exte
             }
             cRow++;
         }
-
-        /* Write file */
-        FileOutputStream fileOutputStream = new FileOutputStream(outputFile);
-        workbook.write(fileOutputStream);
-
-        /* Close file */
-        WorkbookUtility.close(workbook, fileOutputStream, csvReader);
-
-        return outputFile;
     }
 
     private static void isValidCsvExtension(String extension) throws ExtensionNotValidException {

src/main/java/tools/ExcelUtility.java

@@ -11,7 +11,6 @@
 import org.apache.poi.ss.usermodel.Workbook;
 
 import java.io.File;
-import java.io.FileInputStream;
 import java.io.IOException;
 import java.util.LinkedList;
 import java.util.List;