diff -r f7433671fec5 -r 1c62c6009161 src/main/java/de/uapcore/sudoku/Field.java
--- a/src/main/java/de/uapcore/sudoku/Field.java	Sat Jul 25 15:56:48 2020 +0200
+++ b/src/main/java/de/uapcore/sudoku/Field.java	Mon Jul 27 10:56:17 2020 +0200
@@ -36,7 +36,7 @@
  * Cells are identified by zero-based indices from top-left to bottom-right.
  */
 public final class Field extends JPanel {
-    private SudokuTextField[][] cells;
+    private final SudokuTextField[][] cells;
 
     /**
      * Constructs a new 9x9 Sudoku grid.
@@ -124,6 +124,7 @@
      * @param x horizontal position
      * @param y vertical position
      * @param v the cells value
+     * @throws IllegalArgumentException if v is not between 0 and 9
      */
     public void setCellValue(int x, int y, int v) {
         cells[x][y].setValue(v);
@@ -198,6 +199,7 @@
      * @param x horizontal position from 0 to 2
      * @param y vertical position from 0 to 2
      * @return a two-dimensional array containing the square cell values
+     * @throws IllegalArgumentException if the coordinates are out of bounds
      */
     public int[][] getSquare(int x, int y) {
         if (x < 0 || x > 2 || y < 0 || y > 2) {
@@ -219,12 +221,13 @@
      *
      * @param y the row position
      * @return an array containing the row values
+     * @throws IllegalArgumentException if y is not between 0 and 8
      */
     public int[] getRow(int y) {
         if (y < 0 || y > 8) {
             throw new IllegalArgumentException("Invalid row number");
         }
-        int row[] = new int[9];
+        int[] row = new int[9];
 
         for (int x = 0; x < 9; x++) {
             row[x] = getCellValue(x, y);
@@ -238,12 +241,13 @@
      *
      * @param x the column position
      * @return an array containing the column values
+     * @throws IllegalArgumentException if x is not between 0 and 8
      */
     public int[] getColumn(int x) {
         if (x < 0 || x > 8) {
             throw new IllegalArgumentException("Invalid column number");
         }
-        int column[] = new int[9];
+        int[] column = new int[9];
 
         for (int y = 0; y < 9; y++) {
             column[y] = getCellValue(x, y);