@Override
	public Function<SudokuGrid, Integer> fitness() {
		return SudokuGrid::penalties;
	}
Fitness function penalizes issues rows, columns, and issues in sub-boards (3x3).
"Issues" mean repeated numbers.
It is one of the simplest fitness function for sudoku.
jenetics.example/src/main/java/io/jenetics/example/sudoku/SudokuProblem.java