We solve Sudoku by following certain routines in our mind. Thus we have to apply handful of patterns to solve each of these routines. I am taking just one of these patterns good enough for solving easy ones and delve in to detail.

1. Applying Design Patterns
Solving Sudoku, easy ones

2. Problem

3. Design Patterns
Problem Pattern Solution

4. Applying Patterns
• Break the problem down to common
known problems
• Apply Known Solutions

5. A Common Problem
• When a number is placed in a cell, the
adjacent cells loose the opportunity to have
this number
• How to make the adjacent cell react on
placement of a number?

6. A Known Solution
• Cells that are adjacent to a cell can be
made to observe and react on
placement of a number using Observer
Pattern

7. Setup
• To start with, each cell can have any value
between 1 and 9, inclusive, with equal
probability.
public class Cell {
private List values = new ArrayList();
private int row;
private int col;
// Add all possible entries (1 to 9) in each cell
public Cell(int row, int col) {
this.row = row;
this.col = col;
for (int n = 1; n <= 9; n++) {
values.add(new Integer(n));
}
}
}

8. Cell as Observable and Observer
public class Cell extends Observable implements Observer {
//…
public synchronized void addObserver(Cell[][] cells) {
//… loop thru and add adjacent cells as observers
super.addObserver(cells[i][j]);
//…
}
//notify observers while setting the known value in the cell
public void setValue(int value) {
super.notifyObservers(new Integer(value));
}
// Observe and remove the entry set in the observable
public void update(Observable o, Object arg) {
values.remove(arg);
}
}
• Observable implementation in Java has provision for adding
objects implementing Observer interface and notifying them by
calling the update method.

9. Demo
• Below is the code for creating a collection of cell and setting up
the values.
• It also demonstrates with a sample.
• http://code.google.com/p/observerpattern-
sudoku/source/browse/