A Sudoku JAVA Application

  • It’s the final course project of JAVA Programming, THU 2018 Fall.
  • I was a aficionado of Sudoku when I was little, so I implemented a Sudoku application, which can be used to learning, solving and generating sudoku.
  • The source code is available on my github page.


The app has 4 tabbed panels, Load History, Solve Problem, Example Display, Free Fill.

Load History
Solve Problem
Example Display
Free Fill


Load History

  • The history is name by the saved time.
  • User can choose whichever history record and jump to Solve Problem mode by click the Confirm button. The state of the probelm at the saved time will be recovered (Including the problem content, what the user filled and the used time. The time bar would show when you save it). Be free to continue it.
  • The selected history record can also be deleted by Delete button.

Solve Problem

  • One can get into this mode directly or by Confirm in Load History and Solve it! in Free Fill.
  • If you leave this panel, the stopwatch would pause. If you just jumpped to this panel, the boxes are invisible and the stopwatch is suspended. Clicking the Pause/Continue button can stop the stopwatch at any time, and when the stopwatch is suspended, the boxes are all invisible.
  • Click New Question and Start (it would become Pause/Continue later) button and start the game! The Difficulty bar can change the difficulty of the problem. But due to the questions are generated randomly, multi-solution may occur, so if you want to avoid this, DO NOT select something too difficult.
  • The boxes with numbers given by the question is blue and those with the wrong number input is red. The selected box and the unselected boxes are also be displayed in different colors.
  • Click a box (but not the blue ones) to active it, and click the number button at the bottom to put the number into it. If you selected Mark in the bottom right, you can fill anything in the active box.
  • Press Solve you can see all the solutions to this given scheme, but after seeing all the solutions, you are not allowed to continue to solve this :). If you really want, you can Reset, which clear all the filled numbers.
  • Press Hint, it would give you a hint at the active box. But under multi-solution cases, it would only provide a possible solution, which can be conflicted with what you have filled.

Example Display

Everytime you press New Scheme, you can see a newly generated filled scheme. Thanks to the algorithm, it can generate a new scheme very fast, but with the limitation that it cannot cover all the possible schemes.

Free Fill

  • The Pause/Contine, box display and number fill

Implement Details


  • NineBlockBox: create a panel with 9*9 buttons
  • NineBlockBoxFill: extends NineBlockBox; listen to press button. Implemented for Solve Mode and Free (Fill) Mode.
  • NineBlockBoxDisplay: extends NineBlockBox; display generated schemes. Implemented for Display Mode.
  • NumberInputPanel: input 1~9 number to the selected button in NineBlockBoxFill in Solve Mode and Free (Fill) Mode; can also choose to Mark UP.
  • HistoryLoaderPanel: load and display the files saved previously in Solve Mode, so that we can continue with the unsolved problems.
  • The implementation of some button is a little complex, so I wrote some class out of the MainGUI, e.g., QuestionGeneratorButton to generate new questions in Solve Mode, QuickSolveButton to solve the remaining blocks in Solve Mode and Free (Fill) Mode, ResetFilledButton to reset the filled blocks in Solve Mode and Free (Fill) Mode, SudokuGenerateButton to generate a new completed scheme in Display Mode, SaveButton to save the current box state in Solve Mode.
  • MainInterface: the main GUI. Organize all the mode (Load History, Solve Problem, Example Display, Free Fill) with a TabbedPanel.


  • Array99: use ArrayList[81] to realize 9*9 box, can also be declared with genericity to support further usage.

  • Array99Mother: the base class of Array99Solve, Array99Compute, Array99Generate.

  • Array99Solve: 9*9 box for the user to solve the sudoku question.

  • Array99Compute: 9*9 box for the app compute all the solutions of the sudoku. Because of the limited time, I only employed a Backtracking algorithm, which can be very time and space consuming sometimes. So the solve button in Free Fill Mode cannot really work temporarily, or the app would run into a long recursion. To accelerate the checking of whether a number has been filled in a row/column/box, the number i is represented by 2^(i-1), so that the checking process can be completed with bit operation.

  • Array99Generate: generate valid sudoku questions. As is shown in the above picture, I used an algorithm metioned here to generate a new scheme.

    generate method

    That’s very efficient, although it can be solved very quickly if know the scheme. In the Solve Problem Mode, the generate process also need to decide which blocks should be set to null for the user to fill. The difficulty SilerBar in the Solve Problem Panel decides a difficulty constant F. I used to chosen floor(F*81) blocks randomly to achieve that, but that’s too time consuming, for the already chosen blocks can be chosen at a very high possiblity. So I select every block with a possibility of F instead. Although the total selected blocks may not be that accurate, it’s considerably efficient.


  • StopWatch: a timer record class extends from Thread. It starts a sub-thread to count the consumed time from the start. It can also be controled to pause, reset or continue.
  • HistoryArrayLoader: it’s actually a file reader loading data from the chosen file in the Load History Panel. The sudoku origin data, the used time, the last modified time and the former color (presents its state) of the blocks are all recorded.