The course project for JAVA and Object-Oriented Programming, THU, 2018 Fall
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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 demo, which can be used to learning, solving and generating sudoku.


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Load History Solve Problem
Example DisplayFree 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.

    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.