java - 適切な Java 数独ジェネレーターを作成する際の問題

Question

だからここに私が得たものがあります

package sud.util;

import java.util.Random;

public class SudoGrid {

    Random gen;

    private int[][] grid;
    private int[] fail;
    private int[]   succes;

    public void init(){
        gen = new Random();
        grid = new int[9][9];
        fail = new int[3];
        succes = new int[3];

        fail[0] = 0;
        fail[1] = 0;
        fail[2] = 0;

        succes[0] = 0;
        succes[1] = 0;
        succes[2] = 0;
    }

    public int[][] generate() {

        for (int x = 0; x < 9; x++) {
            for (int y = 0; y < 9; y++) {

                boolean isValid = false;


                do{
                    int num = gen.nextInt(9) + 1;


                    if ((checkRow(num, x) != true
                            || checkCol(num, y) != true
                            ||checkSection(num, x, y) != true)){

                        //System.out.println("Row  failed: "+fail[0]+" times, and succeeded:"+succes[0]+" times");
                        //System.out.println("Col  failed: "+fail[1]+" times, and succeeded:"+succes[1]+" times");
                        //System.out.println("Sec failed: "+fail[2]+" times, and succeeded:"+succes[2]+" times\n----------------------------");
                        printBlock();

                    }else if ((checkRow(num, x) == true
                            && checkCol(num, y) == true
                            &&checkSection(num, x, y) == true)){




                        grid[x][y] = num;

                        isValid = true;
                    }

                }while(isValid == false);
            }
        }
        System.out.println("\n######################################################");
        System.out.println("########################CHEESE########################");
        System.out.println("######################################################\n");
        printBlock();
        return grid;
    }

    private boolean checkRow(int num, int row) {//Check a specific row

        boolean valid = true;
        for (int c = 0; c < 9; c++) {
            if (grid[row][c] == num) {
                valid = false;

               break;
            }
        }
        if(valid == false){
            fail[0]++;
        }else{
            succes[0]++;
        }

        return valid;
    }

    private boolean checkCol(int num, int col) {//Checks a specific col

        boolean valid = true;
        for (int r = 0; r < 9; r++) {
            if (grid[r][col] == num) {
                valid = false;

                break;
            }
        }
        if(valid == false){
            fail[1]++;
        }else{
            succes[1]++;
        }
        return valid;
    }

    private boolean checkSection(int num, int xPos, int yPos) {//Checks a 3x3 square

        int[][] section = new int[3][3];
        section = getSection(xPos, yPos);

        boolean valid = true;
        for (int i = 0; i < 3; i++) {
            for (int j = 0; j < 3; j++) {
                if (section[i][j] == num) {
                    valid = false;
                  break;
                }
            }
        }
        if(valid == false){
            fail[2]++;
        }else{
            succes[2]++;
        }
        return valid;
    }


    private int[][] getSection(int xPos, int yPos) {

        int xIndex = 0;
        int yIndex = 0;
        int[][] section = new int[3][3];

        if (xPos == 0 || xPos == 3 || xPos == 6) {
            xIndex = xPos;
        } else if (xPos == 1 || xPos == 4 || xPos == 7) {
            xIndex = xPos - 1;
        } else if (xPos == 2 || xPos == 5 || xPos == 8) {
            xIndex = xPos - 2;
        }

        if (yPos == 0 || yPos == 3 || yPos == 6) {
            yIndex = yPos;
        } else if (yPos == 1 || yPos == 4 || yPos == 7) {
            yIndex = yPos - 1;
        } else if (yPos == 2 || yPos == 5 || yPos == 8) {
            yIndex = yPos - 2;
        }

        int i = 0;
        int j = 0;

        for (int x = xIndex; x < 3; x++) {
            for (int y = yIndex; y < 3; y++) {
                section[x][y] = grid[i][j];
                i++;
            }
            j++;
        }

        return section;

    }

    public void printBlock() {
        String str = "";
        for(int i = 0; i < 9; i++) { 
            for(int j = 0; j < 9; j++) { 
                str += " " + (grid[i][j]);
            }
            str += "\n";
        }
        System.out.println(str);
    }
}

printBlock();しばらくすると戻ってくるのがこれ

9 5 6 1 3 4 8 7 2
7 1 3 8 5 2 9 6 4
6 8 5 7 4 1 3 2 9
3 7 8 6 9 5 1 4 0
0 0 0 0 0 0 0 0 0
0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
0 0 0 0 0 0
0 0
0 0 0 0

セクションチェックに何か問題がありますが、見つけられないようです。

御時間ありがとうございます。