Java solution to check if a Sudoku solution is valid

Here's my solution to checking if a Sudoku board is valid. The algorithm checks the following,

• Rows add up to 45

• Columns add up to 45

• Rows have no duplicates

• Cols have no duplicates

• Every 3X3 sub-grid has no duplicate

Invite comments.

public class Checker {
    private int size;
    private int[][] board;

    public Checker(int size){
        this.size = size;
        board = new int[size][size];
    }

    private boolean rowCheck(){
        int sum = (size*(size+1))/2;
        int count = 0;
        for (int i = 0; i  set = new HashSet<>();
        for (int i = 0; i  set = new HashSet<>();
        for (int i = 0; i  set = new HashSet<>();
        for (int i = rowStart; i < rowFinish  ; i++) {
            for (int j =colStart ; j < colFinish; j++) {
                set.add(board[i][j]);
            }
            if(set.size() != 9) return false;

        }
        return true;
    }
    public boolean isValid(){

        return(rowCheck() && columnCheck() && checkRowDuplicates() && checkColumnDuplicates() && checkSubBoardDuplicates());

    }

    public void trace(){
        for (int i = 0; i < size; i++) {
            for (int j = 0; j < size; j++) {
                System.out.print(board[i][j] + " ");
            }
            System.out.println();
        }
    }
}

Your setting of count = 0 should be immediately before the appropriate for loop, not after it, and I would actually call that variable sum (renaming your sum to target) since it's not really a "count" at all, e.g:

private boolean rowCheck(){
    int target = size * (size + 1) / 2;
    for (int i = 0; i <size; i++) {
        int sum = 0;        // declaration moved, too
        for (int j = 0; j < size ; j++) {
            sum += board[i][j];
        }
        if (sum != target) return false;
    }
    return true;
}

The point being that you should be setting the variable in advance of the loop, not resetting it afterwards (that being a pointless operation on the final iteration).

I would probably also move target into a class member, calculated in the constructor.

StackExchange Code Review Q#115912, answer score: 4