please complete this in Java

One variant of Sudoku has a 44 grid, with the digits 1,2,3, and 4 placed in them so that there are no duplicate digits in any row, column, or 22 subgrid as shown in the diagram below for an unsolved (left) puzzle and different solved puzzle (right): Your task is to implement a 44 Sudoku solver, using backtracking with recursion. Input The input consists of 4 lines, with each line containing 4 characters in the set {1,2,3,4,}. The "-" indicates a blank Sudoku cell. Output The Soduku solution must be output in the format shown below. If a solution is found, this format consists of 4 lines, each containing 4 characters in the set {1,2,3,4}. If no solution is found, output "invalid