Dynamic Programming for board game in Java

BoardGame.java

import java.util.*;
import org.apache.commons.codec.digest.DigestUtils;

public class BoardGame {
    Random rnd;
    List<StringBuilder> board;
    int cols;
    int holes;
    List<Integer> hole_idx;
    List<String> memo;
    int hits;

    public static void main(String argv[]){
        BoardGame game = new BoardGame();
        if(argv.length>0){
            game.start_domino(argv);
        }
    }

    void start_domino(String argv[]){
        cols = Integer.parseInt(argv[0]);
        rnd = new Random();
        holes = rnd.nextInt((cols*cols+1)/2)*2;
        memo = new ArrayList<>();

        System.out.printf("%d x %d : # of holes(+) = %d\n",cols,cols,holes);
        board = new ArrayList<StringBuilder>();
        init_board();
        hole_idx = new ArrayList<>();
        make_hole_idx();
        make_board();
        print();
        if(domino(0,0)){
            System.out.printf("Completed. (# of hits = %d)\n",hits);
            print();
        }else{
            System.out.printf("Not completed... (# of hits = %d)\n",hits);
        }
    }

    void init_board(){
        for(int i=0;i<cols;i++){
            board.add(new StringBuilder(""));
            for(int j=0;j<cols;j++){
                board.get(i).append("_");
            }
        }
    }

    void make_hole_idx(){
        int x;
        while(hole_idx.size()<holes){
            x = rnd.nextInt(cols*cols);
            if(hole_idx.indexOf(x) == -1){
                hole_idx.add(x);
            }
        }
    }

    void make_board(){
        int x;
        for(int i=0;i<hole_idx.size();i++){
            x = hole_idx.get(i);
            board.get(x/cols).replace(x%cols,x%cols+1,"+");
        }
    }

    void print(){
        for(int i=0;i<cols;i++){
            for(int j=0;j<cols;j++){
                System.out.print(board.get(i).charAt(j)+" ");
            }
            System.out.println();
        }
        System.out.println();
    }

    boolean complete(){
        for(int i=0;i<cols;i++){
            for(int j=0;j<cols;j++){
                if(board.get(i).charAt(j) == '_'){
                    return false;
                }
            }
        }
        return true;
    }

    boolean check_redundancy(String hex){
        for(int i=0;i<memo.size();i++){
            //System.out.printf("%s <-> %s\n",hex,memo.get(i));
            if( hex.equals( memo.get(i) ) ){
                return true;
            }
        }
        return false;
    }

    List<StringBuilder> get_copy(){
        char c;
        List<StringBuilder> copy = new ArrayList<>();;
        for(int i=0;i<cols;i++){
            copy.add(new StringBuilder());
            for(int j=0;j<cols;j++){
                c = board.get(i).charAt(j);
                if(c == '^' ||
                   c == 'v' ||
                   c == '<' ||
                   c == '>' ){
                    copy.get(i).append(".");
                }else{
                    copy.get(i).append(c);
                }
            }
        }
        return copy;
    }

    void append_memo(){
        List<StringBuilder> copy = get_copy();
        String hex = DigestUtils.md5Hex(String.join("",copy));
        if(!check_redundancy(hex)){
            //System.out.printf("%s appended to memo.\n",hex);
            memo.add(hex);
        }
    }

    boolean check_memo(){
        List<StringBuilder> copy = get_copy();
        String hex = DigestUtils.md5Hex(String.join("",copy));
        return check_redundancy(hex);
    }

    boolean domino(int x, int y){
        //print();

        if(cols<=y){
            y=0;
            x+=1;
        }
        if(cols<=x){
            return complete();
        }

        if(check_memo()){
            hits += 1;
            return false;
        }

        if(board.get(x).charAt(y) == '_'){
            if(x+1<cols && board.get(x+1).charAt(y) == '_'){
                board.get(x).replace(y,y+1,"^");
                board.get(x+1).replace(y,y+1,"v");
                if(domino(x,y+1)){
                    return true;
                }else{
                    append_memo();
                    board.get(x).replace(y,y+1,"_");
                    board.get(x+1).replace(y,y+1,"_");
                }
            }

            if(y+1<cols && board.get(x).charAt(y+1) == '_'){
                board.get(x).replace(y,y+1,"<");
                board.get(x).replace(y+1,y+2,">");
                if(domino(x,y+2)){
                    return true;
                }else{
                    append_memo();
                    board.get(x).replace(y,y+1,"_");
                    board.get(x).replace(y+1,y+2,"_");
                }
            }
            return false;
        }
        return domino(x,y+1);
    }

}