A rudimentary Knight's Tour using Backtracking algorithm ...

KnightsTour.java

/**
  Compile and run as java KnightsTour a1
 */
import java.util.*;
class KnightsTour {
  public static void main(String[] args) { 
    String start = args[0];
    Board.setStart(start);
    if(solve(start))
      Board.draw();
    else
      System.out.println("No solution for: " + args[0]);
  }
  static boolean solve(String curr) {
    Board.placeKnightAt(curr);
    if (Board.isTourDone(curr)) 
      return true;
    Set<String> candidates = Board.candidateKnightMoves(curr);
    for (String candidate : candidates) {
      if (solve(candidate))
        return true;
    }
    Board.removeKnightFrom(curr);
    return false;
  }
  static final class Board {
    static int[][] board = new int[8][8]; //
    static int counter = 0; 
    static String start;
    static {
      for (int i = 0; i < 8; i++)
        for (int j = 0; j < 8; j++)
          board[i][j] = 0;
    }
    static void draw() {
      for (int i = 7; i >=0 ; i--)
        System.out.println(Arrays.toString(board[i]));
    }
    static void setStart(String start) {
      Board.start = start;
    }
    static boolean isTourDone(String curr) {
      if (counter != 64)
        return false;
      Set<String> possible = candidateKnightMoves(curr, 1);
      //System.out.println("possible: " + possible + ", curr: " + curr);
      return possible.contains(start);
    }
    static void read() {
      try {
        System.in.read();
      } catch (Exception e) {
        e.printStackTrace();
      }
    }
    static void placeKnightAt(String pos) {
      //'a' <= f <= 'h', '1' <= r <= '8'
      char f = pos.charAt(0), r = pos.charAt(1);
      counter += 1;
      board[r-'1'][f-'a'] = counter;
      //draw();
      //read();
    }
    static void removeKnightFrom(String pos) {
      //'a' <= f <= 'h', '1' <= r <= '8'
      char f = pos.charAt(0), r = pos.charAt(1);
      counter -= 1;
      board[r-'1'][f-'a'] = 0;
      //draw();
      //read();
    }
    static Set<String> candidateKnightMoves(String curr) {
      return candidateKnightMoves(curr, 0);
    }
    static Set<String> candidateKnightMoves(String curr, int nextScore) {
      // from a current position like "d4", returns the "available" board positions knight can move to
      //System.out.println("curr: " + curr);
      Set<String> candidates = new HashSet<String>();
      char file = curr.charAt(0);
      char rank = curr.charAt(1);
      char nf, nr;
      nf = (char)(file + 1); 
      nr = (char)(rank + 2);
      if (nf <= 'h' && nr <= '8' && board[nr-'1'][nf-'a'] == nextScore) {
        String c = ""+nf + nr;
        candidates.add(c);
        //System.out.println("cand: " + c);
      }
      nr = (char)(rank - 2);
      if (nf <= 'h' && nr >= '1' && board[nr-'1'][nf-'a'] == nextScore) {
        String c = ""+nf + nr;
        candidates.add(c);
        //System.out.println("cand: " + c);
      }
      nf = (char)(file - 1); 
      nr = (char)(rank + 2);
      if (nf >= 'a' && nr <= '8' && board[nr-'1'][nf-'a'] == nextScore) {
        String c = ""+nf + nr;
        candidates.add(c);
        //System.out.println("cand: " + c);
      }
      nr = (char)(rank - 2);
      if (nf >= 'a' && nr >= '1' && board[nr-'1'][nf-'a'] == nextScore) {
        String c = ""+nf + nr;
        candidates.add(c);
        //System.out.println("cand: " + c);
      }
      nf = (char)(file + 2); 
      nr = (char)(rank + 1);
      if (nf <= 'h' && nr <= '8' && board[nr-'1'][nf-'a'] == nextScore) {
        String c = ""+nf + nr;
        candidates.add(c);
        //System.out.println("cand: " + c);
      }
      nr = (char)(rank - 1);
      if (nf <= 'h' && nr >= '1' && board[nr-'1'][nf-'a'] == nextScore) {
        String c = ""+nf + nr;
        candidates.add(c);
        //System.out.println("cand: " + c);
      }
      nf = (char)(file - 2); 
      nr = (char)(rank + 1);
      if (nf >= 'a' && nr <= '8' && board[nr-'1'][nf-'a'] == nextScore) {
        String c = ""+nf + nr;
        candidates.add(c);
        //System.out.println("cand: " + c);
      }
      nr = (char)(rank - 1);
      if (nf >= 'a' && nr >= '1' && board[nr-'1'][nf-'a'] == nextScore) {
        String c = ""+nf + nr;
        candidates.add(c);
        //System.out.println("cand: " + c);
      }
      return candidates;
    }
  }
}

For some reason, Java 7 runs this algorithm reasonably well (in ~35 seconds, a knight's tour is found with a1 as the starting position), whereas Java 8 just hangs!