Task: Ulam spiral (for primes)
C++: #include <cmath>
#include <iostream>
#include <string>
#include <iomanip>
#include <vector>

class ulamSpiral {
public:
    void create( unsigned n, unsigned startWith = 1 ) {
        _lst.clear();
        if( !( n & 1 ) ) n++;
        _mx = n;
        unsigned v = n * n;
        _wd = static_cast<unsigned>( log10( static_cast<long double>( v ) ) ) + 1;
        for( unsigned u = 0; u < v; u++ )
            _lst.push_back( -1 );

        arrange( startWith );

    }
    void display( char c ) {
        if( !c ) displayNumbers();
        else displaySymbol( c );
    }

private:
    bool isPrime( unsigned u ) {
        if( u < 4 ) return u > 1;
        if( !( u % 2 ) || !( u % 3 ) ) return false;
 
        unsigned q = static_cast<unsigned>( sqrt( static_cast<long double>( u ) ) ),
                 c = 5;
        while( c <= q ) {
            if( !( u % c ) || !( u % ( c + 2 ) ) ) return false;
            c += 6;
        }
        return true;
    }
    void arrange( unsigned s ) {
        unsigned stp = 1, n = 1, posX = _mx >> 1, 
                 posY = posX, stC = 0;
        int dx = 1, dy = 0;
    
        while( posX < _mx && posY < _mx ) {
            _lst.at( posX + posY * _mx ) =  isPrime( s ) ? s : 0;
            s++;
        
            if( dx ) {
                posX += dx;
                if( ++stC == stp ) {
                    dy = -dx;
                    dx = stC = 0;
                }
            } else {
                posY += dy;
                if( ++stC == stp ) {
                    dx = dy;
                    dy = stC = 0;
                    stp++;
                }
            }
        }
    }
    void displayNumbers() {
        unsigned ct = 0;
        for( std::vector<unsigned>::iterator i = _lst.begin(); i != _lst.end(); i++ ) {
            if( *i ) std::cout << std::setw( _wd ) << *i << " ";
            else std::cout << std::string( _wd, '*' ) << " ";
            if( ++ct >= _mx ) {
                std::cout << "\n";
                ct = 0;
            }
        }
        std::cout << "\n\n";
    }
    void displaySymbol( char c ) {
        unsigned ct = 0;
        for( std::vector<unsigned>::iterator i = _lst.begin(); i != _lst.end(); i++ ) {
            if( *i ) std::cout << c;
            else std::cout << " ";
            if( ++ct >= _mx ) {
                std::cout << "\n";
                ct = 0;
            }
        }
        std::cout << "\n\n";
    }

    std::vector<unsigned> _lst;
    unsigned _mx, _wd;
};

int main( int argc, char* argv[] )
{
    ulamSpiral ulam;
    ulam.create( 9 );
    ulam.display( 0 );
    ulam.create( 35 );
    ulam.display( '#' );
    return 0;
}

Java: import java.util.Arrays;

public class Ulam{
	enum Direction{
		RIGHT, UP, LEFT, DOWN;
	}
	
	private static String[][] genUlam(int n){
		return genUlam(n, 1);
	}

	private static String[][] genUlam(int n, int i){
		String[][] spiral = new String[n][n];
		Direction dir = Direction.RIGHT;
		int j = i;
		int y = n / 2;
		int x = (n % 2 == 0) ? y - 1 : y; 
		while(j <= ((n * n) - 1 + i)){
			spiral[y][x] = isPrime(j) ? String.format("%4d", j) : " ---";

			switch(dir){
			case RIGHT:
				if(x <= (n - 1) && spiral[y - 1][x] == null && j > i) dir = Direction.UP; break;
			case UP:
				if(spiral[y][x - 1] == null) dir = Direction.LEFT; break;
			case LEFT:
				if(x == 0 || spiral[y + 1][x] == null) dir = Direction.DOWN; break;
			case DOWN:
				if(spiral[y][x + 1] == null) dir = Direction.RIGHT; break;
			}
			
			switch(dir){
				case RIGHT:	x++; break;
				case UP: 	y--; break;
				case LEFT:	x--; break;
				case DOWN:	y++; break;
			}
			j++;
		}
		return spiral;
	}
	
	public static boolean isPrime(int a){
		   if(a == 2) return true;
		   if(a <= 1 || a % 2 == 0) return false;
		   long max = (long)Math.sqrt(a);
		   for(long n = 3; n <= max; n += 2){
		      if(a % n == 0) return false;
		   }
		   return true;
		}
	
	public static void main(String[] args){
		String[][] ulam = genUlam(9);
		for(String[] row : ulam){
			System.out.println(Arrays.toString(row).replaceAll(",", ""));
		}
		System.out.println();
		
		for(String[] row : ulam){
			System.out.println(Arrays.toString(row).replaceAll("\\[\\s+\\d+", "[  * ").replaceAll("\\s+\\d+", "   * ").replaceAll(",", ""));
		}
	}
}