queens.java

package appito;


import java.util.ArrayList;
import java.util.List;

class Queen
{
    private int xPos = 0;
    private int yPos = 0;
    
    public Queen( int initialX, int initialY ) {
        setXPos( initialX );
        setYPos( initialY );
    }

    public int getXPos() { return xPos; }
    public int getYPos() { return yPos; }
    
    public void setXPos( int x ) {
        if( 0 < x || x < 8 ) {
            xPos = x;
        } else {
            throw new RuntimeException( "New value of xPos outside allowed range." );
        }
    }
    
    
    public void setYPos( int y ) {
        if( 0 < y || y < 8 ) {
            yPos = y;
        } else {
            throw new RuntimeException( "New value of yPos outside allowed range." );
        }
    }
    

    private static boolean closeEnough( double value1, double value2 ) {
        return (value2 - 0.000001) < value1 && value1 < (value2 + 0.000001);
    }


    public boolean canTake( Queen otherQueen ) {
        if( otherQueen == this ) // one cannot take themselves
            return false;

        final double rise = otherQueen.getYPos() - getYPos();
        final double run = otherQueen.getXPos() - getXPos();

        if( closeEnough(run, 0) ) // infinite slope, need to detect here since we can't divide by zero
            return true;

        final double slope = rise / run;
        return ( closeEnough(slope, 0) || closeEnough(slope, -1) || closeEnough(slope, 1) );
    }
    
    public String toString() { return "(" + xPos + "," + yPos + ")"; }
}



public final class Queens {

    static final void findSolution( Queen[] queens, int currentCol ) {
        if( currentCol < 8 ) {
            for( int yPos = 0; yPos < 8; yPos++ ) {
                queens[currentCol].setYPos( yPos );

                boolean beTaken = false;
                for( int i = 0; i < currentCol; i++ )
                    beTaken = queens[i].canTake(queens[currentCol]) || beTaken;

                if( !beTaken ) {
                    int nextCol = currentCol + 1;
                    if( nextCol < 8 )
                        findSolution( queens, nextCol );
                    else {
                        System.out.print("Possible solution - ");
                        for(Queen q: queens )
                            System.out.print( q.toString() );
                            
                        System.out.println(" ");
                    }
                }
            }
        }
    }

    public static void main( String[] args ) {
        Queen[] placedQueens = new Queen[8];
        for( int i = 0; i < 8; i++ )
            placedQueens[i] = ( new Queen( i, 0 ) );
        
        findSolution( placedQueens, 0 );
    }
}