Help in nega max+alpha beta pruning

Question

well I have programmed a connect four which implemented minimax algorithm,but then it took 10 secs(minimum) to reply for maxdepth of 8. More over it still lost few games to online ai's which to my surprise played faster and more accurate than my program.Then by searching in books and internet I came across search funtion called as megamax with alpha-beta pruning.I replaced my minimax with this method but then it plays very bad(doesn't even see the connect 4 even if there are 3 in a row).I am willing to put my source code, pls point out my mistakes and rectify (with explanations if possible). Thank you, and also forgive me for grammatical mistakes(if any) as english is my second language.
I know alpha beta is a bit tedious thing to do(at least for me as I am doing it for the very first time).
So I request you all to please help

Answer 1

Here is the source code
package connect4;
import java.io.*;
import java.util.*;
public class Connect4 {
    private static final int WIDTH=7, HEIGHT=6;
    private static  int MAX_RECURDEPTH=8;
    private static BufferedReader in;
    private  int[][] grid=new int[WIDTH][HEIGHT];
    private  int[] columnHeight=new int[WIDTH];
    private  static int recurDepth=0;
    public static void main(String[] args) {
        in=new BufferedReader(new InputStreamReader(System.in));
        new Connect4();
    public Connect4() {
        Scanner read=new Scanner(System.in);
        int column,p;
        int player=1;
        System.out.println("Who play first?");
        System.out.println("1.Player");
        System.out.println("2.Computer");
        p=read.nextInt();
        player=p;
        double a=-MAX_RECURDEPTH;
        double b=MAX_RECURDEPTH;
        int move=0;
        double value=0;
        Vector moves=new Vector();
        while(true) {
            if(player==1) {
                printGrid();
                do {
                    System.out.println("Make your move (1-7): ");
                    column=readInt()-1;
                } while(column<0 || column >=WIDTH || columnHeight[column]>=HEIGHT);
            } else {
                moves.removeAllElements();
                column=0;
                       value=0;
                double prevValue=-MAX_RECURDEPTH-1;
                for(int x=0; x<WIDTH; x++)
                    if(columnHeight[x]>=HEIGHT)
                        continue;
                   if (move==0)
                   moves.add(new Integer(3));
                    break;
                  if(p==2 && move==1)
                   moves.add(new Integer(3));
                    break;
                    value=minmax_ab(2,x,a,b);
                    if(value>prevValue) {
                        moves.removeAllElements();
                        prevValue=value;
                    if(value==prevValue)
                        moves.add(new Integer(x));
                   move++;
                   System.out.println("VAlue="+prevValue);
                if(moves.size()>0) {
                    Collections.shuffle(moves);
                    column=((Integer)moves.get(0)).intValue();
                if(moves.size()==0) {
                    System.out.println("Its a draw.");
                    break;
            grid[column][columnHeight[column]]=player;
            if(player==2)
            System.out.println("I PUT MY CHECKER IN "+(column+1));
            columnHeight[column]++;
            int haswon=0;
            haswon=fourInARow();
            if(haswon>0) {
                printGrid();
                if(player==2)
                    System.out.println("I  WON");
                else
                    System.out.println("YOU WON");
                break;
            player=3-player;
    double minmax_ab(int player, int column,double a,double b) {
        double value=0;
        if(columnHeight[column]>=HEIGHT)
            return 0;
        grid[column][columnHeight[column]]=player;
        columnHeight[column]++;
if(columnHeight[column]>=HEIGHT)
            return 0;
        recurDepth++;
        if(fourInARow()>0) {
            if(player==2)
                value=(MAX_RECURDEPTH+1)-recurDepth;
            else value=-MAX_RECURDEPTH-1+recurDepth;
        if(recurDepth<MAX_RECURDEPTH&&value==0)
             value=MAX_RECURDEPTH+1;
            for(int x=0; x<WIDTH; x++)
              if(columnHeight[x]>=HEIGHT)
                    continue;
            double v=-minmax_ab(3-player,column,-b,-a);
            if(value==(MAX_RECURDEPTH+1))
                    value=v;
            else if(player==2)
                 if(v>value)
               value=v;
            else if(v>value)
                value=v;
                else if(v>a)
               a=v;
                 else if(a>=b)
                 return value;
        recurDepth--;
        columnHeight[column]--;
        grid[column][columnHeight[column]]=0;
        return a;
    int fourInARow() {
        int num, player;
        for(int y=0; y<HEIGHT; y++) {
            num=0; player=0;
            for(int x=0; x<WIDTH; x++) {
                if(grid[x][y]==player) num++;
                else { num=1; player=grid[x][y]; }
                if(num==4 && player>0)
                    return player;
        for(int x=0; x<WIDTH; x++) {
            num=0; player=0;
            for(int y=0; y<HEIGHT; y++) {
                if(grid[x][y]==player) num++;
                else { num=1; player=grid[x][y]; }
                if(num==4 && player>0) return player;
        for(int xStart=0, yStart=2; xStart<4; ) {
            num=0; player=0;
            for(int x=xStart, y=yStart; x<WIDTH && y<HEIGHT; x++, y++) {
                if(grid[x][y]==player) num++;
                else { num=1; player=grid[x][y]; }
                if(num==4 && player>0) return player;
            if(yStart==0) xStart++;
            else yStart--;
        for(int xStart=0, yStart=3; xStart<4; ) {
            num=0; player=0;
            for(int x=xStart, y=yStart; x<WIDTH && y>=0; x++, y--) {
                if(grid[x][y]==player) num++;
                else { num=1; player=grid[x][y]; }
                if(num==4 && player>0) return player;
            if(yStart==HEIGHT-1) xStart++;
            else yStart++;
        return 0;
    void printGrid() {
        int x=0;
        int y=0;
        for(y=HEIGHT-1; y>=0; y--)
            for(x=0; x<WIDTH; x++)
                if(grid[x][y]==0)
                    System.out.print(" * ");
                else if(grid[x][y]==1)
                    System.out.print(" A ");
                else if(grid[x][y]==2)
                   System.out.print(" B ");
            System.out.println();
        System.out.println();
        System.out.println(" 1  2  3  4  5  6  7 ");
    int readInt() {
        try {
            String input=in.readLine();
            int number=Integer.parseInt(input);
            return number;
        } catch(NumberFormatException e) {
            return -1;
        } catch(IOException e) {
            return -1;