tic-tac-toe/src/PlayField.java

import java.util.Random;

public class PlayField {
    private final int[] fields = new int[9];
    private final Random move = new Random();

    public void setPos(int pos, int player) {
        fields[pos] = player;
    }

    public boolean isDraw() {
        for(int i = 0; i < 9; i++) {
            if(fields[i] == 0) {
                return false;
            }
        }
        return true;
    }

    public boolean hasWon(int player) {
        // Waagrecht
        for(int i = 0; i < 9; i += 3) {
            if(fields[i] == player && fields[i + 1] == player && fields[i + 2] == player) {
                return true;
            }
        }
        // Senkrecht
        for(int i = 0; i < 3; i++) {
            if(fields[i] == player && fields[i + 3] == player && fields[i + 6] == player) {
                return true;
            }
        }
        // Diagonal
        if(fields[4] == player) {
            if(fields[0] == player && fields[8] == player) {
                return true;
            }
            if(fields[2] == player && fields[6] == player) {
                return true;
            }
        }
        return false;
    }

    public void reset() {
        for(int i = 0; i < 9; i++) {
            fields[i] = 0;
        }
    }

    // the opponent player moves
    // updates the field
    public void oppMove() {
        while(true) {
            int pos = move.nextInt(9);
            if(isFree(pos)) {
                setPos(pos, 2);
                return;
            }
        }
    }

    public boolean isFree(int pos) {
        return fields[pos] == 0;
    }

    private void printSlot(String sign) {
        System.out.print("[" + sign + "]");
    }

    public void print() {
        for(int y = 0; y < 3; y++) {
            for(int x = 0; x < 3; x++) {
                int a = fields[x + 3 * y];
                if(a == 0) {
                    printSlot(" ");
                } else if(a == 1) {
                    printSlot("x");
                } else {
                    printSlot("o");
                }
            }
            System.out.println(); // Zeilenumbruch
        }
    }
}