class Life {

  public final static int ROWS = 5;
  public final static int COLS = ROWS;
  private final static double INITIAL_DENSITY = 0.3;
  private java.util.Random randy = new java.util.Random();
  
  boolean[][] board;
 
  /** constructor.
   * @param density The probabibility that any given location is
   *     initially occupied (in [0,1]).
   */
  public Life( double density ) {
    board = new boolean[ROWS][COLS];
    
    // Initialize the entire board randomly:
    for (int r=0; r < ROWS; ++r) {
      this.initRowRandom(r, density);
      }
    }

  /** Constructor.
   * Uses a default initial density.
   * @see Life(double)
   */
  public Life() {
    this(INITIAL_DENSITY);
    }
    
    
  /** Initialize one row of the board.
   * @param rowNum The row to initialize.
   * @param p the probability that any given cell will be filled.
   */
  private void initRowRandom( int rowNum, double p ) {
    for (int c=0; c < COLS; ++c) {
      board[rowNum][c] = (this.randy.nextDouble() < p);
      }
    }

  /**
   * Return a string representing the board visually.
   */
  public String toString() {
    String boardSoFar = "";
    for (int r=0; r < ROWS; ++r) {
      boardSoFar += this.rowToString(r) + "\n";
      }
    return boardSoFar;
    }

  /** Return a string representing one row of the board.
   * @param rowNum which row to represent; must be in [0,ROWS).
   */
  private String rowToString(int rowNum) {
    String rowSoFar = "";
    
    for (int c=0;  c < COLS;  ++c) {
      if (board[rowNum][c]) {
        rowSoFar += "X";
        }
      else {
        rowSoFar += ".";
        }
      // or, just:   rowSoFar += (board[rowNum][c] ? "X" : ".");
      }
      
    return rowSoFar;
    }
    
    
  }