public boolean isGameOver() {
		for (int i = 0; i < YARD_COL; i++) {
			if (blocks[0][i] == true) {
				gameOver = true;
				if (showGameOverMessage == true) {
					showGameOverMessage = false;	//提示对话框只出现一次
					JOptionPane.showMessageDialog(this, "Game Over!按F2重新开始");					
				}
				return true;
			}
		}
		
		gameOver = false;
		return false;
	}