eval.java

	/**
	 * Return the value of the expression
	 */
	public double eval() throws NumberFormatException {
	    if (this.isLeaf()) return Double.parseDouble(this.data()); 
	    
	    Double left = ((ExpressionTree) this.left()).eval();
	    Double right = ((ExpressionTree) this.right()).eval();
	        
	    return eval(this.data(), left, right);
	} 
	
	/**
	 * Return the value of 'x op y'
	 * where 'op' is "+", "*", "/" or "^"
	 */
	private double eval(String op, double x, double y) { 
		switch (op) {
        case "+":
            return x + y;  
        case "-":
            return x - y;
        case "*":
            return x * y;
        case "/":
            return x / y;
        case "^":
            return Math.pow(x, y);
        default:
            break;
        }
	    return 0;
	}