关于括号的优先级这一块还没有完成，目前只支持简单的四则运算。

相关代码如下：

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import java.util.Stack;

public class PostfixExpression {

    // 中缀表达式转化为后缀表达式
    public String suffix(String infix) {
        infix = infix.replaceAll(" ", "");
        StringBuffer suffixExpression = new StringBuffer();
        char[] chars = infix.toCharArray();
        String[] letters = new String[chars.length];
        for (int i = 0; i < letters.length; i++) {
            letters[i] = String.valueOf(chars[i]);
        }
        Stack<String> stack = new Stack<String>();
        for (int i = 0; i < letters.length; i++) {
            if (letters[i].matches("^[0-9]*$")) {
                if (i > 1 && letters[i - 1].matches("^[0-9]*$")) {
                    suffixExpression.append(letters[i]);
                } else {
                    suffixExpression.append(" " + letters[i]);
                }
            } else {
                // 表达式的判断
                if ("(".equals(letters[i])) {
                    stack.push("(");
                } else if (")".equals(letters[i])) {
                    String s = stack.pop();
                    while (!"(".equals(s)) {
                        suffixExpression.append(" " + s + " ");
                        s = stack.pop();
                    }
                } else {
                    if (stack.isEmpty()) {
                        stack.push(letters[i]);
                    } else {
                        String top = stack.firstElement();
                        if (("-".equals(top) || "+".equals(top)) && ("x".equals(letters[i])) || "/".equals(letters[i])) {
                            stack.push(letters[i]);
                        } else {
                            while (!(("-".equals(top) || "+".equals(top)) && ("x".equals(letters[i])) || "/".equals(letters[i]))) {
                                suffixExpression.append(" " + stack.pop() + " ");
                                if (stack.isEmpty()) {
                                    break;
                                } else {
                                    top = stack.firstElement();
                                }
                            }
                            stack.push(letters[i]);
                        }
                    }
                }
            }
        }
        while (!stack.isEmpty()) {
            suffixExpression.append(" " + stack.pop() + " ");
        }
        return suffixExpression.toString();
    }

    // 计算后缀表达式
    public double calculate(String expression) {
        Stack<String> stack = new Stack<String>();
        String[] values = expression.split(" {1,}");
        for (int i = 0; i < values.length; i++) {
            String value = values[i];
            if (value.matches("^[0-9]*$")) {
                stack.push(value);
            } else {
                try {
                    stack.push(this.calculate(stack.pop(), stack.pop(), value));
                } catch (Exception e) {
                    System.out.println("输入的表达式有误");
                }
            }
        }
        return Double.valueOf(stack.pop());
    }

    // 计算两个数的值
    private String calculate(String s2, String s1, String operator) throws Exception {
        double num1 = Double.valueOf(s1);
        double num2 = Double.valueOf(s2);
        double num;
        if ("+".equals(operator)) {
            num = num1 + num2;
        } else if ("x".equals(operator)) {
            num = num1 * num2;
        } else if ("-".equals(operator)) {
            num = num1 - num2;
        } else if ("/".equals(operator)) {
            num = num1 / num2;
        } else {
            throw new Exception("Can't explain this operator");
        }
        return String.valueOf(num);
    }
    
    public static void main(String[] args) {
        PostfixExpression p = new PostfixExpression();
        String expression = "1x100+90x2/2+2";
        String suffixExpression = p.suffix(expression);
        System.out.println(suffixExpression);
        System.out.println(p.calculate(suffixExpression));
    }

}

理论支撑：