问如何实现Android(Java)计算器逻辑？

EN

  public static double evaluate(final String str){
    class Parser {
        int pos = -1;
        int c;

        void eatChar() {
            c = (++pos < str.length()) ? str.charAt(pos) : -1;
        }

        void eatSpace() {
            while (Character.isWhitespace(c)) eatChar();
        }

        //Everything below this point confuses me xD

        double parse() {
            eatChar();
            double v = parseExpression();
            if (c != -1) throw new RuntimeException("Unexpected: " + (char)c);
            return v;
        }


        double parseExpression() {
            double v = parseTerm();
            for (;;) {
                eatSpace();
                if (c == '+') { // addition
                    eatChar();
                    v += parseTerm();
                } else if (c == '-') { // subtraction
                    eatChar();
                    v -= parseTerm();
                } else {
                    return v;
                }
            }
        }

        double parseTerm() {
            double v = parseFactor();
            for (;;) {
                eatSpace();
                if (c == '/' || c == '÷') { // division
                    eatChar();
                    v /= parseFactor();
                } else if (c == '*' || c == '(') { // multiplication
                    if (c == '*') eatChar();
                    v *= parseFactor();
                } else {
                    return v;
                }
            }
        }

        double parseFactor() {
            double v;
            boolean negate = false;
            eatSpace();
            if (c == '+' || c == '-') { // unary plus & minus
                negate = c == '-';
                eatChar();
                eatSpace();
            }
            if (c == '(') { // brackets
                eatChar();
                v = parseExpression();
                if (c == ')') eatChar();
            } else { // numbers
                int startIndex = this.pos;
                while ((c >= '0' && c <= '9') || c == '.') eatChar();
                if (pos == startIndex) throw new RuntimeException("Unexpected: " + (char)c);
                v = Double.parseDouble(str.substring(startIndex, pos));
            }

            eatSpace();
            if (c == '^') { // exponentiation
                eatChar();
                v = Math.pow(v, parseFactor());
            }
            if (negate) v = -v;
            return v;
        }
    }
    return new Parser().parse();
}