java字符串数学公式运算

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在stackoverflow上找到了答案，可用的lib有BeanShell，ScriptEngineManager之类。但也有人写出了简单的：from: http://stackoverflow.com/a/26227947/2193455

public static double eval(final String str) {    return new Object() {        int pos = -1, ch;        void nextChar() {            ch = (++pos < str.length()) ? str.charAt(pos) : -1;        }        boolean eat(int charToEat) {            while (ch == ' ') nextChar();            if (ch == charToEat) {                nextChar();                return true;            }            return false;        }        double parse() {            nextChar();            double x = parseExpression();            if (pos < str.length()) throw new RuntimeException("Unexpected: " + (char)ch);            return x;        }        // Grammar:        // expression = term | expression `+` term | expression `-` term        // term = factor | term `*` factor | term `/` factor        // factor = `+` factor | `-` factor | `(` expression `)`        //        | number | functionName factor | factor `^` factor        double parseExpression() {            double x = parseTerm();            for (;;) {                if      (eat('+')) x += parseTerm(); // addition                else if (eat('-')) x -= parseTerm(); // subtraction                else return x;            }        }        double parseTerm() {            double x = parseFactor();            for (;;) {                if      (eat('*')) x *= parseFactor(); // multiplication                else if (eat('/')) x /= parseFactor(); // division                else return x;            }        }        double parseFactor() {            if (eat('+')) return parseFactor(); // unary plus            if (eat('-')) return -parseFactor(); // unary minus            double x;            int startPos = this.pos;            if (eat('(')) { // parentheses                x = parseExpression();                eat(')');            } else if ((ch >= '0' && ch <= '9') || ch == '.') { // numbers                while ((ch >= '0' && ch <= '9') || ch == '.') nextChar();                x = Double.parseDouble(str.substring(startPos, this.pos));            } else if (ch >= 'a' && ch <= 'z') { // functions                while (ch >= 'a' && ch <= 'z') nextChar();                String func = str.substring(startPos, this.pos);                x = parseFactor();                if (func.equals("sqrt")) x = Math.sqrt(x);                else if (func.equals("sin")) x = Math.sin(Math.toRadians(x));                else if (func.equals("cos")) x = Math.cos(Math.toRadians(x));                else if (func.equals("tan")) x = Math.tan(Math.toRadians(x));                else throw new RuntimeException("Unknown function: " + func);            } else {                throw new RuntimeException("Unexpected: " + (char)ch);            }            if (eat('^')) x = Math.pow(x, parseFactor()); // exponentiation            return x;        }    }.parse();}

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