今天公司產品提出需求。需要把計算公式放到伺服器，然後伺服器可動態化配置。所以找尋各種資料從而由此方法

程式碼如下：

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;
}


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();
}

例子：

String gs = "100*5*0.003";

System.out.println(eval(gs));