java java中如何计算表达式？
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原文地址: http://stackoverflow.com/questions/7185589/
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How to calculate expression in java?
提问by Jey
How to calculate user given Expression in java.
如何在java中计算用户给定的表达式。
E:g, if the given exp is 3*4+(5*6) how to calculate this. can anyone help me out.
E:g，如果给定的 exp 是 3*4+(5*6) 如何计算。谁能帮我吗。
回答by Paul
I found this code after a quick google:
我在快速谷歌后找到了这个代码：
import java.util.Stack;
/**
* Class to evaluate infix and postfix expressions.
*
* @author Paul E. Davis ([email protected])
*/
public class InfixPostfixEvaluator {
/**
* Operators in reverse order of precedence.
*/
private static final String operators = "-+/*";
private static final String operands = "0123456789";
public int evalInfix(String infix) {
return evaluatePostfix(convert2Postfix(infix));
}
public String convert2Postfix(String infixExpr) {
char[] chars = infixExpr.toCharArray();
Stack<Character> stack = new Stack<Character>();
StringBuilder out = new StringBuilder(infixExpr.length());
for (char c : chars) {
if (isOperator(c)) {
while (!stack.isEmpty() && stack.peek() != '(') {
if (operatorGreaterOrEqual(stack.peek(), c)) {
out.append(stack.pop());
} else {
break;
}
}
stack.push(c);
} else if (c == '(') {
stack.push(c);
} else if (c == ')') {
while (!stack.isEmpty() && stack.peek() != '(') {
out.append(stack.pop());
}
if (!stack.isEmpty()) {
stack.pop();
}
} else if (isOperand(c)) {
out.append(c);
}
}
while (!stack.empty()) {
out.append(stack.pop());
}
return out.toString();
}
public int evaluatePostfix(String postfixExpr) {
char[] chars = postfixExpr.toCharArray();
Stack<Integer> stack = new Stack<Integer>();
for (char c : chars) {
if (isOperand(c)) {
stack.push(c - '0'); // convert char to int val
} else if (isOperator(c)) {
int op1 = stack.pop();
int op2 = stack.pop();
int result;
switch (c) {
case '*':
result = op1 * op2;
stack.push(result);
break;
case '/':
result = op2 / op1;
stack.push(result);
break;
case '+':
result = op1 + op2;
stack.push(result);
break;
case '-':
result = op2 - op1;
stack.push(result);
break;
}
}
}
return stack.pop();
}
private int getPrecedence(char operator) {
int ret = 0;
if (operator == '-' || operator == '+') {
ret = 1;
} else if (operator == '*' || operator == '/') {
ret = 2;
}
return ret;
}
private boolean operatorGreaterOrEqual(char op1, char op2) {
return getPrecedence(op1) >= getPrecedence(op2);
}
private boolean isOperator(char val) {
return operators.indexOf(val) >= 0;
}
private boolean isOperand(char val) {
return operands.indexOf(val) >= 0;
}
}
回答by legendofawesomeness
Take a look at Java Expression evaluator: http://java.net/projects/eval/pages/Home
看看 Java 表达式评估器：http: //java.net/projects/eval/pages/Home
回答by Ray Toal
Java does this already. No need to download anything.
Java 已经这样做了。无需下载任何东西。
import javax.script.ScriptEngine;
import javax.script.ScriptEngineManager;
public class EvaluationExample {
public static void main(String[] args) throws Exception{
System.out.println(new ScriptEngineManager().getEngineByName("JavaScript").eval("3*4+(5*6)"));
}
}
(This is not the first SO answer to show how to use scripting in Java. I only added it here in case people looking at this page do not follow the links. Parsing is fun, and valuable to study, but if you just need to evaluate user-supplied expressions, use a script.)
（这不是第一个展示如何在 Java 中使用脚本的 SO 答案。我只是在此处添加它，以防查看此页面的人不遵循链接。解析很有趣，值得研究，但如果您只需要评估用户提供的表达式，使用脚本。）
UPDATEThe OP is looking for a postfix evaluation solution. This has to be done in two steps: first convert the input string into postfix notation, then run the postfix "code" through a (presumably stack-based evaluator). See PaulPRO's answer for this. If you are willing to use JavaCC or another parser generator, you can be much more flexible with the strings you accept, allowing newlines and other whitespace.
更新OP 正在寻找后缀评估解决方案。这必须分两步完成：首先将输入字符串转换为后缀表示法，然后通过（大概是基于堆栈的评估器）运行后缀“代码”。请参阅 PaulPRO 对此的回答。如果您愿意使用 JavaCC 或其他解析器生成器，您可以更灵活地处理您接受的字符串，允许换行符和其他空格。
