package com.math;import java.math.BigDecimal;import java.util.ArrayList;import java.util.Scanner;import java.util.Stack;/** * 表达式求值 */public class Calculator { private static final int MAXCAPACITY = 500; private String expression; public Calculator(String expression) { if (expression.length() > MAXCAPACITY) System.out.println(“字符串长度超出最大值”); else this.expression = expression; } public int getPriority(String str) { int pri = -1; if (str.equals(“(“)) pri = 0; else if (str.equals(“+”) || str.equals(“-“)) pri = 1; else if (str.equals(“*”) || str.equals(“/”)) pri = 2; return pri; } public String[] split() { ArrayList<String> strList = new ArrayList<String>(); int splitIndex = 0; for (int i = 0; i < expression.length(); i++) { if (expression.charAt(i) == ‘+’ || expression.charAt(i) == ‘-‘ || expression.charAt(i) == ‘*’ || expression.charAt(i) == ‘/’ || expression.charAt(i) == ‘(‘ || expression.charAt(i) == ‘)’) { if (splitIndex == i) { //处理连续多个操作符 if (expression.charAt(i) == ‘-‘) { //处理操作符”( + – * /”符号后面遇到”-“情况时，将后面的数字当做负数处理 splitIndex = i; continue; } else strList.add(expression.substring(splitIndex, i + 1)); } else { strList.add(expression.substring(splitIndex, i)); strList.add(expression.substring(i, i + 1)); } splitIndex = i + 1; } if ((i == expression.length() – 1) && (splitIndex <= i)) strList.add(expression.substring(splitIndex)); } return strList.toArray(new String[0]); } public String[] getSuffixExpression(String[] expression) { Stack<String> strStack = new Stack<String>(); ArrayList<String> strList = new ArrayList<String>(); for (int i = 0; i < expression.length; i++) { if (expression[i].equals(“(“)) //遇到'(‘时入栈 strStack.push(expression[i]); else if (expression[i].equals(“+”) || expression[i].equals(“-“) || expression[i].equals(“*”) || expression[i].equals(“/”)) { if (strStack.isEmpty()) //栈空时操作符入栈 strStack.push(expression[i]); else { //否则比较栈顶操作符优先级 while ((!strStack.isEmpty()) && (getPriority(strStack.peek()) >= getPriority(expression[i]))) { strList.add(strStack.pop()); } strStack.push(expression[i]); } } else if (expression[i].equals(“)”)) {//遇到’)’时出栈直至'(‘ while (!strStack.peek().equals(“(“)) { strList.add(strStack.pop()); } strStack.pop(); //抛出'(‘ } else strList.add(expression[i]); } while (!strStack.isEmpty()) strList.add(strStack.pop()); return strList.toArray(new String[0]); } public double getResult(String[] expression) { Stack<Double> strStack = new Stack<Double>(); double num1, num2; double result = 0; for (int i = 0; i < expression.length; i++) { if (expression[i].equals(“+”) || expression[i].equals(“-“) || expression[i].equals(“*”) || expression[i].equals(“/”)) { String str = expression[i]; switch (str) { case “+”: { num1 = strStack.pop(); num2 = strStack.pop(); //BigDcimal的作用：解决计算过程中丢失精度问题 result = (BigDecimal.valueOf(num1).add(BigDecimal.valueOf(num2))).doubleValue(); strStack.push(result); break; } case “-“: { num1 = strStack.pop(); num2 = strStack.pop(); result = (BigDecimal.valueOf(num2).subtract(BigDecimal.valueOf(num1))).doubleValue(); strStack.push(result); break; } case “*”: { num1 = strStack.pop(); num2 = strStack.pop(); result = num2 * num1; result = (BigDecimal.valueOf(num2).multiply(BigDecimal.valueOf(num1))).doubleValue(); strStack.push(result); break; } case “/”: { num1 = strStack.pop(); num2 = strStack.pop(); result = (BigDecimal.valueOf(num2).divide(BigDecimal.valueOf(num1))).doubleValue(); strStack.push(result); break; } } } else strStack.push(Double.valueOf(expression[i])); } result = 0; while (!strStack.isEmpty()) { result += strStack.pop(); } return result; } public double getResult() { return getResult(getSuffixExpression(split())); } /** * 计算包含+、-、*、/、(、)等几种运算符的表达式的值。 */ public static void main(String[] args) { try { //第一步：输入包含表达式的字符串 Scanner scanner = new Scanner(System.in); System.out.println(“请输入表达式并以#结束:”); //第二步：计算表达式的值 StringBuffer sb = new StringBuffer(); while (true) { String s = scanner.next(); sb.append(s); if (sb.toString().endsWith(“#”)) { break; } if (sb.toString().equals(“#”)) { System.out.println(“请输入表达式”); } } String line = sb.toString().substring(0, sb.toString().length() – 1); //第三步：输出计算结果 Calculator exp1 = new Calculator(line); String[] strSplit = exp1.split(); String[] strSuffixExpression = exp1.getSuffixExpression(strSplit); double result = exp1.getResult(strSuffixExpression); System.out.println(“计算结果：” + line + “=” + result); } catch (Exception e) { System.err.println(“请输入合法的表达式！”); } }}
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