The main thing to consider when making a calculator in Java is the priority of the order of addition, subtraction, multiplication, and division ,
such as:
20-2-1.7*4/5-4 *2，
You need to calculate 1.7*4/5 first,
then 4*2, and
finally add and subtract from left to right.
Some other calculators on the Internet use queues and stacks, and convert infix expressions to postfix expressions. For example:
Java Programming Simple Calculator-Implementation Chapter
This writing method is more complicated and difficult to understand.
So I wrote a calculator with the idea of recursion:
import java.io.IOException;
import java.math.BigDecimal;
import java.math.RoundingMode;
import java.util.ArrayList;
import java.util.List;
import java.util.Scanner;
public class Test {
public static void main(String[] args) throws IOException {
System.out.println("请输入要计算的运算：");
Scanner scanner = new Scanner(System.in);
System.out.println(saveNumber_Symbol(scanner.next()));
}
/*分割字符串存储运算符和数字分别到calcuStrList和numberList*/
public static String saveNumber_Symbol(String str) {
str+="$"; //加上"$"用于获取分割最后一个数字
char[] strChar = str.toCharArray();
List<String> calcuStrList = new ArrayList<>();//存储运算符号
List<String> numberList = new ArrayList<>();//存储数字
StringBuilder buff = new StringBuilder();
int i = 0;
for (char ch : strChar) {
if (ch == '+' || ch == '-' || ch == '*' || ch == '/') {
calcuStrList.add(String.valueOf(ch));
numberList.add(buff.toString());
buff.delete(0, buff.length());
} else if (ch == '$') {
//判断是不是最后一个数
numberList.add(buff.toString());
} else {
buff.append(ch);
}
}
return calculationOrder(calcuStrList, numberList);
}
/*控制加减乘除的运算优先级顺序*/
public static String calculationOrder(List<String> calcuStr, List<String> number) {
if (calcuStr.size() > 0) {
int multiply = calcuStr.indexOf("*");
int except = calcuStr.indexOf("/");
//除在前乘在后，或有除无乘，则先算除
if ((-1 != multiply && -1 != except && except < multiply) || (calcuStr.contains("/") && !calcuStr.contains("*"))) {
calcuResult("/", calcuStr, number);
}
//乘在前除在后，或有乘无除，先算乘
if ((-1 != multiply && -1 != except && multiply < except) || (calcuStr.contains("*")&& !calcuStr.contains("/"))) {
calcuResult("*", calcuStr, number);
}
//无乘除只有加减，则从左往右计算
if (calcuStr.size() > 0) {
calcuResult(calcuStr.get(0), calcuStr, number);
}
}
return number.get(0);
}
/*通过symbol找到运算符在calcuStr出现的位置,根据位置在number中找到符号前后的两个数进行计算*/
public static String calcuResult(String symbol, List<String> calcuStr, List<String> number) {
int position = calcuStr.indexOf(symbol);
BigDecimal a = new BigDecimal(number.get(position));
BigDecimal b = new BigDecimal(number.get(position + 1));
String calculateNumber = "";
switch (symbol) {
case "-":
calculateNumber = String.valueOf(a.subtract(b));
break;
case "*":
calculateNumber = String.valueOf(a.multiply(b));
break;
case "/":
calculateNumber = String.valueOf(a.divide(b, 2, RoundingMode.HALF_UP));//除法保留两位小数
break;
default:
calculateNumber = String.valueOf(a.add(b));
}
number.set(position, calculateNumber);//两个数运算完后将计算结果覆盖到第一个数的位置
calcuStr.remove(position);//每用完一个运算符就删除该运算符
number.remove(position + 1);//两个数运算完后删除后面的数
return calculationOrder(calcuStr, number);
}
}
Test Results:
请输入要计算的四则运算：
30-4*7/2+2
18.00
Regarding recursion, to
30-4*7/2+2
For example:
Continuously recursively calculate the multiplication and division first, and then calculate the addition and subtraction in order from left to right.