import com.yzk18.GUI.GUI;
import java.util.ArrayList;
import java.util.Stack;

public class 计算机 {
    public static String trespass(String exp) {

        StringBuilder out = new StringBuilder();
        exp = exp.trim();
        for (String x : exp.split("\\s+"))
            out.append(x);
        return out.toString();

    }
    public static ArrayList<String> infix(String exp) {
        ArrayList<String> sb = new ArrayList<>();
        char[] ch = exp.toCharArray();
        for (int i = 0; i < ch.length; i++) {
            StringBuilder in = new StringBuilder(String.valueOf(ch[i]));
            while (Character.isDigit(ch[i]) && (i + 1) != ch.length && Character.isDigit(ch[i + 1])) {
                i++;
                in.append(ch[i]);
            }
            sb.add(in.toString());
        }
        return sb;
    }

    public static boolean digital(String x) {
        boolean ju = true;
        for (int i = 0; i < x.length(); i++) {
            if (!Character.isDigit(x.charAt(i))) {
                ju = false;
                break;
            }
        }
        return ju;
    }
    public static ArrayList<String> inflicts(ArrayList<String> infix) {
        ArrayList<String> post = new ArrayList<>();
        Stack<String> tack = new Stack<>();
        for (String s : infix) {
            if (digital(s)) {
                post.add(s);
            } else if (s.equals("(")) {
                tack.push(s);
            } else if (s.equals(")")) {

                while (!tack.peek().equals("(")) {
                    post.add(tack.pop());
                }
                tack.pop();
            } else {
                if (s.equals("+") || s.equals("-")) {
                    while (!tack.empty()) {
                        if (!tack.peek().equals("("))
                            post.add(tack.pop());
                        else
                            break;
                    }
                    tack.push(s);
                }
                if (s.equals("*") || s.equals("/")) {
                    while (!tack.empty()) {
                        if (!tack.peek().equals("(") && !tack.peek().equals("+") && !tack.peek().equals("-"))
                            post.add(tack.pop());
                        else
                            break;
                    }
                    tack.push(s);
                }

            }

        }
        while (!tack.empty())
        {
            post.add(tack.pop());
        }
        return post;
    }
    public static Double PostCalculate(ArrayList<String> post) {
        Stack<Double> calculate = new Stack<>();
        for (String s : post) {
            if (digital(s)) {
                calculate.push(Double.valueOf(s));
            }
            if (s.equals("+")) {
                Double x = calculate.pop();
                Double y = calculate.pop();
                Double z = y + x;
                calculate.push(z);
            }
            if (s.equals("-")) {
                Double x = calculate.pop();
                Double y = calculate.pop();
                Double z = y - x;
                calculate.push(z);
            }
            if (s.equals("*")) {
                Double x = calculate.pop();
                Double y = calculate.pop();
                Double z = y * x;
                calculate.push(z);
            }
            if (s.equals("/")) {
                Double x = calculate.pop();
                Double y = calculate.pop();
                Double z = y / x;
                calculate.push(z);
            }

        }
        return calculate.peek();
    }
    public static void main(String[] args) {
        String s= GUI.inputBox("请输入表达式");
        GUI.inputBox("结果为：",String.format("%.5f",PostCalculate(inflicts(infix(s)))));
        System.out.print(s);
    }
}

 

com.yzk18.GUI.GUI这个库需自行到https://mvnrepository.com/中下载  具体如何添加自己搜MAven