将字符串变成算数式

import java.math.BigDecimal;

import java.math.RoundingMode;

import java.util.*;



import javax.script.ScriptEngine;

import javax.script.ScriptEngineManager;

import javax.script.ScriptException;

import javax.security.auth.kerberos.KerberosKey;

public class Main{

    private static String multipleBigDecimal(String a,String b)

    {

        double a1=Double.parseDouble(a);

        double b1=Double.parseDouble(b);

        BigDecimal a2=BigDecimal.valueOf(a1);

        BigDecimal b2=BigDecimal.valueOf(b1);

        BigDecimal s=a2.multiply(b2);

        return s.toString();

    }

    private static String divideBigDecimal(String a,String b)

    {

        double a1=Double.parseDouble(a);

        double b1=Double.parseDouble(b);

        BigDecimal a2=BigDecimal.valueOf(a1);

        BigDecimal b2=BigDecimal.valueOf(b1);

        BigDecimal s=a2.divide(b2,5,RoundingMode.HALF_UP);//当这里可能出现无限循环小数时，就必须指定保留位数

        return s.toString();

    }

    private static String addBigDecimal(String a,String b)

    {

        double a1=Double.parseDouble(a);

        double b1=Double.parseDouble(b);

        BigDecimal a2=BigDecimal.valueOf(a1);

        BigDecimal b2=BigDecimal.valueOf(b1);

        BigDecimal s=a2.add(b2);

        return s.toString();

    }

    private static String substractBigDecimal(String a,String b)

    {

        double a1=Double.parseDouble(a);

        double b1=Double.parseDouble(b);

        BigDecimal a2=BigDecimal.valueOf(a1);

        BigDecimal b2=BigDecimal.valueOf(b1);

        BigDecimal s=a2.subtract(b2);

        return s.toString();

    }

    //an example   9+3*3-6/2+3

    private static String sizeyunsuan(String s)

    {

        //1、先将  +-*/找出来

        int p=0;//the count of (+-*/)

        for(int i=0;i<s.length();i++)

        {

            if(s.charAt(i)=='+' || s.charAt(i)=='-' ||s.charAt(i)=='*' ||s.charAt(i)=='/')

                p++;

        }

        //将字符串按照运算符进行切割总共有  2*p+1个段   example:p=5

        String[] piece=new String[2*p+1];// save pieces divided by operator   11 pieces

        int start=0,index=0;//

        for(int i=0;i<s.length();i++)

        {

            if(s.charAt(i)=='+' || s.charAt(i)=='-' ||s.charAt(i)=='*' ||s.charAt(i)=='/')

            {

                piece[index]=s.substring(start,i);//index=0  i=1  p[0]="9"

                index++;

                piece[index]=""+s.charAt(i);    //index=1  i=1  p[1]="+"

                index++;

                start=i+1;   //start=2

            }

        }

        // last piece;

        piece[index]=s.substring(start,s.length());

        ///

        int count=p;

        while(count>0)

        {

            // first calculate */

            for(int i=0;i<piece.length;i++)

            {

                if(piece[i].equals("*") || piece[i].equals("/"))

                {

                    //find strs in piece has not calculated

                    //find strs left not equals "p"

                    int l=0;

                    for(l=i-1;l>-1;l--)

                    {

                        if(!piece[l].equals("p"))

                            break;

                    }

                    //find strs right not equals "p"

                    int r=0;

                    for(r=i+1;r<piece.length;r++)

                    {

                        if(!piece[r].equals("p"))

                            break;

                    }

                    if(piece[i].equals("*"))

                    {

                        piece[i]=multipleBigDecimal(piece[l], piece[r]);

                        piece[l]="p";

                        piece[r]="p";

                        count--;

                    }

                    else

                    {

                        piece[i]=divideBigDecimal(piece[l], piece[r]);

                        piece[l]="p";

                        piece[r]="p";

                        count--;

                    }

                    //break;

                }

            }

            //then calculate +-

            for(int i=0;i<piece.length;i++)

            {

                if(piece[i].equals("+") || piece[i].equals("-"))

                {

                    //find strs in piece has not calculated

                    //find strs left not equals "p"

                    int l=0;

                    for(l=i-1;l>-1;l--)

                    {

                        if(!piece[l].equals("p"))

                            break;

                    }

                    //find strs right not equals "p"

                    int r=0;

                    for(r=i+1;r<piece.length;r++)

                    {

                        if(!piece[r].equals("p"))

                            break;

                    }

                    if(piece[i].equals("+"))

                    {

                        piece[i]=addBigDecimal(piece[l], piece[r]);

                        piece[l]="p";

                        piece[r]="p";

                        count--;

                    }

                    else

                    {

                        piece[i]=substractBigDecimal(piece[l], piece[r]);

                        piece[l]="p";

                        piece[r]="p";

                        count--;

                    }

                    //break;

                }

            }

        }

        String r="";

        //find the string not equals "p"

        for(int i=0;i<piece.length;i++)

        {

            if(!piece[i].equals("p"))

            {

                r=piece[i];

                break;

            }

        }

        return r;

    }

    private static void calculate(String s)

    {

        while(true)

        {

            //find the innerest ()

            int start=0,last=0;

            for(int i=0;i<s.length();i++)

            {

                if(s.charAt(i)=='(')

                    start=i;

                if(s.charAt(i)==')')

                {

                    last=i;

                    break;

                }

            }

            if(last==0)

            {

                System.out.println("自定义方法"+sizeyunsuan(s));

                return;

            }

            else

            {

                String s1=s.substring(0,start);

                String s2=s.substring(start+1,last);

                String s3=s.substring(last+1,s.length());

                s=s1+sizeyunsuan(s2)+s3;

            }

        }

    }

      public static void main(String[] args) throws ScriptException{

          String s="3.6+(1.4+2*32/(3-2.1))*2.3";

          calculate(s);

          

          ScriptEngineManager manager=new ScriptEngineManager();

          ScriptEngine engine=manager.getEngineByName("js");

          Object result=engine.eval(s);

          System.out.println("脚本方法 类型"+result.getClass().getName()+"  值："+result);

      }

      

}

输出

自定义方法170.375553
脚本方法 类型java.lang.Double 值：170.37555555555556