I want to fix error of parsing into double in a mathematical expression solving java program

Question

I'm a beginner in java. And I don't have ability of complex coding in java. I want to make a program to solve mathematical expression given in string form which must contain only numbers and four basic arithmetic operators. I have written the below code with the little basics that I know. But when i run the code, it shows error. It says i can't parse object into double and so on...

I have tried to give precedence for four basic operators according to BODMAS rule. But i don't know how to give precedence for brackets if the user encloses the expression within brackets.

the below code might seem funny for experts those who can make it with just a few lines, but I am really a beginner.

Here is the code that i made: (comments are not written as this code is simple and anyone can understand)

import java.util.*;
import java.util.concurrent.TimeUnit;
class Calculator
{
public static void main(String[] args)
throws Exception
{
    int aaa =0;
    while(aaa==0){
        Scanner sc = new Scanner(System.in);
        System.out.println("\nEnter expression to solve : ");
        String expression = sc.nextLine();
        if(!expression.isEmpty()){
            int charlength = expression.length();
            String charat0 = ""+expression.charAt(0);
            String charatlast = ""+expression.charAt(charlength-1);
            ArrayList forops = new ArrayList();
            ArrayList forchars = new ArrayList();
            if(expression.contains(" ")){
                pWD("\nSpaces are not allowed",TimeUnit.MILLISECONDS,30);
            }else if(!expression.matches("[-+/*0-9]+")){
                pWD("\nEnter only numbers and operators",TimeUnit.MILLISECONDS,30);
            }else if(!charat0.matches("[0-9]+")||!charatlast.matches("[0-9]+")){
                pWD("first and last characters must be\nnumbers",TimeUnit.MILLISECONDS,30);
            }else if(expression.contains("//")||expression.contains("**")||expression.contains("--")||expression.contains("++")||
                     expression.contains("/+")||expression.contains("*/")||expression.contains("-*")||expression.contains("+-")||
                     expression.contains("/-")||expression.contains("*+")||expression.contains("-/")||expression.contains("+*")||
                     expression.contains("/*")||expression.contains("*-")||expression.contains("-+")||expression.contains("+/")){
                pWD("There must be SINGLE operator between\ntwo adjacent numbers",TimeUnit.MILLISECONDS,30);
            }else{
                String[] chararr = expression.split("(?<=[-+*/])|(?=[-+*/])");
                int chararrl =chararr.length;
                for(int nn=0;nn<chararrl;nn++){
                    forchars.add(chararr[nn]);
                }for(int mn=0;mn<chararrl;mn+=2){
                    String willparse = ""+forchars.get(mn);
                    double parseint= Double.parseDouble(willparse);
                    forchars.set(mn,parseint);
                }while(forchars.contains("/")){
                    int divindex = forchars.indexOf("/");
                    double prediv = forchars.get(divindex-1);
                    double nexdiv = forchars.get(divindex+1);
                    forchars.set(divindex,(prediv/nexdiv));
                    forchars.remove(forchars.indexOf(prediv));
                    forchars.remove(forchars.indexOf(nexdiv));
                }while(forchars.contains("*")){
                    int mulindex = forchars.indexOf("*");
                    double premul = forchars.get(mulindex-1);
                    double nexmul = forchars.get(mulindex+1);
                    forchars.set(mulindex,(premul*nexmul));
                    forchars.remove(forchars.indexOf(premul));
                    forchars.remove(forchars.indexOf(nexmul));
                }while(forchars.contains("-")){
                    int subindex = forchars.indexOf("-");
                    double presub = forchars.get(subindex-1);
                    double nexsub = forchars.get(subindex+1);
                    forchars.set(subindex,(presub-nexsub));
                    forchars.remove(forchars.indexOf(presub));
                    forchars.remove(forchars.indexOf(nexsub));
                }while(forchars.contains("+")){
                    int addindex = forchars.indexOf("+");
                    double preadd = forchars.get(addindex-1);
                    double nexadd = forchars.get(addindex+1);
                    forchars.set(addindex,(preadd+nexadd));
                    forchars.remove(forchars.indexOf(preadd));
                    forchars.remove(forchars.indexOf(nexadd));}
                double answer1 = forchars.get(0);
                String answer = Double.toString(answer1);
                pWD("\nFinal answer is : "+answer,TimeUnit.MILLISECONDS,80);
            }}else{
            pWD("Please enter something",TimeUnit.MILLISECONDS,30);
        }}
}public static void pWD(String data, TimeUnit unit, long delay)
throws InterruptedException {
    for (char ch:data.toCharArray()) {
        System.out.print(ch);
        unit.sleep(delay);
        }
    }
}

Please help me how to fix the error of parsing into double. And explain me in a little detail on how to solve expression if user encloses the expression in brackets.

Thank You.

What line does the error occur on? Even better, can you make an example with less code, isolating the point where the error occurs? — Phil Freihofner, Feb 24 '17 at 08:27
You should print out the results of your `split` operation. Probably one of the substrings cannot be converted to a number. — Frank Puffer, Feb 24 '17 at 08:28

score 0 · Accepted Answer · answered Feb 24 '17 at 08:30

0

Since you are declaring a double variable, you cannot assign an int value to it. So I assume the best option you should have is to:

cast your int to double

double prediv = (double) forchars.get(divindex - 1);

It is just the type mismatch which you should take into account.

answered Feb 24 '17 at 08:30

Dani_NJ

48
1
7

You might want to refer to [this](http://stackoverflow.com/questions/5289393/casting-variables-in-java) link to get to know more on casting. – Dani_NJ Feb 24 '17 at 08:33
yes it worked. but when i run this in command prompt, it shows like this : Note: Calculator.java uses unchecked or unsafe operations. Note: Recompile with -Xlint:unchecked for details. I don't have idea what to do. help me... – Abhilash Feb 24 '17 at 08:45
What I can see is that you are using an ArrayList without specifying the type. If you wanna get rid of it try to manipulate your code according to this [link](http://stackoverflow.com/questions/197986/what-causes-javac-to-issue-the-uses-unchecked-or-unsafe-operations-warning). Your code was working in `Eclipse` IDE though! – Dani_NJ Feb 25 '17 at 04:33

I want to fix error of parsing into double in a mathematical expression solving java program

1 Answers1