Android Calculator给出了错误的答案

Question

I'm making a android one-line calculator. So far this calculator works when I write it in javascript, c++ and c#, but I can't get it to work in android (java). Here is my code:

            String seq = String.valueOf(sec.getText());
            ArrayList<Integer> allNums = new ArrayList<Integer>();
            int i = 0; 
            ArrayList<String> allSigns = new ArrayList<String>();
            String currentNums = "";
            for (i = 0; i< seq.length(); i++)
            {
                if (seq.charAt(i) != '+' && seq.charAt(i) != '-' && seq.charAt(i) != '*' && seq.charAt(i) != '/')
                {
                    currentNums+=seq.charAt(i);
                }
                else
                {
                    allNums.add(Integer.parseInt(currentNums));
                    currentNums="";
                    allSigns.add(String.valueOf(seq.charAt(i)));
                }
            }
            allNums.add(Integer.parseInt(currentNums));

            int result = (Integer)allNums.get(0);
            for (i = 1; i <= allNums.size(); i++)
            {
                if (allSigns.get(i-1) == "+")
                    result+=allNums.get(i);
                else if (allSigns.get(i-1) == "-")
                    result-=allNums.get(i);
                else if (allSigns.get(i-1) == "*")
                    result*=allNums.get(i);
                else if (allSigns.get(i-1) == "/")
                    result/=allNums.get(i);
                else
                {
                    sec.setText(String.valueOf(result));
                    break;
                }
            }

I'm calling the function by onClick event. The app doesn't crash, but only gives me as answer the first number. For example: 34+31-4*8/1 It returns 34. Any Ideas?

Answer 1

You are using the == operator to compare string values, which is not correct. For objects (including Strings), == compares the references to determine if they refer to the same object.

To compare string values, use String#equals instead:

if ("+".equals(allSigns.get(i-1)))

Answer 2

String comparison in java uses "oneString".equals("another") not ==

            ...
            if (allSigns.get(i-1).equals("+"))
                result+=allNums.get(i);
            else if (allSigns.get(i-1).equals("-"))
                result-=allNums.get(i);
            else if (allSigns.get(i-1).equals("*"))
                result*=allNums.get(i);
            else if (allSigns.get(i-1).equals("/"))
                result/=allNums.get(i);
            else
            ....

Should do it.