How to insert characters inside a string in Java

Question

In my application, a user will select multiple options out of 10 options. Numbers of selected options may vary from 1 to 10. Now I am trying to separate these selected options by inserting a comma in between the numbers. I am getting the numbers like this:

Now I am trying to convert them like this:

 1,2,3,4,5
 3,4,6
 1,2
 5(no comma)

For this I am trying StringBuffer, but I'm getting the wrong output:

 For 12 output is 1,2
 For 5 output is 5
 For 123 output is 1,,23
 For 123456 output is 1,,,,,23456

Can you help me find the mistake in my code?

String str = jTextField1.getText();
StringBuffer sb = new StringBuffer(str);
int x = 0;    
for (int i = 0; i < str.length() - 1; i++) {    
   sb.insert(++x, ",");
}
System.out.println(sb);

I feel like pedagogically, this should be hinted at, but that the questioner should solve. Think what happens as the StringBuffer grows in length with that comma: what does that mean about the index of everything after it? — BaseZen, Mar 01 '15 at 07:28
Is using `StringBuffer` necessary? It seems like you are using it locally within a method, thus a `StringBuilder` might be more fit for the job. — initramfs, Mar 01 '15 at 07:31
@BaseZen Thank you for your comment but I can't understand what you want to say? Kindly tell me in easy understandable version. — Vishal Deb, Mar 01 '15 at 07:31
@CPUTerminator Thank you I am using these classes first time as learner I will be thankful if you can put more lights on your comment — Vishal Deb, Mar 01 '15 at 07:33
As explained clearly in [this question](http://stackoverflow.com/questions/355089/stringbuilder-and-stringbuffer), a `StringBuffer` is useful when multiple threads are writing to the object (which is almost never the case) whilst a `StringBuilder` is the same object without synchronization overhead, allowing for better performance. — initramfs, Mar 01 '15 at 07:37

Abhijeet Dhumal · Answer 1 · 2015-03-01T07:59:44.027

1

You can use regex to achieve it with very simple and fast code:

    String num = "123456789";
    String regex = "(\\d)(?=(\\d{1})+$)";
    String commaSaperatedNums = num.replaceAll(regex, "$1,");
    System.out.println( commaSaperatedNums);

edited Mar 01 '15 at 07:59

answered Mar 01 '15 at 07:40

Abhijeet Dhumal

1,799
13
24

score 0 · Accepted Answer · answered Mar 01 '15 at 07:36

0

@SMA's answer works, but another solution using your code as a base is as follows:

Your StringBuffer is growing as you are inserting commas, which makes your x index out of sync with the actual length of the buffer, therefore you'll need to increment it another time after inserting the comma.

class Test {

  public static void main(String[] args) {
    String str="123456";
    StringBuffer sb=new StringBuffer(str);
    int x=0;
    for (int i = 0;i < str.length()-1; i++)
      {
        sb.insert(++x, ",");
        x += 1;
      }
      System.out.println(sb);  
    }

}

answered Mar 01 '15 at 07:36

Sanketh Katta

5,961
2
29
30

Since people have handed a beginner the answer rather than letting him find it himself (oh well, that's how this site works), I challenge @Vishal Deb to rewrite the loop by walking *backwards* through the `StringBuffer` and observing how the indexing logic changes. – BaseZen Mar 01 '15 at 07:40
@Sanketh Katta Thank U I understood what I missed. – Vishal Deb Mar 01 '15 at 07:43

score 0 · Answer 3 · answered Mar 01 '15 at 07:37

Actually whats happening is, your x=0 initially, but then you insert at '++x' so it goes in 'x-1'. But in next iteration you again do '++x', and it stores comma in 'x=2' but it should be storing in 'x=3' actually, as the size of your array has increased.

Try this.

String str=jTextField1.getText();
StringBuffer sb=new StringBuffer(str);
int x=-1;    
for (int i = 0;i < str.length()-1; i++)
{    
x+=2
sb.insert(x, ",");
}
System.out.println(sb);

How to insert characters inside a string in Java

3 Answers3