How does Integer.toString() works internally?

Question

I found that a similar question has been asked before here : how does Float.toString() and Integer.toString() works?

But this doesn't speak about how that function internally works. When I opened the internally source code of Integer.toString(), it is not understandable for normal junior java programmer.

Can somebody please explain what happens internally in short description ?

NOTE : This was one of the interview questions that I was asked recently. I had no idea about how to answer such question !

It's by no means simple and probably too broad for this site. Probably best to Google it. — Bathsheba, Aug 26 '14 at 12:24
Iterating over all numbers in the provided base, every number is converted to its corresponding `char`. The array of chars is then converted to a `String`. — Pphoenix, Aug 26 '14 at 12:30
"this...has been asked before...But this doesn't speak about how that function internally works." Go look at that other question again, and specifically, look at Stephen C's answer. He explains the heart of the algorithm in two sentences. If you know the math, it will be obvious. Unfortunately, If you don't know the math, then StackOverflow may not be the best place for you to seek that knowledge. — Solomon Slow, Aug 26 '14 at 12:47

Rudi Kershaw · Accepted Answer · 2014-08-26T14:15:00.923

The no arg call of integer.toString() simply calls the static method Integer.toString(int i) (using the integer variables own primitive value), which is implemented as below;

  public static String toString(int i) {
       if (i == Integer.MIN_VALUE)
           return "-2147483648";
       int size = (i < 0) ? stringSize(-i) + 1 : stringSize(i);
       char[] buf = new char[size];
       getChars(i, size, buf);
       return new String(0, size, buf);
   }

First it checks whether it's value is == the lowest possible integer, and returns that if it is equal. If not, then it checks what size the String needs to be using the stringSize() method of Integer to use as the size of an array of characters.

stringSize() implementation below;

  static int stringSize(int x) {
       for (int i=0; ; i++)
           if (x <= sizeTable[i])
               return i+1;
   }

Once it has a char[] of the correct size, it then populates that array using the getChars() method, implemented below;

  static void getChars(int i, int index, char[] buf) {
       int q, r;
       int charPos = index;
       char sign = 0;

       if (i < 0) {
           sign = '-';
           i = -i;
       }

       // Generate two digits per iteration
       while (i >= 65536) {
           q = i / 100;
       // really: r = i - (q * 100);
           r = i - ((q << 6) + (q << 5) + (q << 2));
           i = q;
           buf [--charPos] = DigitOnes[r];
           buf [--charPos] = DigitTens[r];
       }

       // Fall thru to fast mode for smaller numbers
       // assert(i <= 65536, i);
       for (;;) {
           q = (i * 52429) >>> (16+3);
           r = i - ((q << 3) + (q << 1));  // r = i-(q*10) ...
           buf [--charPos] = digits [r];
           i = q;
           if (i == 0) break;
       }
       if (sign != 0) {
           buf [--charPos] = sign;
       }
   }

Explaining each individual step would take far too long for for a stackoverflow answer. The most pertinent section however (as pointed out in the comments) is the getChars() method which, complicated bit shifting aside, is essentially process of elimination for finding each character. I am afraid I can't go into any greater detail than that without going beyond my own understanding.

@assylias - Updated answer to be as in-depth as a SO answer can be on this subject (given the limited space). Added implementations for `stringSize()` and `getChars()`. — Rudi Kershaw, Aug 26 '14 at 12:37

How does Integer.toString() works internally?

1 Answers1

Linked