FULL PRODUCT VERSION :
java version " 1.7.0_10 "
Java(TM) SE Runtime Environment (build 1.7.0_10-b18)
Java HotSpot(TM) Server VM (build 23.6-b04, mixed mode)

ADDITIONAL OS VERSION INFORMATION :
Linux mima-nb 3.2.0-35-generic-pae #55-Ubuntu SMP Wed Dec 5 18:04:39 UTC 2012 i686 i686 i386 GNU/Linux

A DESCRIPTION OF THE PROBLEM :
The contract for String.indexOf(String str, int fromIndex) as stated by javadoc is as follows:
   " The returned index is the smallest value k for which:
       k >= fromIndex && this.startsWith(str, k)
       
  If no such value of k exists, then -1 is returned. "

Formally it will look like this:
  ( (k >= fromIndex) && this.startsWith(str, k) ) || (k == -1)
which implicates
  (k >= fromIndex) || (k == -1)

And here is where this contract is broken:
given:
  s instanceof String
  fromIndex > s.length()
when
  int k = s.indexOf( " " , fromIndex)
then
  k == s.length()

0 <= s.length() == k < fromIndex
  which implicates
0 <= k < fromIndex
  which implicates
!(k == -1) && !(k >= fromIndex)
  which implicates
!(k >= fromIndex || k == -1)

and contracts states exactly opposite:
  k >= fromIndex || k == -1

This finishes the proof that the contract is broken in case presented above.

The root cause of the bug is the first if in the following method:
    static int indexOf(char[] source, int sourceOffset, int sourceCount,
            char[] target, int targetOffset, int targetCount,
            int fromIndex) {
        if (fromIndex >= sourceCount) {
            return (targetCount == 0 ? sourceCount : -1);
        }

STEPS TO FOLLOW TO REPRODUCE THE PROBLEM :
String s = " bug " ;
int fromIndex = s.length() + 1;
int k = s.indexOf( " " , fromIndex);
System.out.println ( " fromIndex= " + fromIndex + " , k= " + k);


EXPECTED VERSUS ACTUAL BEHAVIOR :
EXPECTED -
// system output: fromIndex=4, k=-1
ACTUAL -
// system output: fromIndex=4, k=3

REPRODUCIBILITY :
This bug can be reproduced always.

---------- BEGIN SOURCE ----------
public class StringIndexOfContractBug {

    public static void main(String[] args) {
        String s = " bug " ;
        int fromIndex = s.length() + 1;
        int k = s.indexOf( " " , fromIndex);
        System.out.println ( " fromIndex= " + fromIndex + " , k= " + k);
    }

}
---------- END SOURCE ----------