Uploaded image for project: 'JDK'
  1. JDK
  2. JDK-4096273

java.lang.String.indexOf(String) fails on two empty strings

    XMLWordPrintable

    Details

    • Subcomponent:
    • Resolved In Build:
      1.2beta3
    • CPU:
      sparc
    • OS:
      solaris_2.5
    • Verification:
      Not verified

      Description



      Name: mgC56079 Date: 12/02/97



      > From ###@###.### Tue Dec 2 06:47 MSK 1997
      > To: "'JCK-comments'" <###@###.###>
      >
      > In this code:
      > String s1 = ""; // empty string
      > String s2 = ""; // empty string
      > int n1 = s1.indexOf( s2 );
      > int n2 = s1.indexOf( s2, 0 );
      >
      > the JDK sets both the variables "n1" and "n2" to -1.
      > That is, the JDK reports that the empty string cannot
      > occur within the empty string.
      >
      > This result both defies logic and is **NOT** in accord
      > with the language specification.
      >
      > Consider: The JDK and the spec agree that the empty
      > string is a subset of any non-empty string. That is,
      > "xyz".indexOf( "" )
      > is always zero. And if we put that in terms of sets, it
      > makes sense: "The empty set is the subset of every
      > set."
      >
      > But the JDK does not follow through on the full and proper
      > definition: "The empty set is the subset of every set,
      > INCLUDING the empty set."
      >
      > And, although it is obscure and difficult to follow, the JDK
      > spec agrees with this.
      >
      > Specifically, in 20.12.26, the spec reads (in part):
      >
      > [The method indexOf returns] the smallest value
      > "k" such that
      > this.startsWith(str, k ) && ( k >= fromIndex )
      > is true.
      >
      > In either of the cases shown above, "fromIndex" is zero,
      > so "k" must simply be greater to or equal to zero, so
      > the task is to find
      >
      > ...the smallest non-negative value "k" such that
      > this.startsWith( str, k )
      > is true.
      >
      > Well, the smallest non-negative value is, of course, zero.
      > So let's test to see what
      > this.startsWith( str, 0 )
      > means.
      >
      > We now go to section 20.12.21, which says (after ignoring
      > the subcases that do not apply in this situation):
      >
      > [the method startsWith returns] true if and only if
      > this.subString( toffset ).startsWith( prefix )
      > [is true].
      >
      > But here the value of "toffset" is zero, and 20.12.31 that,
      > effectively, says that
      > substring( 0 )
      > returns a String object that is identical to the original.
      >
      > So we are finally effectively reduced to finding out if
      > this.startsWith( prefix )
      > is true, given that "prefix" is the empty string.
      >
      > And section 20.12.20 says SPECFICALLY
      > "Note that the result will be true is the argument
      > is an empty string..."
      >
      > In other words,
      >
      > "".startsWith( "" ) is TRUE
      > so "".substring( 0 ).startsWith("") is TRUE
      > so "".startsWith( "", 0 ) is TRUE
      > so "".startsWith( "", k ) is TRUE for k==0
      > and 0 is the minimum value of k such that k is >= 0
      > so "".indexOf( "", 0 ) is 0.
      >
      > ***************
      >
      > Note that our in-house tests found this discrepancy between the
      > JDK and the spec. So far as I know, the JCK String tests did
      > NOT find this.
      >
      > Our version of class java.lang.String follows the spec, not the JDK,
      > in this matter.


      ======================================================================

        Attachments

          Activity

            People

            Assignee:
            apalanissunw Anand Palaniswamy (Inactive)
            Reporter:
            mgorshen Mikhail Gorshenev (Inactive)
            Votes:
            0 Vote for this issue
            Watchers:
            0 Start watching this issue

              Dates

              Created:
              Updated:
              Resolved:
              Imported:
              Indexed: