Optimizing Opportunities with Java Streams

Question

I was looking through some code and came across this method that takes an HTML Header value (i.e. Content-Disposition=inline;filename=foo.bar) and parses it into a map separated by the semi-colon's into key=value pairs. At first it looked like a good candidate for optimization using a stream, but after I implemented it, the fact that I can't reuse the computed String.indexOf('=') value means the string must be scanned 3 times, which is actually less optimal than the original. I'm perfectly aware that there are many instances where Streams aren't the right tool for the job, but I was wondering if I had just missed some technique that could allow the Stream to be as performant/more performant than the initial code.

  /**
   * Convert a Header Value String into a Map
   *
   * @param value The Header Value
   * @return The data Map
   */
  private static Map<String,String> headerMap (String value) {
    int eq;
    Map<String,String> map = new HashMap<>();
    for(String entry : value.split(";")) {
      if((eq = entry.indexOf('=')) != -1) {
        map.put(entry.substring(0,eq),entry.substring(eq + 1));
      }
    }
    return map;

    return Stream.of(value.split(";")).filter(entry -> entry.indexOf('=') != -1).collect(Collectors.));
  } //headerMap

My attempt at Streaming it:

  /**
   * Convert a Header Value String into a Map
   *
   * @param value The Header Value
   * @return The data Map
   */
  private static Map<String,String> headerMap (String value) {
    return Stream.of(value.split(";")).filter(entry -> entry.indexOf('=') != -1).collect(Collectors.toMap(entry -> entry.substring(0,entry.indexOf('=')),entry -> entry.substring(entry.substring(entry.indexOf('=') + 1))));
  } //headerMap

Answer 1

This solution looks for '=' only once:

private static Map<String, String> headerMap(String value) {
    return Stream.of(value.split(";"))
            .map(s -> s.split("=", 2))
            .filter(arr -> arr.length == 2)
            .collect(Collectors.toMap(arr -> arr[0], arr -> arr[1]));
}

Note that here the fast-path for String.split is used, thus regular expression is not actually created.

Note that using Guava you can do this in quite clean way even prior to Java-8:

private static Map<String, String> headerMap(String value) {
    return Splitter.on( ';' ).withKeyValueSeparator( '=' ).split( value );
}

In general I would advise you against manual parsing of HTTP headers. There are many caveats there. See, for example, how it's implemented in Apache HTTP library. Use libraries.