fahd.blog

Sunday, October 01, 2017

Java 9: Eclipse Installation

Java 9 is out! This post shows you how to get developing with Java 9 in Eclipse straightaway!

Install JDK 9
Install Eclipse Oxygen
Install Java 9 Support for Oxygen in Eclipse
Finally, go to "Window > Preferences > Java > Installed JREs" in Eclipse and add your Java 9 JRE

That's it!

Update (14-Oct-2017): Eclipse Oxygen.1a (4.7.1a) supports Java 9, so the "Java 9 Support for Oxygen" plugin is no longer required.

Sunday, August 27, 2017

Java: Enforcing a minimum number of method arguments using varargs

Sometimes you need to write a method that requires at least a certain number of arguments. For example, let's say you have a calculation that requires three or more integer arguments. You could write it like this:

public static int calculate(int... args) {
  if (args.length < 3) {
    throw new IllegalArgumentException("At least 3 arguments expected");
  }
  // do something
  return result;
}

The problem with this is that the client cannot tell how many arguments are required and, if they are pass in too few arguments, the method will fail at runtime rather than compile time. You also have to explicity validate the number of arguments passed into the method.

A better way is to declare the method to take 3 normal arguments and 1 varargs arguments, like this:

public static int calculate(int a, int b, int c, int... rest) {
  // do something
  return result;
}

Saturday, July 29, 2017

Java 8: Multimaps

A multimap is a Map which maps a single key to multiple values e.g. HashMap<String, List<String>>.

Java 8 introduces a Map.computeIfAbsent method, which makes inserting values into a multimap much simpler:

Map<String, List<String>> multimap = new HashMap<>();
multimap.computeIfAbsent(key, k -> new ArrayList<>()).add(value);

Prior to Java 8, a multimap was usually created as follows:

// create the map
Map<String, List<String>> multimap = new HashMap<>();

// put a key/value into the map
List<String> list = multimap.get(key);
if (list == null) {
  multimap.put(key, list = new ArrayList<>());
}
list.add(value);

Or with Guava's Multimap class:

ListMultimap<String, String> multimap = ArrayListMultimap.create();
multimap.put(key, value);

You can read more about Java 8 updates to the Map class in my previous blog post.

Saturday, July 22, 2017

Java: Splitting a Pipe-delimited String - Fast!

This post shows how you can efficiently split a pipe-delimited string e.g. "foo|bar|baz". There are many ways to do this - I could even write my own - but I will only use those that are available in the JDK (or commonly used libraries) and will measure the performance of each.

Remember that, since the pipe symbol (|) is a special character in regular expressions, it needs to be escaped if necessary.

1. String.split

The most obvious way to split a string on the pipe character is to use Java's String.split:

public static String[] split(String s) {
  return s.split("\\|");
}

2. String.split with Pattern.quote

Instead of escaping the pipe ourselves, we can use Pattern.quote to do it for us. (Note: Pattern.quote("|") returns "\Q|\E".)

public static String[] splitWithPatternQuote(String s) {
  return s.split(Pattern.quote("|"));
}

3. Pattern.split

Create a static Pattern and use it to split the string.

private static final Pattern SPLITTER = Pattern.compile("\\|");

public static String[] splitWithPattern(String s) {
  return SPLITTER.split(s);
}

4. StringUtils.split

Apache Commons provides StringUtils.split, which splits a string on a single character:

import org.apache.commons.lang3.StringUtils;

public static String[] splitWithStringUtils(String s) {
  return StringUtils.split(s, '|');
}

So, which one is fastest?

I ran each method on 1 million pipe-delimited strings of different lengths - RandomStringUtils.randomAlphabetic is great for generating random strings - and the table below shows how long each one took:

Method	Time (ms)
`split`	485
`splitWithStringUtils`	520
`splitWithPattern`	643
`splitWithPatternQuote`	936

An interesting observation is that splitWithPatternQuote is so much slower than split, even though they both call String.split internally! If we delve into the source code for String.split, we can see that there is an optimisation (a "fastpath") if the provided regex has two-chars and the first char is a backslash. This applies to "\\|" but, since Pattern.quote produces \Q|\E, it does not use the fastpath and instead creates a new Pattern object for every split. This also explains why it is slower than splitWithPattern, which re-uses the same Pattern object.