As in the Perl API, a mapper or a reducer can increment various counters by using context.getCounter(“Group”, “Name”).increment(value):

public void map(Text key, Text value, Context context) throws IOException, InterruptedException {
  ...
  context.getCounter("Group", "Name").increment(value);
  ...
}

The getCounter method returns a Counter object, so if a counter is incremented frequently, the getCounter method can be called only once:

public void reduce(Text key, Iterable<IntWritable> values, Context context) throws IOException, InterruptedException {
  ...
  Counter values = context.getCounter("Reducer", "Number of values");
  for (IntWritable value : values) {
    ...
    values.increment(1);
  }
}

The job properties can be set:

• on the command line – the ToolRunner parses options in format -Dname=value. See the syntax of the hadoop script.
• using the Job.getConfiguration() a Configuration object is retrieved. It provides following methods:
  • String get(String name) – get the value of the name property, null if it does not exist.
  • String get(String name, String defaultValue) – get the value of the name property
  • getBoolean, getClass, getFile, getFloat, getInt, getLong, getStrings – return a typed value of the name property (i.e., number, file name, class name, …).
  • set(String name, String value) – set the value of the name property to value.
  • setBoolean, setClass, setFile, setFloat, setInt, setLong, setStrings – set the typed value of the name property (i.e., number, file name, class name, …).

Apart from already mentioned brief list of Hadoop properties, there is one important Java-specific property: