...for instance what is the equivalent to 'Data::Dumper' ?

Not exactly equivalent... but when I feel like needing Data::Dumper in Java (to get an idea of what some complex data structure contains), I usually use XStream with the JSON backend (which I find a little easier on my eyes than the default XML representation).

The usual

  use Data::Dumper;
  ...
  print Dumper $data;
[download]

then becomes something like

  import com.thoughtworks.xstream.XStream;
  import com.thoughtworks.xstream.io.json.JsonHierarchicalStreamDriver
+;
  ...
  XStream dumper = new XStream(new JsonHierarchicalStreamDriver());
  System.out.println(dumper.toXML(data));
[download]

Not ideal, but better than nothing...