How do I dump the contents of a hash map?

Question

How can I dump the contents of a Java HashMap(or any other), for example to STDOUT ?

As an example, suppose that I have a complex HashMap of the following structure :

( student1 => Map( name => Tim,         
                   Scores => Map( math => 10,
                                  physics => 20,
                                  Computers => 30),
                   place => Miami,
                   ranking => Array(2,8,1,13),
                  ),
 student2 => Map ( 
                   ...............
                   ...............
                 ),
............................
............................
);

So I would like to print it to the screen in order to get an idea about the data structure. I am looking for something similar to PHP's var_dump() or Perl's dumper().

Answer 1

Use HashMap.toString() (docs here):

System.out.println("HASH MAP DUMP: " + myHashMap.toString());

Generally, use Object.toString() to dump data like this.

Answer 2

A good way to dump data structures (e.g. made of nested maps, arrays and sets) is by serializing it to formatted JSON. For example using Gson (com.google.gson):

Gson gson = new GsonBuilder().setPrettyPrinting().create();
System.out.println(gson.toJson(dataStructure));

This will print out the most complex data structures in a fairly readable way.