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

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

I often use function like this one ( it might need to be expanded to accommodate prety-print of other types of object inside of Map ).

@SuppressWarnings("unchecked")
private static String hashPP(final Map<String,Object> m, String... offset) {
    String retval = "";
    String delta = offset.length == 0 ? "" : offset[0];
    for( Map.Entry<String, Object> e : m.entrySet() ) {
        retval += delta + "["+e.getKey() + "] -> ";
        Object value = e.getValue();
        if( value instanceof Map ) {
            retval += "(Hash)\n" + hashPP((Map<String,Object>)value, delta + "  ");
        } else if( value instanceof List ) {
            retval += "{";
            for( Object element : (List)value ) {
                retval += element+", ";
            }
            retval += "}\n";
        } else {
            retval += "["+value.toString()+"]\n";
        }
    }
    return retval+"\n";
} // end of hashPP(...)

Thus following code

public static void main(String[] cmd) {
    Map<String,Object> document = new HashMap<String, Object>();

    Map<String,Object> student1 = new LinkedHashMap<String, Object>();
    document.put("student1", student1);
    student1.put("name", "Bob the Student");
    student1.put("place", "Basement");
    List<Integer> ranking = new LinkedList<Integer>();
    student1.put("ranking", ranking);
    ranking.add(2);
    ranking.add(8);
    ranking.add(1);
    ranking.add(13);
    Map<String,Object> scores1 = new HashMap<String, Object>();
    student1.put("Scores", scores1);
    scores1.put("math", "0");
    scores1.put("business", "100");

    Map<String,Object> student2= new LinkedHashMap<String, Object>();
    document.put("student2", student2);
    student2.put("name", "Ivan the Terrible");
    student2.put("place", "Dungeon");

    System.out.println(hashPP(document));
}

will produce output

[student2] -> (Hash)
  [name] -> [Ivan the Terrible]
  [place] -> [Dungeon]

[student1] -> (Hash)
  [name] -> [Bob the Student]
  [place] -> [Basement]
  [ranking] -> {2, 8, 1, 13, }
  [Scores] -> (Hash)
    [math] -> [0]
    [business] -> [100]

Again. You may need to modify this for your particular needs.

Answer 3

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.