Pretty-print a Map in Java

Question

I am looking for a nice way to pretty-print a Map   map toString   gives me   key1 value1  key2 value2  key3 value3   I want more freedom in my map entry values and am looking for something more like this  key1  value1   key2  value2   key3  value3   I wrote this little piece of code   StringBuilder sb   new StringBuilder    Iterator lt Entry lt String  String gt  gt  iter   map entrySet   iterator    while  iter hasNext          Entry lt String  String gt  entry   iter next        sb append entry getKey         sb append      append           sb append entry getValue         sb append           if  iter hasNext              sb append      append               return sb toString      But I am sure there is a more elegant and concise way to do this

User · Answer

I prefer to convert the map to a JSON string it is:

a standard
human readable
supported in editors like Sublime, VS Code, with syntax highlighting, formatting and section hide/show
supports JPath so editors can report exactly which part of the object you have navigated to

supports nested complex types within the object

import com.fasterxml.jackson.core.JsonProcessingException;
import com.fasterxml.jackson.databind.ObjectMapper;

public static String getAsFormattedJsonString(Object object)
{
    ObjectMapper mapper = new ObjectMapper();
    try
    {
        return mapper.writerWithDefaultPrettyPrinter().writeValueAsString(object);
    }
    catch (JsonProcessingException e)
    {
        e.printStackTrace();
    }
    return "";
}

User · Answer

Simple and easy  Welcome to the JSON world  Using Google s Gson   new Gson   toJson map    Example of map with 3 keys     array   null  Some string    just string   Yo   number  999

User · Answer

You should be able to do what you want by doing     System out println map  for example    As long as ALL your objects in the map have overiden the toString method you would see   key1 value1  key2 value2  in a meaningfull manner  If this is for your code  then overiding toString is a good habit and I suggest you go for that instead     For your example where your objects are Strings you should be fine without anything else  I e  System out println map  would print exactly what you need without any extra code

User · Answer

Or put your logic into a tidy little class   public class PrettyPrintingMap lt K  V gt        private Map lt K  V gt  map       public PrettyPrintingMap Map lt K  V gt  map            this map   map             public String toString             StringBuilder sb   new StringBuilder            Iterator lt Entry lt K  V gt  gt  iter   map entrySet   iterator            while  iter hasNext                  Entry lt K  V gt  entry   iter next                sb append entry getKey                 sb append      append                   sb append entry getValue                 sb append                   if  iter hasNext                      sb append      append                                       return sb toString               Usage   Map lt String  String gt  myMap   new HashMap lt String  String gt      System out println new PrettyPrintingMap lt String  String gt  myMap      Note  You can also put that logic into a utility method

User · Answer

Apache libraries to the rescue   MapUtils debugPrint System out   myMap   map     All you need Apache commons-collections library  project link   Maven users can add the library using this dependency    lt dependency gt       lt groupId gt commons-collections lt  groupId gt       lt artifactId gt commons-collections lt  artifactId gt       lt version gt 3 2 1 lt  version gt   lt  dependency gt

User · Answer

When I have org json JSONObject in the classpath  I do   Map lt String  Object gt  stats        System out println new JSONObject stats  toString 2       this beautifully indents lists  sets and maps which may be nested

User · Answer

public void printMapV2  Map  lt      gt  map        StringBuilder sb   new StringBuilder 128       sb append           for  Map Entry lt     gt  entry   map entrySet              if  sb length   gt 1                sb append                          sb append entry getKey    append      append entry getValue               sb append           System out println sb

User · Answer

I guess something like this would be cleaner  and provide you with more flexibility with the output format  simply change template        String template     s    s          StringBuilder sb   new StringBuilder        for  Entry e   map entrySet              sb append String format template  e getKey    e getValue                if  sb length    gt  0            sb deleteCharAt sb length   - 1      Ugly way to remove the last comma           return sb toString      I know having to remove the last comma is ugly  but I think it s cleaner than alternatives like the one in this solution or manually using an iterator

User · Answer

Since java 8 there is easy way to do it with Lambda     yourMap keySet   forEach key - gt        Object obj   yourMap get key       System out println  obj

User · Answer

As a quick and dirty solution leveraging existing infrastructure  you can wrap your uglyPrintedMap into a java util HashMap  then use toString       uglyPrintedMap toString       ugly System out println  uglyPrintedMap       prints in an ugly manner  new HashMap lt Object  Object gt  jobDataMap  toString       pretty System out println  new HashMap lt Object  Object gt  uglyPrintedMap        prints in a pretty manner

User · Answer

Does not answer exactly the question  but it is worth mentioning Lombodok  ToString annotation  If you annotate with  ToString the key   value classes  then doing System out println map  will return something meaningful    It also works very well with maps-of-maps  for example   Map lt MyKeyClass  Map lt String  MyValueClass gt  gt  will be printed as    MyKeyClass properties      string1 MyValuesClass properties      string2 MyValuesCalss properties

User · Answer

String result   objectMapper writeValueAsString map  - as simple as this   Result     2019-07-04T03 00  1  2019-07-04T04 00  1  2019-07-04T01 00  1  2019-07-04T02 00  1  2019-07-04T13 00  1  2019-07-04T06 00  1        P S  add Jackson JSON to your classpath

User · Answer

Look at the code for HashMap toString   and AbstractMap toString   in the OpenJDK sources   class java util HashMap Entry lt K V gt  implements Map Entry lt K V gt           public final String toString                return getKey           getValue                   class java util AbstractMap lt K V gt         public String toString              Iterator lt Entry lt K V gt  gt  i   entrySet   iterator             if    i hasNext                return                StringBuilder sb   new StringBuilder            sb append               for                    Entry lt K V gt  e   i next                K key   e getKey                V value   e getValue                sb append key      this     this Map     key               sb append                   sb append value    this     this Map     value               if    i hasNext                    return sb append      toString                sb append                            So if the guys from OpenJDK did not find a more elegant way to do this  there is none  -

User · Answer

Arrays toString map entrySet   toArray

User · Answer

Have a look at the Guava library   Joiner MapJoiner mapJoiner   Joiner on      withKeyValueSeparator       System out println mapJoiner join map

User · Answer

Using Java 8 Streams   Map lt Object  Object gt  map   new HashMap lt  gt      String content   map entrySet                        stream                        map e - gt  e getKey             e getValue                                collect Collectors joining          System out println content

[java] Pretty-print a Map in Java

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