How do I access nested HashMaps in Java

Question

I have a HashMap in Java  the contents of which  as you all probably know  can be accessed by  HashMap get  keyname      If a have a HashMap inside another HashMap i e  a nested HashMap  how would i access the contents  Can i do this like this  inline   HashMap get  keyname   get  nestedkeyname      Thank you

User · Answer

I prefer creating a custom map that extends HashMap. Then just override get() to add extra logic so that if the map doesnt contain your key. It will a create a new instance of the nested map, add it, then return it.

public class KMap<K, V> extends HashMap<K, V> {

    public KMap() {
        super();
    }

    @Override
    public V get(Object key) {
        if (this.containsKey(key)) {
            return super.get(key);
        } else {
            Map<K, V> value = new KMap<K, V>();
            super.put((K)key, (V)value);
            return (V)value;
        }
    }
}

Now you can use it like so:

Map<Integer, Map<Integer, Map<String, Object>>> nestedMap = new KMap<Integer, Map<Integer, Map<String, Object>>>();

Map<String, Object> map = (Map<String, Object>) nestedMap.get(1).get(2);
Object obj= new Object();
map.put(someKey, obj);

User · Answer

import java util    public class MyFirstJava   public static void main String   args      Animal dog   new Animal      dog Info  Dog   Breezi   Lab   Chicken liver      dog Getname      Animal dog2  new Animal      dog2 Info  Dog    pumpkin    POM    Pedigree      dog2 Getname      HashMap lt String   HashMap lt String  Object gt  gt  dogs   new HashMap lt  gt       dogs put  dog1   new HashMap lt  gt      put  Name  dog name      put  Food  dog food  put  Age  3         dogs put  dog2   new HashMap lt  gt      put  Name  dog2 name      put  Food  dog2 food  put  Age  6           dogs get  dog1      System out print dogs     n      System out print dogs get  dog1   get  Age

User · Answer

I came to this StackOverflow page looking for a something ala valueForKeyPath known from objc  I also came by another post -  quot Key-Value Coding quot  for Java  but ended up writing my own    I m still looking for at better solution than PropertyUtils getProperty in apache s beanutils library   Usage  Map lt String  Object gt  json       public String getOptionalFirstName         return MyCode getString json   contact    firstName        Implementation  public static String getString Object object  String key0  String key1        if  key0    null            return null            if  key1    null            return null            if  object instanceof Map    false            return null             SuppressWarnings  unchecked       Map lt Object  Object gt  map    Map lt Object  Object gt  object      Object object1   map get key0       if  object1 instanceof Map    false            return null             SuppressWarnings  unchecked       Map lt Object  Object gt  map1    Map lt Object  Object gt  object1      Object valueObject   map1 get key1       if  valueObject instanceof String    false            return null            return  String valueObject

User · Answer

You can get the nested value by repeating  get    but with deeply nested maps you have to do a lot of casting into Map  An easier way is to use a generic method for getting a nested value   Implementation  public static  lt T gt  T getNestedValue Map map  String    keys        Object value   map       for  String key   keys            value     Map  value  get key              return  T  value      Usage     Map contents with string and even a list       data          vehicles            list                          registration                  owner                    id    3643619                                                            List lt Map gt  list   getNestedValue mapContents   data    vehicles    list    Map first   list get 0   String id   getNestedValue first   registration    owner    id

User · Answer

I hit this discussion while trying to figure out how to get a value from a nested map of unknown depth and it helped me come up with the following solution to my problem   It is overkill for the original question but maybe it will be helpful to someone that finds themselves in a situation where you have less knowledge about the map being searched   private static Object pullNestedVal          Map lt Object  Object gt  vmap          Object     keys        if   keys length    0      vmap size      0             return null        else if  keys length    1            return vmap get keys 0               Object stageObj   vmap get keys 0        if  stageObj instanceof Map            Map lt Object  Object gt  smap    Map lt Object  Object gt   stageObj          Object   skeys   Arrays copyOfRange keys  1  keys length           return pullNestedVal smap  skeys         else           return null

User · Answer

Yes  if you use the proper generic type signature for the outer hashmap   HashMap lt String  HashMap lt String  Foo gt  gt  hm   new HashMap lt String  HashMap lt String  Foobar gt  gt        populate the map hm get  keyname   get  nestedkeyname      If you re not using generics  you d have to do a cast to convert the object retrieved from the outer hash map to a HashMap  or at least a Map  before you could call its get   method   But you should be using generics  -

User · Answer

If you plan on constructing HashMaps with variable depth  use a recursive data structure    Below is an implementation providing a sample interface   class NestedMap lt K  V gt         private final HashMap lt K  NestedMap gt  child      private V value       public NestedMap             child   new HashMap lt  gt             value   null             public boolean hasChild K k            return this child containsKey k              public NestedMap lt K  V gt  getChild K k            return this child get k              public void makeChild K k            this child put k  new NestedMap                public V getValue             return value             public void setValue V v            value   v            and example usage   class NestedMapIllustration       public static void main String   args             NestedMap lt Character  String gt  m   new NestedMap lt  gt              m makeChild  f            m getChild  f   makeChild  o            m getChild  f   getChild  o   makeChild  o            m getChild  f   getChild  o   getChild  o   setValue  bar             System out println               nested element at  f  - gt   o  - gt   o  is                 m getChild  f   getChild  o   getChild  o   getValue

User · Answer

As others have said you can do this but you should define the map with generics like so   Map lt String  Map lt String  String gt  gt  map   new HashMap lt String  Map lt String String gt  gt       However  if you just blindly run the following   map get  keyname   get  nestedkeyname      you will get a null pointer exception whenever keyname is not in the map and your program will crash   You really should add the following check   String valueFromMap   null  if map containsKey  keyname       valueFromMap   map get  keyname   get  nestedkeyname

User · Answer

Yes   See    public static void main String args           HashMap lt String  HashMap lt String  Object gt  gt  map   new HashMap lt String  HashMap lt String Object gt  gt         map put  key   new HashMap lt String  Object gt          map get  key   put  key2    val2         System out println map get  key   get  key2

User · Answer

You can do it like you assumed  But your HashMap has to be templated   Map lt String  Map lt String  String gt  gt  map        new HashMap lt String  Map lt String  String gt  gt       Otherwise you have to do a cast to Map after you retrieve the second map from the first   Map map   new HashMap      Map map get   keyname     get   nestedkeyname

[java] How do I access nested HashMaps in Java?

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