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Printing HashMap of HashMaps : Map.Entry or java8

I have a method which returns out hashmap of hashmaps
HashMap<String, HashMap<String, String>> mapofmaps = abcd(<String>, <Integer>);
I am trying to print the the outer hashmap using the following code
for (Entry<String, HashMap<String, String>> entry : mapofmaps.entrySet()) {
String key = entry.getKey();
System.out.println(key);
HashMap<String, String> value = entry.getValue();
System.out.println(key + "\t" + value);
}
I would like to iterate through the inner map. What would be the entryset variable there (??? in the code).
for (Entry<String, HashMap<String, String>> entry : mapofmaps.entrySet()) {
String key = entry.getKey();
System.out.println(key);
for(Entry<String, HashMap<String, String>> entry : ????.entrySet()){
HashMap<String, String> value = entry.getValue();
System.out.println(key + "\t" + value);
}}
Is my logic for printing the hashmaps correct? or is there a better way to do that?

It will be entry.getValue().entrySet()
so
for(Entry<String, String> innerEntry : entry.getValue().entrySet()){
then you can use
String key = innerEntry.getKey();
String value = innerEntry.getValue();
It is worth mentioning that, this can also be done Using java 8 Streams and lambda expressions
HashMap<String, HashMap<String, String>> mapofmaps = new HashMap<>();
HashMap<String,String> map1 = new HashMap<>();
map1.put("map1_key1", "map1_value1");
HashMap<String,String> map2 = new HashMap<>();
map2.put("map2_key1", "map2_value1");
mapofmaps.put("map1", map1);
mapofmaps.put("map2", map2);
// To print the keys and values
mapofmaps.forEach((K,V)->{ // mapofmaps entries
V.forEach((X,Y)->{ // inner Hashmap enteries
System.out.println(X+" "+Y); // print key and value of inner Hashmap
});
});
mapofmaps.forEach((K,V) : This expects a lambda expressions which takes two inputs i.e Key (String) and Value (HashMap)
V.forEach((X,Y)->{ : As this is applied on inner (V: fetched through previous foreach) hashmap so both Key and Value will be strings
Reference for further reading :
Lambda Expressions
Map foreach description

A Straight forward example with data
Map<String, Integer> map = new HashMap<>();
map.put("Apple", 10);
map.put("Motorolla", 20);
map.put("RealMe", 30);
map.put("Oppo", 40);
map.put("Sony", 50);
map.put("OnePlus", 60);
for (Map.Entry<String, Integer> entry : map.entrySet()) {
System.out.println(entry.getKey() + ", Stock : " + entry.getValue());
}
Using lambda expression
map.forEach((K,V) -> System.out.println(K + ", Stock : " + V));

Java: Iterate through a HashMap which is inside another HashMap

I want to iterate through a HashMap which is inside another HashMap
Map<String, Map<String, String>> PropertyHolder
I was able to iterate through the parent HashMap as following,
Iterator it = PropertyHolder.entrySet().iterator();
while (it.hasNext()) {
Map.Entry pair = (Map.Entry) it.next();
System.out.println("pair.getKey() : " + pair.getKey() + " pair.getValue() : " + pair.getValue());
it.remove(); // avoids a ConcurrentModificationException
}
but could not able to iterate through the child Map, It can be done by converting pair.getValue().toString() and separated using , and =. Is there any other way of iterating it?

for (Entry<String, Map<String, String>> entry : propertyHolder.entrySet()) {
Map<String, String> childMap = entry.getValue();
for (Entry<String, String> entry2 : childMap.entrySet()) {
String childKey = entry2.getKey();
String childValue = entry2.getValue();
}
}

You could iterate the child map similar to how you've done the parent:
Iterator<Map.Entry<String, Map<String, String>>> parent = PropertyHolder.entrySet().iterator();
while (parent.hasNext()) {
Map.Entry<String, Map<String, String>> parentPair = parent.next();
System.out.println("parentPair.getKey() : " + parentPair.getKey() + " parentPair.getValue() : " + parentPair.getValue());
Iterator<Map.Entry<String, String>> child = (parentPair.getValue()).entrySet().iterator();
while (child.hasNext()) {
Map.Entry childPair = child.next();
System.out.println("childPair.getKey() : " + childPair.getKey() + " childPair.getValue() : " + childPair.getValue());
child.remove(); // avoids a ConcurrentModificationException
}
}
I've presumed you want to call .remove() on the child map, which will lead to a ConcurrentModificationException if done while looping the entrySet - it looks as though you discovered this already.
I've also swapped out your use of casting with strongly-typed generics as suggested in the comments.

It's obvious - you need two nested loops:
for (String key1 : outerMap.keySet()) {
Map innerMap = outerMap.get(key1);
for (String key2: innerMap.keySet()) {
// process here.
}
}

Iterate over List<Map<String, String>> in Java

I'm trying to iterate List<Map<String, String>> in Java. However, I'm not able to iterate it properly. Can any one guide me?
Iterator it = list.iterator();
while (it.hasNext()) {
Map.Entry pairs = (Map.Entry) it.next();
System.out.println(pairs.getKey() + " = " + pairs.getValue());
}
Thanks,

public static void main(String[] args) {
List<Map<String, String>> myListOfMaps = new ArrayList<Map<String, String>>();
Map<String, String> map1 = new HashMap<String, String>();
map1.put("Fname", "Ankur");
Map<String, String> map2 = new HashMap<String, String>();
map2.put("Lname", "Singhal");
myListOfMaps.add(map1);
myListOfMaps.add(map2);
for (int i = 0 ; i < myListOfMaps.size() ; i++) {
Map<String, String> myMap = myListOfMaps.get(i);
System.out.println("Data For Map" + i);
for (Entry<String, String> entrySet : myMap.entrySet()) {
System.out.println("Key = " + entrySet.getKey() + " , Value = " + entrySet.getValue());
}
}
}
output
Data For Map0
Key = Fname , Value = Ankur
Data For Map1
Key = Lname , Value = Singhal

Forget using the iterator directly, why not simply this:
List<Map<String,String>> l = new ArrayList<>();
...
// add map elements to list
...
for (Map<String,String> m:l) {
for (Map.Entry<String,String> e:m.entrySet()) {
String key = e.getKey();
String value = e.getValue();
// Do something with key/value
}
}
This is called an Enhanced for Loop. Internally it will handle it as a for loop traversing the iterator of any collection, or any other implementation of the Iterable Interface.
It was already used for traversing the Map Entries in one answer, so why not for the list of maps?
Of course, for nested collections, you also need to know how to nest your for-loops (how you put one for loop inside the other).

You iterate over a list with elements of type Map<String, String>. So casting to Map.Entry will give you a ClassCastException.
Try it like this
Iterator it = list.iterator();
while (it.hasNext()) {
Map<String, String> map = (Map<String, String>) it.next();
for (Map.Entry entry : map.entrySet()) {
System.out.println(entry.getKey() + " = " + entry.getValue());
}
}
It would be easier for you, if you didn't use the raw types Iterator and Map.Entry. Use generics wherever possible. So the code would look like this:
Iterator<Map<String, String>> it = list.iterator();
while (it.hasNext()) {
Map<String, String> map = it.next(); //so here you don't need a potentially unsafe cast
for (Map.Entry<String, String> entry : map.entrySet()) {
System.out.println(entry.getKey() + " = " + entry.getValue());
}
}

list has no entySet() method!
Try this:
final Iterator<Map<String, String>> it = list.iterator();
while (it.hasNext())
{
Map<String, String> mapElement = it.next();
// do what you want with the mapElement
}
Of course, you will need another loop to iterate over the elements in the map.

issue with hashmap getting concurrent modification exception

I am getting the below error while using map and performing some remove.How to avoid this ?
Caused by: java.util.ConcurrentModificationException
at java.util.HashMap$HashIterator.nextEntry(HashMap.java:793)
at java.util.HashMap$EntryIterator.next(HashMap.java:834)
at java.util.HashMap$EntryIterator.next(HashMap.java:832)
Map<FormField, Object> ItemMap = domainItem.getValues();
for (Map.Entry<FormField, Object> ValMap : ItemMap.entrySet()) {
List<Field> groupIdList = Mapper.getGroupId(groupFieldId);
for (Field field : groupIdList) {
ItemMap.put(new FormField(field), domainItem.getDomainItemLinkId());
}
ItemMap.remove(ValMap.getKey());
}

Update:
Use Iterator and ConcurrentHashMap to avoid this scenario
Following won't throw exception
Map<Integer, String> map = new ConcurrentHashMap<Integer, String>();
map.put(1, "a");
map.put(2, "b");
map.put(3, "c");
map.put(4, "d");
for (Iterator<Integer> keys = map.keySet().iterator(); keys.hasNext();) {
Integer key = keys.next();
String val = map.get(key);
map.remove(key);
}
or use another map while iterating and at the end copy it to source
for example:
Map<Integer, String> dummy = new HashMap<Integer, String>();
map.put(1, "a");
map.put(2, "b");
map.put(3, "c");
map.put(4, "d");
dummy.putAll(map);
for (Iterator<Integer> keys = dummy.keySet().iterator(); keys.hasNext();) {
Integer key = keys.next();
String val = map.get(key);
map.remove(key);
}
System.out.println(map);

A Map is sorted by the keys in the key-value pairs. When you add or remove elements from the Map while you are iterating through them, the program essentially loses track of "where" in the Map it is.
To get around this, try making a separate, temporary transfer Map. There is also a class called Iterator which might suit your needs.

One way to avoid this issue is to iterate over a copy.
for (Map.Entry<FormField, Object> ValMap :
new HashMap<FormField, Object>(ItemMap).entrySet()) {

can be done without a copy of the map, execute this example and take a look at the code:
public static void main(String[] args)
{
System.out.println("creating map ...");
Map<String, String> dummyMap = new HashMap<>();
while (dummyMap.size() < 10)
{ dummyMap.put(String.valueOf(new Random().nextInt()), String.valueOf(new Random().nextInt())); }
System.out.println("start, map size: " + dummyMap.size() + ", keys=" + dummyMap.keySet());
System.out.print("going to remove: ");
for (Iterator<String> keys = dummyMap.keySet().iterator(); keys.hasNext(); )
{
final String key = keys.next();
// delete map entries per random
if(new Random().nextInt(3)>1)
{
System.out.print(key+" ");
keys.remove();
}
}
System.out.print("\n");
System.out.println("done, map size: " + dummyMap.size() + ", keys=" + dummyMap.keySet());
}
and take a look at this similar question.
HTH,

Get Key value from iterator

I have a HashMap, which contains another HashMap. I want to iterate over the first HashMap and use the Key values from that. Then, as I iterate over the first HashMap I want to start an inner loop iterating over the second HashMap, getting all the values.
The problem I have so far is that I can't figure out how to get the keys from the Iterator.
HashMap<String, HashMap<Integer, String>> subitems = myHashMap.get("mainitem1");
Collection c = subitems.values();
Iterator itr = c.iterator();
while(itr.hasNext())
{
// Get key somehow? itr.getKey() ???
// contains the sub items
HashMap productitem = (HashMap)itr.next();
}
The data that i get from subitems is this:
{Item1{0=sub1, 1=sub2}, Item2{0=sub3, 1=sub4}}
Then, in the while loop productitem contains the 'sub items'. But i can't find out where i can get the key value 'Item1' and 'Item2' from.
How can i get those?

You can't get the key from values().iterator().
You need to use entrySet().iterator(). That will return Map.Entry<K,V> objects on which you can call getKey() and getValue().
for (Map.Entry<Integer,Key> entry : subitems.keySet()) {
Integer key = entry.getKey();
String value = entry.getValue();
// do stuff
}
I'd also like to add that having deeply nested maps of lists of maps is usually a sign that you really want to write custom classes to hold your data. Especially when the maps have pre-defined keys to be used and interpretation of the values in the lists depends on the position within the list! I call this code smell "object denial".

You can't go from value to key in a map. (There may be several keys mapping to the same value!)
You can iterate over the map entries though using subitems.entrySet().iterator(), or you can iterate over the keys, and in each iteration retrieve the associated value through subitems.get(key).

You could do something like this (using iterators):
Set<Entry<String, HashMap<Integer, String>>> c = subitems.entrySet();
Iterator<Entry<String, HashMap<Integer, String>>> iterator = c.iterator();
while(iterator.hasNext())
{
Entry<String, HashMap<Integer, String>> entry = iterator.next();
System.out.println("key:" + entry.getKey());
HashMap<Integer, String> innerMap = entry.getValue();
if (innerMap == null) {
continue;
}
Iterator<Entry<Integer, String>> innerIterator = innerMap.entrySet().iterator();
while (innerIterator.hasNext()) {
Entry<Integer, String> innerEntry = innerIterator.next();
System.out.println("key:" + innerEntry.getKey() + " value: " + innerEntry.getValue());
}
}
or like this using foreach structure:
for (Entry<String, HashMap<Integer,String>> entry : subitems.entrySet())
{
System.out.println("key:" + entry.getKey());
HashMap<Integer, String> innerMap = entry.getValue();
if (innerMap == null) {
continue;
}
for (Entry<Integer, String> innerEntry : innerMap.entrySet())
System.out.println("key:" + innerEntry.getKey() + " value: " + innerEntry.getValue());
}
}

java Collections provide facility of EntrySet. This is a list of objects which contain individual keys and values as its properties. You can take a iterator out of this list.
You can get keys as follows.
Iterator i= subitems.entrySet().iterator();
while(i.hasNext()){
String key= i.next().getkey();
}

You can iterate over entries using entrySet().iterator() on the first HashMap or get the keys and iterate over them: Instead of subitems.values().iterator() use subitems.keys().iterator() and use the next key to get the inner hashmap.