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How to achieve a multi valued hashmap like HashMap<String,Hashmap<String,String>> in Java [closed]

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I want an outer map to have duplicate keys, therefore cannot use a traditional hashmap.
I want to achieve something like below:
Key Value
Col--->apple-->Apple
Col--->ball--->Ball
Col1--->tree--->Tree
so map would look like,
Key. Value
Col-->[apple-->Apple],[ball-->Ball]
Col1-->[tree--->Tree]
Please help!

A nice, compact way to do this in modern Java is:
Map<String, Map<String, String>> map = new HashMap<>();
map.computeIfAbsent("Col", k -> new HashMap<String,String>()).put("apple", "Apple");
map.computeIfAbsent("Col", k -> new HashMap<String,String>()).put("ball", "Ball");
map.computeIfAbsent("Col", k -> new HashMap<String,String>()).put("tree", "Tree");
A little verbose but I'd assume you'd actually do this in a like in this example:
String[][] values = {
{"Col", "apple", "Apple"},
{"Col", "ball", "Ball"},
{"Col1", "tree", "Tree"},
};
Map<String, Map<String, String>> map = new LinkedHashMap<>();
for (String[] row : values) {
map.computeIfAbsent(row[0], k -> new LinkedHashMap<String, String>()).put(row[1], row[2]);
}
Note: I used LinkedHashMap to preserve order.

I don't see that you would have duplicate keys...
HashMap<String, String> innerMap = new HashMap<>();
innerMap.put("apple", "Apple");
innerMap.put("ball", "Ball");
HashMap<String, String> innerMap1 = new HashMap<>();
innerMap1.put("tree", "Tree");
HashMap<String, HashMap<String, String>> outerMap = new HashMap<>();
outerMap.put("Col", innerMap);
outerMap.put("Col1", innerMap1);
What's wrong with this option - I mean apart from the fact like this seems to be some kind of homework or theoretical use case...

Refactor this code snippet [closed]

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I am unable to refactor and make this snippet cleaner and smaller.
For example i want to add a new value to the listOfValues in one line, instead of first initliazing it to an empty arraylist then calling add on it on a seperate line and finally putting listofvalues in the hasmap on the third line. Also in the else statement is the hasMap.put necessary since we are getting a reference to listOfValues and if we add something to it hashMap will contain it automatically?
MyObject:
key: String
values: List<Value>
Value:
Value1: String
Value2: String
HashMap<String, MyObject> hashMap = new HashMap<>();
List<Value> listOfvalues;
if (!hashMap.containsKey(key)) {
listOfvalues = new ArrayList<>();
listOfvalues.add(Value.builder().withValue1(Value1).withValue2(Value2).build());
hashMap.put(key, MyObject.builder().withkey(key).withvalues(listOfvalues).build());
} else {
listOfvalues = hashMap.get(key).getvalues();
log.info(String.format("Duplicate key: %s. Previous Value(s): %s", key,
listOfvalues));
listOfvalues.add(Value.builder().withValue1(Value1).withValue2(Value2).build());
hashMap.put(key, MyObject.builder().withkey(key).withvalues(listOfvalues).build());
}

The common part with list and map working can be moved out from if/else:
Map<String, MyObject> hashMap = new HashMap<>();
List<Value> listOfvalues;
if (!hashMap.containsKey(key)) {
listOfvalues = new ArrayList<>();
} else {
listOfvalues = hashMap.get(key).getvalues();
log.info(String.format("Duplicate key: %s. Previous Value(s): %s", key, listOfvalues));
}
listOfvalues.add(Value.builder().withValue1(Value1).withValue2(Value2).build());
hashMap.put(key, MyObject.builder().withkey(key).withvalues(listOfvalues).build());

i want to get the keys and values of each set using java [closed]

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Am new to java i have formed a set of result in
`Map<String, Map<String, List<String>>>`
now i want to get the keys and values of each set
How should get this. anyone please suggest me
thanks in advance.

You will want to look at the documentation for Maps.
myArbitrarilyNamedMap = new Map<String, Map<String, List<String>>>();
//do stuff so that myArbitrarilyNamedMap contains values
Set firstLevelKeys = myArbitrarilyNamedMap.keySet(); //this bit

For me is much easier to study with examples. I can give you a little example. May be it will be usefull.
public class MapHierarchy {
public static void main(String[] args) {
// preparation
Map<String, Map<String, List<String>>> myTestMap = new HashMap<>();
ArrayList<String> listOfValues = new ArrayList<>();
listOfValues.add("Hello");
listOfValues.add("my");
listOfValues.add("little");
listOfValues.add("friend");
HashMap<String, List<String>> innerMap = new HashMap<>();
innerMap.putIfAbsent("innerMapKey", listOfValues);
myTestMap.put("outerKey", innerMap);
// where the magic happens
System.out.println("Keys of outer map: " + myTestMap.keySet().toString());
for (Map.Entry<String, List<String>> innerMapItem : innerMap.entrySet()) {
String innerMapItemKey = innerMapItem.getKey();
System.out.println("Key of inner map: " + innerMapItemKey);
System.out.println("Values of inner map: " + innerMapItem.getValue().toString());
}
}}

How can I convert a Map<String, List<Object>> to a List? [closed]

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I like to convert a map that contains a string and a list of object into a list.
Map<String, List<Object>>
How can I do that?
Thank you in advance!

Assuming you want a list of all the objects in the map values this would be a way to do it:
List<Object> all = new LinkedList<>();
for (Collection<Object> l : map.values()) {
all.addAll(l);
}

Map<String, List<Object>> myMap = new HashMap<String, List<Object>>();
// Assuming map containing a key with "Apple"
// This is how you get the list indicated by key Apple
List<Object> myObjects = myMap.get("Apple");

You can convert list of Object into your DTO List as follows.
ArrayList<DTOtoMAP> mylist = (ArrayList<DTOtoMAP>) (Object) hashMap.get(0);

How do I declare multi-dimensional arrays or structures in Java [closed]

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I have this structure in PHP and I'm not really sure how to translate it into Java, I need to mention that all keys and values are strings.
For bonus points, please show an example
private $group = array(
"group1" = array("item1","item2"),
"group2" = array("item3","item4"),
....
"groupn" = array("itemn","itemn+1"),
....
);

Map<String, List<String>> group = new LinkedHashMap<String, List<String>>();
group.put("group1", new ArrayList<String>(Arrays.asList("item1", "item2")));
group.put("group2", new ArrayList<String>(Arrays.asList("item3", "item4")));
In php, if you iterate over the elements with a foreach
foreach ($group as $key => $subarray)
You're guaranteed the first $key is 'group1' because a php array is implicitly order by key creation time. If you want the same guarantee in java you must use LinkedHashMap. Otherwise a regular HashMap is fine.

An array in PHP is actually an ordered map. A map is a type that
associates values to keys. Source
Java uses the true definition of an array. What you want is a Map.

Use a Map<String, Collection<String>>. If values in the collection are unique, use a Set, otherwise a List.

You can use a Map as your solution :
Map<String, List<String>> map = new HashMap<String, List<String>>();
map.put("group1", Arrays.asList("item1", "item2", "item3");
etc...

HashMap<String, ArrayList<String>> map = new HashMap<String, ArrayList<String>>();
ArrayList<String> list = new ArrayList<String>();
list.add("item1");
list.add("item2");
map.add("group1", list);
And so on...