Storing and Retrieving ArrayList values from hashmap

Our variable:

Map<String, List<Integer>> map = new HashMap<String, List<Integer>>();

To store:

map.put("mango", new ArrayList<Integer>(Arrays.asList(0, 4, 8, 9, 12)));

To add numbers one and one, you can do something like this:

String key = "mango";
int number = 42;
if (map.get(key) == null) {
    map.put(key, new ArrayList<Integer>());
}
map.get(key).add(number);

In Java 8 you can use putIfAbsent to add the list if it did not exist already:

map.putIfAbsent(key, new ArrayList<Integer>());
map.get(key).add(number);

Use the map.entrySet() method to iterate on:

for (Entry<String, List<Integer>> ee : map.entrySet()) {
    String key = ee.getKey();
    List<Integer> values = ee.getValue();
    // TODO: Do something.
}

The modern way (as of 2020) to add entries to a multimap (a map of lists) in Java is:

map.computeIfAbsent("apple", k -> new ArrayList<>()).add(2);
map.computeIfAbsent("apple", k -> new ArrayList<>()).add(3);

According to Map.computeIfAbsent docs:

If the specified key is not already associated with a value (or is mapped to null), attempts to compute its value using the given mapping function and enters it into this map unless null.

Returns: the current (existing or computed) value associated with the specified key, or null if the computed value is null

The most idiomatic way to iterate a map of lists is using Map.forEach and Iterable.forEach:

map.forEach((k, l) -> l.forEach(v -> /* use k and v here */));

Or, as shown in other answers, a traditional for loop:

for (Map.Entry<String, List<Integer>> e : map.entrySet()) {
    String k = e.getKey();
    for (Integer v : e.getValue()) {
        /* use k and v here */
    }
}

Iterator it = map.entrySet().iterator();
while (it.hasNext()) {
     Map.Entry pairs = (Map.Entry)it.next();

     if(pairs.getKey().equals("mango"))
     {
        map.put(pairs.getKey(), pairs.getValue().add(18));
     }

     else if(!map.containsKey("mango"))
     {
        List<Integer> ints = new ArrayList<Integer>();
        ints.add(18);
        map.put("mango",ints);
     }

     it.remove(); // avoids a ConcurrentModificationException
}

EDIT: So inside the while try this:

map.put(pairs.getKey(), pairs.getValue().add(number))

You are getting the error because you are trying to put an integer to the values, whereas it is expected an ArrayList.

EDIT 2: Then put the following inside your while loop:

if(pairs.getKey().equals("mango"))
{
    map.put(pairs.getKey(), pairs.getValue().add(18));
}

else if(!map.containsKey("mango"))
{
     List<Integer> ints = new ArrayList<Integer>();
     ints.add(18);
     map.put("mango",ints);
 }

EDIT 3: By reading your requirements, I come to think you may not need a loop. You may want to only check if the map contains the key mango, and if it does add 18, else create a new entry in the map with key mango and value 18.

So all you may need is the following, without the loop:

if(map.containsKey("mango"))
{
    map.put("mango", map.get("mango).add(18));
}

else
{
    List<Integer> ints = new ArrayList<Integer>();
    ints.add(18);
    map.put("mango", ints);
}

You can use like this(Though the random number generator logic is not upto the mark)

public class WorkSheet {
    HashMap<String,ArrayList<Integer>> map = new HashMap<String,ArrayList<Integer>>();

public static void main(String args[]) {
    WorkSheet test = new WorkSheet();
    test.inputData("mango", 5);
    test.inputData("apple", 2);
    test.inputData("grapes", 2);
    test.inputData("peach", 3);
    test.displayData();

}
public void displayData(){
    for (Entry<String, ArrayList<Integer>> entry : map.entrySet()) {
        System.out.print(entry.getKey()+" | ");
        for(int fruitNo : entry.getValue()){
            System.out.print(fruitNo+" ");
        }
        System.out.println();
    }
}
public void inputData(String name ,int number) {
    Random rndData = new Random();
    ArrayList<Integer> fruit = new ArrayList<Integer>();
    for(int i=0 ; i<number ; i++){
        fruit.add(rndData.nextInt(10));
    }
    map.put(name, fruit);
}
}

OUTPUT

grapes | 7 5 
apple | 9 5 
peach | 5 5 8 
mango | 4 7 1 5 5 

You could try using MultiMap instead of HashMap

Initialising it will require fewer lines of codes. Adding and retrieving the values will also make it shorter.

Map<String, List<Integer>> map = new HashMap<String, List<Integer>>();

would become:

Multimap<String, Integer> multiMap = ArrayListMultimap.create();

You can check this link: http://java.dzone.com/articles/hashmap-%E2%80%93-single-key-and

Method1 : Use putIfAbsent

    Map<String, List<Integer>> map = new HashMap();
    map.putIfAbsent("mango",new ArrayList<>());
    map.get("mango").add(5);

Method 2: Check key present in Map

    Map<String, List<Integer>> map = new HashMap();
    if(! map.containsKey("mango"){
          map.put("mango",new ArrayList<>());
    }
    List<Integer> list = map.get("mango");
    list.add(3);
  
    

Method 3: Use getOrDefault

    Map<String, List<Integer>> map = new HashMap();
    List<Integer> list = map.getOrDefault("mango",new ArrayList<>());
    list.add(4)

      
  

for (Map.Entry<String, ArrayList<Integer>> entry : map.entrySet()) {
 System.out.println( entry.getKey());     
 System.out.println( entry.getValue());//Returns the list of values
}

static HashMap<Integer, ArrayList<Integer>> has(int arr[], int target) {
    HashMap<Integer, ArrayList<Integer>> hm = new HashMap<Integer, ArrayList<Integer>>();
    for (int i = 0; i < arr.length; i++) {
        if (!hm.containsKey(arr[i])) {
            ArrayList<Integer> res = new ArrayList<Integer>();
            res.add(i + 1);
            hm.put(arr[i], res);
        } else {
            hm.get(arr[i]).add(i);
        }
    }
    return hm;
}

Fetch all at once =

List<Integer> list = null;

if(map!= null) 
{ 
  list = new ArrayList<Integer>(map.values()); 
}

For Storing =

if(map!= null) 
{ 
  list = map.get(keyString); 
   if(list == null)
    {
         list = new ArrayList<Integer>();
    }
  list.add(value);
  map.put(keyString,list);
}