Best way to create a hashmap of arraylist

Question

You don't need to re-add the ArrayList back to your Map. If the ArrayList already exists then just add your value to it.

An improved implementation might look like:

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

while processing each line:

String user = user field from line
String value = value field from line

Collection<String> values = map.get(user);
if (values==null) {
    values = new ArrayList<String>();
    map.put(user, values)
}
values.add(value);

Follow-up April 2014 - I wrote the original answer back in 2009 when my knowledge of Google Guava was limited. In light of all that Google Guava does, I now recommend using its Multimap instead of reinvent it.

Multimap<String, String> values = HashMultimap.create();
values.put("user1", "value1");
values.put("user2", "value2");
values.put("user3", "value3");
values.put("user1", "value4");

System.out.println(values.get("user1"));
System.out.println(values.get("user2"));
System.out.println(values.get("user3"));

Outputs:

[value4, value1]
[value2]
[value3]

Use Multimap from Google Collections. It allows multiple values for the same key

https://google.github.io/guava/releases/19.0/api/docs/com/google/common/collect/Multimap.html

Since Java 8 you can use map.computeIfAbsent

https://docs.oracle.com/javase/8/docs/api/java/util/Map.html#computeIfAbsent-K-java.util.function.Function-

Collection<String> values = map.computeIfAbsent(user, k -> new ArrayList<>());
values.add(value);

The ArrayList values in your HashMap are references. You don't need to "put it back to HashMap". You're operating on the object that already exists as a value in the HashMap.

If you don't want to import a library.

package util;    

import java.util.ArrayList;    
import java.util.HashMap;    
import java.util.List;    

/**    
 * A simple implementation of a MultiMap. This implementation allows duplicate elements in the the    
 * values. (I know classes like this are out there but the ones available to me didn't work).    
 */    
public class MultiMap<K, V> extends HashMap<K, List<V>> {    

  /**    
   * Looks for a list that is mapped to the given key. If there is not one then a new one is created    
   * mapped and has the value added to it.    
   *     
   * @param key    
   * @param value    
   * @return true if the list has already been created, false if a new list is created.    
   */    
  public boolean putOne(K key, V value) {    
    if (this.containsKey(key)) {    
      this.get(key).add(value);    
      return true;    
    } else {    
      List<V> values = new ArrayList<>();    
      values.add(value);    
      this.put(key, values);    
      return false;    
    }    
  }    
}    

i think what you want is the Multimap. You can get it from apache's commons collection, or google-collections.

http://commons.apache.org/collections/

http://code.google.com/p/google-collections/

"collection similar to a Map, but which may associate multiple values with a single key. If you call put(K, V) twice, with the same key but different values, the multimap contains mappings from the key to both values."