I know, this is an old question. But just for the sake of completeness, the lambda version.

Map<String, List<Item>> items = new HashMap<>();
items.computeIfAbsent(key, k -> new ArrayList<>()).add(item);

problem:

prodHashMap.put("prod", tvProd.getText().toString());

You are using the same key each time you are adding an element to the the arraylist with the same reference to the HashMap thus changing its values.

Solution:

create a new instance of HashMap each time you want to add it to the ArrayList to avoid changing its values upon calling addProd

public void addProd(View ap)
{
    // test arraylist of hashmaps
    HashMap<String, String> prodHashMap = new HashMap<String, String>();
    prodHashMap.put("prod", tvProd.getText().toString());

    prodArrayList.add(prodHashMap);

    tvProd.setText("");

    // check data ///

    Log.e("myLog","Data prodArrayList in ADD Method Size = "+prodArrayList.size());

    for(int i=0; i< prodArrayList.size();i++)
    {
         Log.e("myLog","Data prodArrayList in ADD Method = "+prodArrayList.get(i).toString());
    }
}

The general methodology would be to iterate through the ArrayList, and insert the values into the HashMap. An example is as follows:

HashMap<String, Product> productMap = new HashMap<String, Product>();
for (Product product : productList) {
   productMap.put(product.getProductCode(), product);
}

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]

I think what you need is a Map of Map of List.ie.,

Map< String, Map< String, List< String>>>

Sample code below:

import java.io.IOException;
import java.util.ArrayList;
import java.util.HashMap;
import java.util.List;
import java.util.Map;
import java.util.Map.Entry;

public class Test {
public static void main(String[] args) throws IOException {

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

    //Adding
    addSchedule(scheduleMap, "1111", "Engineering", "Tue3");
    addSchedule(scheduleMap, "1111", "Engineering", "Wed2");
    addSchedule(scheduleMap, "1111", "Math", "Wed4");
    addSchedule(scheduleMap, "1111", "Physics", "Fri10");
    addSchedule(scheduleMap, "2222", "Engineering", "Wed2");
    addSchedule(scheduleMap, "2222", "Chemistry ", "Wed4");

    //Printing
    printSchedule(scheduleMap);

}

private static void printSchedule(Map<String, Map<String, List<String>>> scheduleMap) {
    //Sample printing
    for(Entry<String, Map<String, List<String>>> staff : scheduleMap.entrySet()) {
        for(Entry<String,List<String>> subject:staff.getValue().entrySet()) {
            StringBuilder sb=new StringBuilder();
            sb.append(staff.getKey()).append(" : ").append(subject.getKey()).append(" : ");
            for(String sessionTime: subject.getValue()) {
                sb.append(sessionTime).append(", ");
            }
            sb.setLength(sb.length()-2);
            System.out.println(sb.toString());
        }
    }
}

public static void addSchedule(Map<String,Map<String,List<String>>> scheduleMap, String id,String subject,String sessionTime) {
    if(!scheduleMap.containsKey(id)) {
        scheduleMap.put(id, new HashMap<String,List<String>>());
    }
    if(!scheduleMap.get(id).containsKey(subject)) {
        scheduleMap.get(id).put(subject, new ArrayList<String>());
    }
    scheduleMap.get(id).get(subject).add(sessionTime);
}
}

You could either use the Google Guava library, which has implementations for Multi-Value-Maps (Apache Commons Collections has also implementations, but without generics).

However, if you don't want to use an external lib, then you would do something like this:

if (map.get(id) == null) { //gets the value for an id)
    map.put(id, new ArrayList<String>()); //no ArrayList assigned, create new ArrayList

map.get(id).add(value); //adds value to list.