Removing item from a list inside hashmap

Question

I have an List<String> in a HashMap as value. The key is a String . When I loop through the HashMap and remove items from the list of a particular key it updates the lists mapped to all the keys in the map.

The code is below

public class ListClass {
public static void main(String[] args) {
    ListClass lc = new ListClass();
    for(Map.Entry<String, List<String>> entry : lc.postProcessList().entrySet()) {
        System.out.println("Date : "+entry.getKey());
        List<String> data = entry.getValue();
        for (int i = 0; i < data.size(); i++) {
            System.out.println("    Value : "+data.get(i));
        }
    }
} 

private Map<String,List<String>> postProcessList() {
    Map<String,List<String>> map = new HashMap<String, List<String>>();
    populateMap(map);
    for(Map.Entry<String, List<String>> entry : map.entrySet()) {
        String dateKey = entry.getKey();
        System.out.println("Date key : "+dateKey);
        List<String> data = entry.getValue();
        System.out.println("List before modification : "+data);
        for (int i = 0; i < data.size(); i++) {
            String dateNoTime = data.get(i).split(" ")[0];
            if(!dateNoTime.equalsIgnoreCase(dateKey)) {
                System.out.println("Removing : "+data.get(i));
                data.remove(i);
            }   
        }
        System.out.println("List after modification: "+data+"\n\n");
    }
    return map;
}

private Map<String,List<String>> populateMap(Map<String,List<String>> map) {
    List<String> list = new ArrayList<String>();
    list.add("2015-01-13 09:30:00");
    list.add("2015-01-12 05:45:10");
    list.add("2015-01-13 06:22:12");
    list.add("2015-01-12 01:52:40");
    list.add("2015-01-12 02:23:45");    
    map.put("2015-01-12", list);
    map.put("2015-01-13", list);
    return map;
}
}

In the above code, the Map has key which is a string and holds date values. List, which is the value of the map, holds date and time.

First key is "2015-01-12" and I'm trying to remove the items in the list that are not "2015-01-12". Now the list in both the keys get's updated. Why is that so?

Answer 1

You are adding the same list instance to the map against both keys. Therefore when you retrieve it using key "2015-01-12" and edit it, you'll see the changes in the entire map.

You need to clone the list using something like list.clone() or new ArrayList<String>(list) before you add it to each key.

eg

ArrayList<String> list = new ArrayList<String>();
list.add("2015-01-13 09:30:00");
list.add("2015-01-12 05:45:10");
list.add("2015-01-13 06:22:12");
list.add("2015-01-12 01:52:40");
list.add("2015-01-12 02:23:45");    

map.put("2015-01-12", (List<String>)list.clone());
map.put("2015-01-13", (List<String>)list.clone());