Adding a value to a list to an already existing key in Map

Question

Evening!

I have the following Map:

HashMap<String, ArrayList> myMap = new HashMap<String, ArrayList>();

I then added the following data to it:

ArrayList myList = new ArrayList();
myList.add("Test 1");
myList.add("Test 2");
myList.add("Test 3");
myMap.put("Tests", myList);

This left me with the following data:

Key: Tests

---

Values: Test 1, Test 2, Test 3

My question is, how do I then add new values on to my already existing key? So for example, how could I add the value "Test 4" onto my key "Tests".

Thanks.

Solution 1

Simply get the list from the map and then add the element to the list:

ArrayList list = myMap.get("Tests");
list.add("Test4");

There are some other things that can be remarked about your code. First of all, don't use the raw type ArrayList. Use generics:

HashMap<String, ArrayList<String>> myMap = new HashMap<String, ArrayList<String>>();

ArrayList<String> myList = new ArrayList<String>();
myList.add("Test 1");
myList.add("Test 2");
myList.add("Test 3");
myMap.put("Tests", myList);

Second, program to interfaces, not implementations. In other words, program using interfaces Map and List rather than the implementations HashMap and ArrayList. This is a well-known OO programming principle, which makes it for example easier to switch to a different implementation, if necessary.

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

List<String> myList = new ArrayList<String>();
myList.add("Test 1");
myList.add("Test 2");
myList.add("Test 3");
myMap.put("Tests", myList);

Finally, a syntax tip: if you're using Java 7 or newer you can use <> and you don't have to repeat the type arguments:

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

List<String> myList = new ArrayList<>();
myList.add("Test 1");
myList.add("Test 2");
myList.add("Test 3");
myMap.put("Tests", myList);

myMap.get("Tests").add("Test 4");

Solution 2

I'd recommend using Map#computeIfAbsent, to always ensure retrieving a List from the map:

private final Map<String, List<String>> example = new HashMap<>();

private List<String> getList(String key) {
    return this.example.computeIfAbsent(key, k -> new ArrayList<>());
}

//elsewheres
getList("test").add("foobar");
getList("test").forEach(System.out::println); // "foobar"

This means that if the map doesn't contain an entry for the key, it will use the provided lambda to generate a new value for the key and return that.

Solution 3

import java.util.*;

class M {
    public static void main( String ... args ) {
        List<String> l = new ArrayList<>();
        l.add("Test 1");
        l.add("Test 2");
        l.add("Test 3");

        Map<String,List<String>> m = new HashMap<>();
        m.put("Tests", l );

        // some time later that day ... 
        m.computeIfAbsent("Tests", k -> new ArrayList<>()).add("Test 4");
        System.out.println(m);
    }
}

Comments

[Boris the Spider]

Or even myMap.computeIfAbsent("Tests", k -> new ArrayList<>()), for the next question of "why did I get a NullPointerException with a different key".

[dabadaba]

@BoristheSpider wow I didn't know this one. Was it introduced in Java 8? Also is there a similar one for put?

[Boris the Spider]

@dabadaba, yes - as it has a lambda. What would the one for put do exactly?

[Jorn Vernee]

@BoristheSpider Are you sure getOrDefault can take a supplier? HashMap Javadoc. There are no overloads.

[Joe]

Thank you so much for your help everyone! I will make sure to follow this information.

[Jorn Vernee]

@BoristheSpider That would be a nice feature though ;) I was wondering why I never heard about it (in my mind maybe some 3rd party libray or something).

[dabadaba]

@BoristheSpider imagine you have a Map<K, List<V>>. If you want to add a new value to a existing or not key you need to check if containsKey(k) and if the value is an initalized list (not null). So I am talking about a method that would a) create a new list, add the element to it and then set is as ks value or b) simply add the element to the existing value list

[Boris the Spider]

@dabadaba isn't that exactly what the above does?

[dabadaba]

@BoristheSpider no... what you said is a "getter", I am talking about a "setter". edit: Wait you just changed the method, I was talking about getOrDefault...

[Boris the Spider]

@dabadaba you want to get a value from a Map, and if there is no value present compute a new value a put it into the Map. The method then either returns the get or the new value. Finally you do whatever you want with that value - in your example it's a List so you add. I'm not convinced you've read the documentation.

[dabadaba]

@BoristheSpider and I am convinced you edited your comment without letting me know so what I said above referred to the content of your comments before your edit ;)

[dabadaba]

But that's putting the value in the map right? I am not sure I like the idea of putting data in a collection if we're using a getter method.

[Rogue]

It places the value if it is absent, the idea is you're defaulting. If the resource isn't available for you, it will make it and provide the new resource. Perfectly valid.

[Jan]

I like the one-liner (second to last line).