访问Java映射内的ArrayList

Question

This is the first question I've posted on this site!

In the final part of a Java assignment for university, my class has been asked to show we understand how to use different types of arrays.

At one point, we'll need to make and maintain a Map(key, value). However, we'll need to use an ArrayList(String) as the Map's value.

Adding to this Map appears easy enough: Just check that the Map in question does not already contain the specified key and then create the array with its values filled in and 'put' it, with the key String to the Map.

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

public void addToMap(String key, String value) {
  if (!this.map.containsKey(key) {
    ArrayList<String> array = new ArrayList<String>();
    array.add(value);
    this.map.put(key, array);
  }
  else {
    this.map.get(key).add(value); // Does this add a new item to the current ArrayList?
  }
}

But what if I wanted to edit the contents of that array? I've only just started looking at Maps today, so I'm a little new to them, but I see they have a 'put' method - and I understand what put, pop and push are usually for, so this is slightly familiar. Would I have to copy the contents of the array into a new temporary one, remove() the old "key => value" pair and make a new one with the additional data?

Your help is very appreciated! :D

Answer 1

Your Map contains references to ArrayList s.

String key = "some list name";
// This assumes this entry is in the map; doesn't check for null
ArrayList<String> listInMap = map.get(key);
listInMap.add("new thing to add to list");

When you use Map.get() it returns the reference to the ArrayList in the map, not a copy of it. You can then modify that List via the returned reference.

Answer 2

To edit the contents of one particular arrayList you need first to get it from the map, and then just add / remove from the array:

ArrayList<String> list = map.get("key");
list.get(index);
list.add("something");

Answer 3

From the Javadoc of Map.#put

Associates the specified value with the specified key in this map (optional operation). If the map previously contained a mapping for the key, the old value is replaced by the specified value . (A map m is said to contain a mapping for a key k if and only if m.containsKey(k) would return true.)

So you can see that if you modify your array , you don't need to delete the pair. Just re-map it with the previous key.

And There is an syntactical error in your code. You can't instantiate an interface. So your code should be

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

or TreeMap or any direct concrete subclass of Map interface.