HashMap updating ArrayList

Question

I'm just starting to learn to use HashMap and reading the java tutorial, but I'm having trouble.

I'm trying to update the List inside a HashMap but I want to get the List of that key, is there a way to update a specific List of the key instead of having to make...5 different Lists and updating those?

 HashMap<String, ArrayList<String>> mMap = new HashMap<String, ArrayList<String>>();
        ArrayList<String> list = new ArrayList<String>();
        mMap.put("A", list);
        mMap.put("B", list);
        mMap.put("C", list);
        mMap.put("D", list);

        Iterator iter = mMap.entrySet().iterator();

        if (mMap.containsKey("A"))
        {   
            Map.Entry mEntry = (Map.Entry) iter.next();
            list.add("test");
            mMap.put("A",list);
            System.out.println(mEntry.getKey() + " : " + mEntry.getValue());
        }
        else if (mMap.containsKey("B"))
        {   
            Map.Entry mEntry = (Map.Entry) iter.next();
            list.add("entry");
            mMap.put("B",list);
            System.out.println(mEntry.getKey() + " : " + mEntry.getValue());
        }

Solution 1

You could use something like:

mMap.get("A").add("test");
mMap.get("B").add("entry");

Solution 2

1. Like @Tudor said, use mMap.get("A").add("foo");

2. You put the same exact list into each map entry. You initial lines should be

    mMap.put("A", new ArrayList());
    mMap.put("B", new ArrayList()); 
    ...
    mMap.put("Z", new ArrayList());

Alternatvely, write a method that checks on the fly

public synchronized void myAdd(String key, String value) {
   List<String> there = mMap.get(key);
   if (there == null) {
      there = new ArrayList<String>();
      mMap.put(key, there);
   }
   there.add(value);
}

Solution 3

you probably mean:

HashMap<String, List<String>> mMap = new HashMap<String, List<String>>();
    mMap.put("A", new ArrayList<String>());
    mMap.put("B", new ArrayList<String>());
    mMap.put("C", new ArrayList<String>());
    mMap.put("D", new ArrayList<String>());

    if (mMap.containsKey("A"))
    {   
        mMap.get("A").add("test");
        System.out.println(mEntry.getKey() + " : " + mEntry.getValue());
    }
    else if (mMap.containsKey("B"))
    {   
        mMap.get("B").add("entry");
        System.out.println(mEntry.getKey() + " : " + mEntry.getValue());
    }
   ...

I wonder if you really need those containsKey checks either! HTH!

Solution 4

I think that in your case you can use Google Guava's Multimap and ListMultimap interfaces and ArrayListMultimap implementation.

Choosing right collection (in link there are only standard collections but Multimap is right in this case) makes code more readable:

ListMultimap<String, String> myMultimap = ArrayListMultimap.create();
myMultimap.put("A", "test");
myMultimap.put("B", "entry");

then myMultimap.get("A") gives you list (ArrayList instance in fact) with one element: "test", while myMultimap.get("C") gives you empty list.

Comparing to Map<String, List<String>> approach:

• you don't have to initialize "C" key with empty list for it,
• you don't have nulls checks (no NullPointerExceptions),
• you don't have to do other checks such as myMap.containsKey("A"),
• you write less code,
• so your code is less bug-prone,
• etc., etc.

---

P.S. Instead of:

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

use interfaces, not classes when using collections i.e.:

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

and even better Guava's "static constructors" (you don't repeat code):

Map<String, List<String>> mMap = Maps.newHashMap()

when knowledge about implementation is not necessary.

Solution 5

I would do something like this.

private final ReentrantLock lock = new ReentrantLock();
    Map<String ,List<String>> mMap=new HashMap<>();
    public  void myAdd(String key, String value) {
        try {
            lock.lock();
        List<String> there =mMap.get(key)==null?new ArrayList<>():mMap.get(key);
        there.add(value);
        mMap.put(key, there);
        }finally {
            lock.unlock();
        }
    }

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Comments

[aishwarya]

ah! you beat me to the answer ;-)

[user949300]

How can this get 3 upvotes when it misses the fact that he put the same List into every map entry? :-)

[Tudor]

I just answered his basic question: Is there a simpler method to update the list that what I'm doing?