Can an array be used as a HashMap key?

Question

If a HashMap's key is a String[] array:

HashMap<String[], String> pathMap;

Can you access the map by using a newly created String[] array, or does it have to be the same String[] object?

pathMap = new HashMap<>(new String[]{"korey", "docs"}, "/home/korey/docs");
String path = pathMap.get(new String[]{"korey", "docs"});

Answer 1

It will have to be the same object. A HashMap compares keys using equals() and two arrays in Java are equal only if they are the same object.

If you want value equality, then write your own container class that wraps a String[] and provides the appropriate semantics for equals() and hashCode(). In this case, it would be best to make the container immutable, as changing the hash code for an object plays havoc with the hash-based container classes.

EDIT

As others have pointed out, List<String> has the semantics you seem to want for a container object. So you could do something like this:

HashMap<List<String>, String> pathMap;

pathMap.put(
    // unmodifiable so key cannot change hash code
    Collections.unmodifiableList(Arrays.asList("korey", "docs")),
    "/home/korey/docs"
);

// later:
String dir = pathMap.get(Arrays.asList("korey", "docs"));

Answer 2

No, but you can use List<String> which will work as you expect!

Answer 3

Arrays in Java use Object's hashCode() and don't override it (the same thing with equals() and toString()). So no, you cannot shouldn't use arrays as a hashmap key.

Answer 4

You cannot use a plain Java Array as a key in a HashMap. (Well you can, but it won't work as expected.)

But you could write a wrapper class that has a reference to the Array and that also overrides hashCode() and equals().

Answer 5

In most cases, where the Strings inside your array are not pathological and do not include commas followed by a space, you can use Arrays.toString() as a unique key. i.e. your Map would be a Map<String, T>. And the get/put for an array myKeys[] would be

T t = myMap.get(Arrays.toString(myKeys));

myMap.put(Arrays.toString(myKeys), myT);

Obviously you could put in some wrapper code if desired.

A nice side effect is that your key is now immutable. Of course, of you change your array myKeys and then try a get(), you won't find it.

Hashing of Strings is highly optimized. So my guess is that this solution, though it feels a bit slow and kludgy, will be both faster and more memory efficient (less object allocations) than @Ted Hopp solution using an immutable List. Just think about whether Arrays.toString() is unique for your keys. If not, or if there is any doubt, (e.g. the String[] comes from user input) use the List.

Answer 6

Like said you need a wrapper class around your array which overrides equality and hashCode.

e.g.

/** 
 * We can use this instance as HashKey,
 * the same anagram string will refer the same value in the map.
 */
class Anagram implements CharSequence {

    private final char[] anagram;

    public Anagram(String anagram) {

        this.anagram = anagram.toCharArray();
        Arrays.sort(this.anagram);
    }

    @Override
    public boolean equals(Object o) {

        if (this == o) {
            return true;
        }
        if (o == null || getClass() != o.getClass()) {
            return false;
        }
        Anagram that = (Anagram) o;
        return Arrays.equals(this.anagram, that.anagram);
    }

    @Override
    public int hashCode() {

        return Arrays.hashCode(this.anagram);
    }

    @Override
    public int length() {

        return anagram.length;
    }

    @Override
    public char charAt(int index) {

        return anagram[index];
    }

    @Override
    public CharSequence subSequence(int start, int end) {

        return new String(anagram).subSequence(start, end);
    }

    @Override
    public String toString() {

        return Arrays.toString(anagram);
    }
}

Otherwise declare your map as IdentityHashMap, then the user knows we need to use the same instance for your CRUD.

Answer 7

Ted Hopp is right it will have to be same object.

For information see this example:

public static void main(String[] args) {
    HashMap<String[], String> pathMap;
    pathMap = new HashMap<String[], String>();
    String[] data = new String[]{"korey", "docs"};
    pathMap.put(data, "/home/korey/docs");
    String path = pathMap.get(data);
    System.out.println(path);
}

When you run the above code, it will print "docs".

Answer 8

Since Java 9, you can use Arrays::compare method as a comparator for TreeMap that compares the contents of arrays.

Map<String[], String> map = new TreeMap<>(Arrays::compare);

String[] key1 = {"one", "two"};
String[] key2 = {"one", "two"};
String[] key3 = {"one", "two"};

map.put(key1, "value1");
map.put(key2, "value2");

System.out.println(map.size()); // 1
System.out.println(map.get(key1)); // value2
System.out.println(map.get(key2)); // value2
System.out.println(map.get(key3)); // value2

---

^{See also: How to make a Set of arrays in Java?}

Answer 9

A running example using the Arrays utility and the hash code it provides:

String[] key1 = { "korey", "docs" };
String value1 = "/home/korey/docs";
HashMap<Integer, String> map = new HashMap<Integer, String>();
map.put(Arrays.hashCode(key1), value1);
System.out.println(map);

{-1122550406=/home/korey/docs}

This approach is useful if your focus is in storing only. Retrieving using the readable (original) key is simple:

String retrievedValue = map.get(Arrays.hashCode(key1));
System.out.println(retrievedValue);

/home/korey/docs