CODEWITHSUNDEEP
java
RandomUser
RandomUser

Reputation: 4220

Can anyone explain this method signature? What does <K> and K... represent?

Howdy, I am trying to understand this method signature:

  public <K> Map<K, String> getMulti(Serializer<K> keySerializer, K... keys)

For the following code block:

public <K> Map<K, String> getMulti(Serializer<K> keySerializer, K... keys) {
    MultigetSliceQuery<K, String, String> q = createMultigetSliceQuery(keyspace, keySerializer, serializer, serializer);
    q.setColumnFamily(CF_NAME);
    q.setKeys(keys);
    q.setColumnNames(COLUMN_NAME);

    QueryResult<Rows<K, String, String>> r = q.execute();
    Rows<K, String, String> rows = r.get();
    Map<K, String> ret = new HashMap<K, String>(keys.length);
    for (K k : keys) {
        HColumn<String, String> c = rows.getByKey(k).getColumnSlice().getColumnByName(COLUMN_NAME);
        if (c != null && c.getValue() != null) {
            ret.put(k, c.getValue());
        }
    }
    return ret;
}

I am not sure what <K> in the method declaration represents, or what K... is suppose to mean. Can anyone shed some light on these?

Upvotes: 2

Views: 2397

Answers (4)

BalusC
BalusC

Reputation: 1108782

The K is the type parameter of this method. It can be anything you want. E.g. Integer. In that case you need to visually (in your mind) substitute all K's in the method signature with Integer to understand what it's taking and returning. With Integer as K, the method will then behave as follows:

public Map<Integer, String> getMulti(Serializer<Integer> keySerializer, Integer... keys)

The ... in turn is the varargs syntax. It allows you to pass zero-more arguments of or an array of the given type into the method. E.g.

getMulti(serializer);
getMulti(serializer, k1);
getMulti(serializer, k1, k2, k3);

Upvotes: 4

Alexander Pogrebnyak
Alexander Pogrebnyak

Reputation: 45576

K here stands for 'key'.

Generic map declaration usually have a signature Map<K,V>, where K stands for arbitrary key type and V for arbitrary value type.

In your case value type is bounded to String.

Upvotes: 0

Sean Owen
Sean Owen

Reputation: 66886

K is a generic type. <K> just declares that the declaration will talk about some generic type called K. Here the coder can declare restrictions on K like <K extends List>, or not.

Then K is simply used as a placeholder, a generic type. When you call this with a Serializer<String> for example, then imagine that the method will be invoked as if K were replaced with the type String throughout the method.

That's all that's going on as regards K.

If you want an interpretation of the method signature in English: it takes a Serializer that operates on Ks, and zero or more Ks. It returns a mapping from Ks to Strings.

Upvotes: 3

Liv
Liv

Reputation: 6124

K simply denotes the presence of generics -- this method operates on a generic class K. It takes a serializer operating on the generic class K and a variable array of arguments of type K and it returns a map which maps instances of K to Strings.

Upvotes: 0

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