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title: "WithKeys"
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# WithKeys
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<br>
Takes a `PCollection<V>` and produces a `PCollection<KV<K, V>>` by associating
each input element with a key.

There are two versions of `WithKeys`, depending on how the key should be determined:

* `WithKeys.of(SerializableFunction<V, K> fn)` takes a function to
  compute the key from each value.
* `WithKeys.of(K key)` associates each value with the specified key.

## Examples
**Example**
```java
PCollection<String> words = Create.of("Hello", "World", "Beam", "is", "fun");
PCollection<KV<Integer, String>> lengthAndWord =
  words.apply(WithKeys.of(new SerialiazableFunction<String, Integer>() {
    @Override
    public Integer apply(String s) {
      return s.length();
    }
  });
```

## Related transforms 
* [Keys]({{ site.baseurl }}/documentation/transforms/java/elementwise/keys) for extracting the key of each component.
* [Values]({{ site.baseurl }}/documentation/transforms/java/elementwise/values) for extracting the value of each element.
* [KvSwap]({{ site.baseurl }}/documentation/transforms/java/elementwise/kvswap) swaps key-value pair values.