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Activates an A/B test for the specified user to start an experiment: determines whether they qualify for the experiment, buckets a qualified user into a variation, and sends an impression event to Optimizely.

## Version

3.1.1 and higher

## Description

This method requires an experiment key, user ID, and (optionally) attributes. The experiment key must match the experiment key you created when you set up the experiment in the Optimizely app. The user ID string uniquely identifies the participant in the experiment.

If the user qualifies for the experiment, the method returns the variation key that was chosen. If the user was not eligible—for example, because the experiment was not running in this environment or the user didn't match the targeting attributes and audience conditions—then the method returns null.

Activate respects the configuration of the experiment specified in the datafile. The method:

  • Evaluates the user attributes for audience targeting.

  • Includes the user attributes in the impression event to support [results segmentation](🔗).

  • Hashes the user ID or bucketing ID to apply traffic allocation.

  • Respects forced bucketing and whitelisting.

  • Triggers an impression event if the user qualifies for the experiment.

Activate also respects customization of the SDK client. Throughout this process, this method:

  • Logs its decisions via the logger.

  • Triggers impressions via the event dispatcher.

  • Raises errors via the error handler.

  • Remembers variation assignments via the User Profile Service.

  • Alerts notification listeners, as applicable.

Note

For more information on how the variation is chosen, see [How bucketing works](🔗).

## Parameters

The parameter names for Java are listed below.

ParameterTypeDescription
**experiment key** _required_stringThe experiment to activate.
**user ID** _required_stringThe user ID.
**attributes** _optional_mapA map of custom key-value string pairs specifying attributes for the user that are used for audience targeting and results segmentation. Non-string values are only supported in the 3.0 SDK and above.

## Returns

@return variation

## Example



## Exception
SDKExceptionMeaning
Java**UnknownExperimentException**The experiment is malformed or doesn't exist in the datafile.
## Side effects
The table lists other other Optimizely functionality that may be triggered by using this method.
FunctionalityDescription
ImpressionsAccessing this method triggers an impression if the user is included in an active A/B test.

See [Decisions and impressions](🔗) for guidance on when to use Activate versus [Get Variation](🔗).
Notification ListenersIn SDKs v3.0 and earlier: Activate invokes the `ACTIVATE` notification listener if the user is included in an active A/B test.

In SDKs v3.1 and later: Invokes the `DECISION` notification listener if this listener is enabled.
## Notes
### Activate versus Get Variation
For details on whether to use Activate or Get Variation, see [Activate versus Get Variation](🔗).
## Source files
The language/platform source files containing the implementation for Java is [Optimizely.java](🔗).