public void setCollapseKey(String collapseKey) {
        this.collapseKey = collapseKey;
    }
<p>
An arbitrary string that identifies a group of messages that can be collapsed to ensure that only the last
message is sent when delivery can resume. This helps avoid sending too many instances of the same messages when
the recipient's device comes online again or becomes active.
</p>
<p>
Amazon Pinpoint specifies this value in the Firebase Cloud Messaging (FCM) collapse_key parameter when it sends
the notification message to FCM.
</p>

@param collapseKey
       An arbitrary string that identifies a group of messages that can be collapsed to ensure that only the last
       message is sent when delivery can resume. This helps avoid sending too many instances of the same messages
       when the recipient's device comes online again or becomes active.</p>
       <p>
       Amazon Pinpoint specifies this value in the Firebase Cloud Messaging (FCM) collapse_key parameter when it
       sends the notification message to FCM.
aws-java-sdk-pinpoint/src/main/java/com/amazonaws/services/pinpoint/model/GCMMessage.java