public String getCollapseKey() {
        return this.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>

@return 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