public ChannelMessageCallback addMessageAttributesEntry(String key, MessageAttributeValue value) {
        if (null == this.messageAttributes) {
            this.messageAttributes = new java.util.HashMap<String, MessageAttributeValue>();
        }
        if (this.messageAttributes.containsKey(key))
            throw new IllegalArgumentException("Duplicated keys (" + key.toString() + ") are provided.");
        this.messageAttributes.put(key, value);
        return this;
    }
Add a single MessageAttributes entry

@see ChannelMessageCallback#withMessageAttributes
@returns a reference to this object so that method calls can be chained together.
aws-java-sdk-chimesdkmessaging/src/main/java/com/amazonaws/services/chimesdkmessaging/model/ChannelMessageCallback.java