public SendMessageRequest addMessageSystemAttributesEntry(String key, MessageSystemAttributeValue value) {
        if (null == this.messageSystemAttributes) {
            this.messageSystemAttributes = new com.amazonaws.internal.SdkInternalMap<String, MessageSystemAttributeValue>();
        }
        if (this.messageSystemAttributes.containsKey(key))
            throw new IllegalArgumentException("Duplicated keys (" + key.toString() + ") are provided.");
        this.messageSystemAttributes.put(key, value);
        return this;
    }
Add a single MessageSystemAttributes entry

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