public GetObjectRequest withMatchingETagConstraint(String eTag) {
        this.matchingETagConstraints.add(eTag);
        return this;
    }
Sets a single ETag constraint to this request.
Returns this {@link GetObjectRequest}, enabling additional method
calls to be chained together.
<p>
Multiple ETag constraints can be added to a request, but one must match the object's
current ETag in order for this request to be executed. If none of the
ETag constraints added to this request match the object's current ETag,
this request will not be executed by Amazon S3.
</p>

@param eTag
           The matching ETag constraint to add to this request.

@return This {@link GetObjectRequest}, enabling additional method
        calls to be chained together.

@see GetObjectRequest#getMatchingETagConstraints()
@see GetObjectRequest#setMatchingETagConstraints(List)
aws-java-sdk-s3/src/main/java/com/amazonaws/services/s3/model/GetObjectRequest.java