public void setStackId(String stackId) {
        this.stackId = stackId;
    }
<p>
A stack ID. If you use this parameter, <code>DescribeInstances</code> returns descriptions of the instances
associated with the specified stack.
</p>

@param stackId
       A stack ID. If you use this parameter, <code>DescribeInstances</code> returns descriptions of the
       instances associated with the specified stack.
aws-java-sdk-opsworks/src/main/java/com/amazonaws/services/opsworks/model/DescribeInstancesRequest.java