title: Java DynamoDB -- Only insert if key not already present (without mapper)
body: <p>I only want to insert this row if the key is not present.  I don't want to override the row if the key already exists.</p>
  
  <p>My syntax is this:</p>
  
  <pre><code>new PutItemRequest().withTableName(myTableName).withItem(myItem).withConditionExpression(?)
  </code></pre>
  
  <p>As per <a href="http://docs.aws.amazon.com/amazondynamodb/latest/developerguide/Expressions.SpecifyingConditions.html" rel="noreferrer">the AWS docs</a>, I'd use something like attribute ATTRIBUTE_NOT_EXISTS. I could also use ComparisonOperator.NULL, etc. That's as far as I can understand.</p>
  
  <p>Syntax tips? Some explanation of this withConditionExpression mechanism?</p>