public final FunctionCondition exists() {
        return attribute_exists(this);
    }
Returns a <a href=
"http://docs.aws.amazon.com/amazondynamodb/latest/developerguide/Expressions.SpecifyingConditions.html#ConditionExpressionReference.Functions"
>function condition</a> (that evaluates to true if the attribute referred
to by this path operand exists) for building condition expression.
aws-java-sdk-dynamodb/src/main/java/com/amazonaws/services/dynamodbv2/xspec/PathOperand.java