@Override
    protected E applyStringOperation(String item) {
        return (E) Integer.valueOf(item.length());
    }
Reference implementation for length() step, a mid-traversal step which returns the length of the incoming string
traverser. Null values are not processed and remain as null when returned. If the incoming traverser is a non-String
value then an {@code IllegalArgumentException} will be thrown.

@author David Bechberger (http://bechberger.com)
@author Yang Xia (http://github.com/xiazcy)
gremlin-core/src/main/java/org/apache/tinkerpop/gremlin/process/traversal/step/map/LengthLocalStep.java