wffweb/src/main/java/com/webfirmframework/wffweb/tag/html/attribute/core/AbstractAttribute.java
protected Set<String> getAttributeValueSet() {
        if (attributeValueSet == null) {
            synchronized (this) {
                if (attributeValueSet == null) {
                    // because the load factor is 0.75f
                    attributeValueSet = new LinkedHashSet<>(2);
                }
            }
        }
        return attributeValueSet;
    }
protected Set<String> getAttributeValueSet() {
        if (attributeValueSet == null) {
            synchronized (this) {
                if (attributeValueSet == null) {
                    // because the load factor is 0.75f
                    attributeValueSet = new LinkedHashSet<>(2);
                }
            }
        }
        return attributeValueSet;
    }
NB:- this is only for getting values. Use addToAttributeValueSet method
for adding

@return the attributeValueSet
@since 1.0.0
@author WFF