private void validateChildName(String childName) throws IllegalArgumentException, NullPointerException {
    	if (childName == null) {
    		throw new NullPointerException("null child name"); 
    	}
    	if (childName.isEmpty()) {
    		throw new IllegalArgumentException("empty child name");
    	}
    }
private void validateChildName ( String childName ) throws IllegalArgumentException , NullPointerException { if ( childName == null ) { throw new NullPointerException ( "null child name" ) ; } if ( childName . isEmpty ( ) ) { throw new IllegalArgumentException ( "empty child name" ) ; } }
extension methods