Monday, April 14, 2014

How to show Sinhala characters in Mac OS X Terminal?

UTF-7 is the character encoding that represents Sinhala text.

    • Terminal > Preferences > Settings > Advanced 
    • International
    • Set Character encoding as Unicode (UTF-7)

    Tuesday, April 8, 2014

    Basic SOLR concepts explained with a simple use case

    This is an example scenario to understand the basic concepts behind SOLR/ Lucene indexing and search using advertising web site[4].

    Use case:

    Searcher: I want to search cars by different aspects such as car model, location, transmission, special features etc. Also, I want to see the similar cars that belong to same model as recommendations.

    SOLR uses index which is an optimized data structure for fast retrieval.

    To create an index, we need to come up with a set of documents with fields in it. How do we create a document for the following advertisement?

    Title: Toyota Rav4 for sale
    Category: Jeeps
    Location: Seeduwa
    Model: Toyota Rav4
    Transmission: Automatic
    Description: find later

    SOLR document:

    document 1

    Toyota Rav4 for sale

    Toyota Rav4
    Brought Brand New By Toyota Lanka-Toyota Rav4 ACA21, YOM-2003, HG-XXXX Auto, done approx 79,500 Km excellent condition, Full Option, Alloy Wheels, Hood Railings,
    call No brokers please.

    Some more documents based on advertisements...

    document 2

    Nissan March for sale

    A/C, P/S, P/W, Center locking, registered year 1998, full option, Auto, New battery, Alloys, 4 doors, Home used car, Mint condition, Negotiable,

    document 3

    Nissan March K12 for rent
    A/C, P/S, P/W, Center locking, registered year 2004, full option, Auto, New battery, Alloys, 4 doors, cup holder, Doctor used car, Mint condition, Negotiable,

    Inverted Index

    Then SOLR creates an inverted index as given below: (Lets take example field as Title)

    toyota doc1(1x)
    rav4 doc1(1x)
    sale doc1(1x) doc2(1x)
    nissan doc2(1x)
    march doc2(1x)

    1x means the term frequency of the document for that particular field.

    Lucene Analyzers

    Note that “for” term here is eliminated during Lucene stop word removal process using Lucene text analysers. You can come up with your own analyser based on your preference as well.

    Field configuration and search

    You can configure, which fields your documents can contain, and how those fields should be dealt with when adding documents to the index, or when querying those fields using schema.xml.

    For example, if you need to index description field as well and the description value of the field should be retrievable during search, what you need to do is add the following line in schema.xml [1].

    Now, assume a user search for a vehicle.

    Search query: “nissan cars for rent”

    SOLR query would be /solr/select/?q=title:”nissan cars for rent"

    Ok what about the other fields (Category, location, transmission etc. ?)?

    By default, SOLR standard query parser can only search one field. To use multiple fields such as title and description and give them a weight to consider during retrieval based on their significance (boosts) we should use Dismax parser [2, 3]. Simply said, using Dismax parser you can make title field more important than description field. 

    Anatomy of a SOLR query

    q - main search statement
    fl - fields to be returned
    wt - response writer (response format)

    - select all the advertisements

    http://localhost:8983/solr/select?q=*:*&fl=title,category,location,transmission&sort=title desc
    - select title,category,location,transmission and sort by title in descending order

    wt parameter = response writer
    http://localhost:8983/solr/select?q=*:*&wt=json - Display results in json format
    http://localhost:8983/solr/select?q=*:*&wt=xml - Display results in XML format

    http://localhost:8983/solr/select?q=category:cars&fl=title,category,location,transmission -
    Give results related to cars only

    more option can be found at [5].

    Coming up next...

    • Extending SOLR functionality using RequestHandlers and Components
    • SOLR more like this

    Wednesday, April 2, 2014

    How would you decide if a class should be abstract class or interface?

    It depends :)

    In my opinion, to implement methods in abstract class you need to inherit the abstract class.One of the key benefits of inheritance is to minimise the amount of duplicate code by implement common functionalities in parent classes. so if the abstract class have some common generic behaviour that can be shared with its concrete classes, then using abstract class would be optimal.

    However, if all methods are abstract and those methods do not represent any unique/significant behaviour related to  the class instances, it may be better to use interface instead.

    Use abstract classes to define planned inheritance hierarchies. Classes with already defined inheritance hierarchy can extend their behavior in terms of the “roles” they can play, which are not common to its parents all the other children, using interfaces. Abstract classes will not help in this situation because of multiple inheritance restriction in  java language.

    How interfaces avoid “Deadly Diamond of Death” problem?

    A key difference between interface and abstract class is, “Interfaces simulate multiple inheritance” for languages where multiple inheritance is not supported due to “Deadly Diamond of Death” problem.

    How interfaces avoid “Deadly Diamond of Death” problem?

    Since interface methods do not have their underlying implementation, unlike the inherited class methods, there won’t be this problem as there can be multiple method signatures that are same, but there can be only one implementation for a particular class instance as duplicate methods cannot be compiled without any errors.

    Head First Java