ScalaKnol: Understanding Uniform Access Principle

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The ScalaKnol series attempts to share a nugget of Scala wisdom every few days or sometimes few times a day. Keep tuned. Today we would look at the Uniform Access Principle.

The principle states that the client code or the calling code should not be affected by the decision to implement an attribute as a field or a method. Hence essentially irrespective of whether the attribute is defined with a val or a def, the calling client should be agnostic of the same.

For example, if we have a class like

and we have the calling code like this

As always, you can see the output because we are coding in worksheet. Now suppose we have a change which results in the val salary now actually becoming a method. In this case let us assume that people whose name starts with a V get a higher salary. (Ok, I am biased, don’t curse your parents yet, because the bonus percentage goes down 😉

So, the new way to define our class then becomes something like this, and you would notice the change in the

If you notice, we did not change the client / calling code even though we changed the attribute from val to def. This is the Uniform Access Principle which Scala supports. Java on the other hand does not support this principle. If you recall, in Java we have array.length. Now string is also an array but for String we have to write string.length() because it is a method and not a field.

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Vikas is the CEO and Co-Founder of Knoldus Inc. Knoldus does niche Reactive and Big Data product development on Scala, Spark, and Functional Java. Knoldus has a strong focus on software craftsmanship which ensures high-quality software development. It partners with the best in the industry like Lightbend (Scala Ecosystem), Databricks (Spark Ecosystem), Confluent (Kafka) and Datastax (Cassandra). Vikas has been working in the cutting edge tech industry for 20+ years. He was an ardent fan of Java with multiple high load enterprise systems to boast of till he met Scala. His current passions include utilizing the power of Scala, Akka and Play to make Reactive and Big Data systems for niche startups and enterprises who would like to change the way software is developed. To know more, send a mail to or visit

3 thoughts on “ScalaKnol: Understanding Uniform Access Principle2 min read

  1. Great article, but please add “M” to the higher paying initials ;)! I think it’s fair to give a tip of the hat to Bertrand Meyer here, who already advocated for the UAP in his book (Object Oriented Software Construction) in the 80ies. This topic, and Meyer’s treatment of Abstract Data Types are really worth a read.
    There are some less defensible positions in Meyer’s book as well, e.g. arguing for mutating setters (which weakens the UAP — what if an “assignment” fails?), and for covariant _parameter_ types in overridden methods (the first edition was released before Barbara Liskov’s eponymous principle), but I think that makes for an even more interesting read ;).

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