Why and what is Scala?

Is Scala for you? You will have to see and decide for yourself. We have found that there are actually many reasons besides scalability to like programming in Scala. Four of the most important aspects will be discussed in this section: compatibility, brevity, high-level abstractions, and advanced static typing.

Scala is compatible Scala doesn’t require you to leap backwards off the Java platform to step forward from the Java language. It allows you to add value to existing code—to build on what you already have—because it was designed for seamless interoperability with Java.8 Scala programs compile to JVM bytecodes. Their run-time performance is usually on par with Java programs.

Scala code can call Java methods, access Java fields, inherit from Java classes, and implement Java interfaces. None of this requires special syntax, explicit interface descriptions, or glue code. In fact, almost all Scala code makes heavy use of Java libraries, often without programmers being aware of this fact.

Another aspect of full interoperability is that Scala heavily re-uses Java types. Scala’s Ints are represented as Java primitive integers of type int, Floats are represented as floats, Booleans as booleans, and so on. Scala arrays are mapped to Java arrays. Scala also re-uses many of the standard Java library types. For instance, the type of a string literal "abc" in Scala is java.lang.String, and a thrown exception must be a subclass of java.lang.Throwable.

Scala not only re-uses Java’s types, but also “dresses them up” to make them nicer. For instance, Scala’s strings support methods like toInt or toFloat, which convert the string to an integer or floating-point number. So you can write str.toInt instead of Integer.parseInt(str). How can this be achieved without breaking interoperability? Java’s String class certainly has no toInt method!

In fact, Scala has a very general solution to solve this tension between advanced library design and interoperability. Scala lets you define implicit conversions, which are always applied when types would not normally match up, or when non-existing members are selected. In the case above, when looking for a toInt method on a string, the Scala compiler will find no such member of class String, but it will find an implicit conversion that converts a Java String to an instance of the Scala class StringOps, which does define such a member. The conversion will then be applied implicitly before performing the toInt operation.

Scala code can also be invoked from Java code. This is sometimes a bit more subtle, because Scala is a richer language than Java, so some of Scala’s more advanced features need to be encoded before they can be mapped to Java.

Scala is concise:

Scala programs tend to be short. Scala programmers have reported reductions in number of lines of up to a factor of ten compared to Java. These might be extreme cases. A more conservative estimate would be that a typical Scala program should have about half the number of lines of the same program written in Java. Fewer lines of code mean not only less typing, but also less effort at reading and understanding programs and fewer possibilities of defects. There are several factors that contribute to this reduction in lines of code. First, Scala’s syntax avoids some of the boilerplate that burdens Java programs. For instance, semicolons are optional in Scala and are usually left out. There are also several other areas where Scala’s syntax is less noisy. As an example, compare how you write classes and constructors in Java and Scala. In Java, a class with a constructor often looks like this:

class MyClass { private int index; private String name; public MyClass(int index, String name) { this.index = index; this.name = name; } } In Scala, you would likely write this instead: class MyClass(index: Int, name: String)

Given this code, the Scala compiler will produce a class that has two private instance variables, an Int named index and a String named name, and a constructor that takes initial values for those variables as parameters.

The code of this constructor will initialize the two instance variables with the values passed as parameters. In short, you get essentially the same functionality as the more verbose Java version.9 The Scala class is quicker to write, easier to read, and most importantly, less error prone than the Java class. Scala’s type inference is another factor that contributes to its conciseness.

Repetitive type information can be left out, so programs become less cluttered and more readable.

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