Scala interview questions

The collection of key-value pairs where the key can retrieve the values present in a map is known as a scala map. The keys, unlike the values in a scala map are unique. There are two types of maps present in scala:

• Mutable
• Immutable

By default, immutable map is supported by scala. To use the mutable map, the programmer needs to import the “scala.collection.mutable”. It is an explicit class. “mutable.map” is the syntax for using the mutable and immutable class in the same program. To access the immutable class, we just have to use the name of the map and it can be accessed.

Some of the advantages due to which scala is preferred over other functional programming languages are listed below:

• As the name suggests, it is a scalable programming language. It is highly scalable. Its high maintainability, productivity, and testability features make it more usable which turns out to be an advantage and hence it is preferred more.
• Scala consists of singleton and companion objects. These objects in scala, unlike the JVM languages provide a clearer solution to every problem.
• The need of ternary operator gets eliminated in scala as “if blocks”, “for-yield loops”, and “code” in braces return a value.

Instead of having static methods or variables, scala has singleton or companion objects. These objects are then compiled to classes which have static methods. Some advantages of these companion objects are listed below:

• These are beneficial as they can be used for encapsulating things. They also act as a bridge and hence functional and object-oriented programs can be written easily.
• These companion objects also help in keeping the scala programming code more concise because the static keyword need not be added to each and every attribute.
• A clear separation between the static and non-static methods is maintained with the help of these companion objects.

pure functional data structure is present in the scala library. It also complements the standard Scala library. It also has a pre-defined set of foundational type classes’ like- Monad, Functor, etc. The standard library of Scala is automatically set as a dependency by default. The Scala library is used for compiling scala sources and for that it needs to be on the classpath.

The difference between these four attributes in scala is given below:

• Nil- it is used to initialize an empty list since it is an object which extends list.
• Null- null in scala is used to provide compatibility with the java null keyword or to provide a type for the null keyword. It also represents the absence of type information for complex types.
• None- the “none” pattern in scala is used to remove null values from the scala code.
• Nothing- it is used for providing the return type for the operations that can affect the normal flow of program.

These classes in scala are the classes which are declared by using a special modifier case. The constructor parameters are exported using these case classes which hence provide a recursive decomposition mechanism through pattern matching. We treat the constructor parameters of the case classes as public values and so they can be accessed directly. The companion objects and its related methods can also be generated automatically for the case classes. All the methods in the class and the companion objects are generated based on the parameter list. It helps in automatically generating the methods from the parameter list. By default, case objects and classes are serializable.

These methods in scala are used for mapping and unmapping the data between the form and model data.

• Apply method- this method is used to assemble an object from its components. For eg.- if an employee object needs to be created, then the two components namely- first name and last name should be used and should be composed using the apply method.
• Unapply method- when we want to decompose the objects from its components, then we make use of this unapply method. Reverse process is followed while making use of the unapply method. So, the employee object can be decomposed into two components namely- first name and last name.

The differences between array and list in Scala are listed below-

• An array is a sequential mutable data structure. Whereas, a list is an immutable recursive data structure.
• In scala, an array is an invariant. Whereas, a list is a covariant data structure.
• The size of an array is fixed and cannot be changed easily. Whereas, the size of a list may increase or decrease based on the operations performed by it.

Implicit parameters are used when we want to invoke a function without passing all the parameters. The default values of all the parameters or the parameters which we want to use are set as implicit. When implicit parameters are not used the local value of that parameter is used. An implicit keyword needs to be used if want to make a value, function parameter, or a variable as an implicit parameter. Once the value becomes implicit, we need not pass all the parameters to invoke a particular function.

The programmers face various situations where they need to write codes which are recursive in nature. But when we use the recursive functions, we may face a problem in which the function may eat up all the stack space. So, to overcome this problem, Scala provides this particular feature of tail recursion which optimizes the recursive function. After optimization, these functions do not create a new stack space, instead uses the current function stack space. “@annotation.tailrec” annotation has to be used before defining the function and for the last statement, recursive call can be used. After all this, your function will be compiled successfully or else it will give an error.