Basics of Kotlin

Introduction#

This topic covers the basics of Kotlin for beginners.

Remarks#

1. Kotlin file has an extension .kt.
2. All classes in Kotlin have a common superclass Any, that is a default super for a class with no supertypes declared(similar to Object in Java).
3. Variables can be declared as val(immutable- assign once) or var(mutables- value can be changed)
4. Semicolon is not needed at end of statement.
5. If a function does not return any useful value, its return type is Unit.It is also optional.

6.Referential equality is checked by the === operation. a === b evaluates to true if and only if a and b point to the same object.

Basic examples

1.The Unit return type declaration is optional for functions. The following codes are equivalent.

    fun printHello(name: String?): Unit {
        if (name != null)
            println("Hello ${name}")
    }
    
    fun printHello(name: String?) {
        ...
    }

2.Single-Expression functions:When a function returns a single expression, the curly braces can be omitted and the body is specified after = symbol

  fun double(x: Int): Int = x * 2

Explicitly declaring the return type is optional when this can be inferred by the compiler

  fun double(x: Int) = x * 2

3.String interpolation: Using string values is easy.

In java:
    int num=10
    String s  = "i =" + i;

In Kotlin
    val num = 10
    val s = "i = $num"

4.In Kotlin, the type system distinguishes between references that can hold null (nullable references) and those that can not (non-null references). For example, a regular variable of type String can not hold null:

var a: String = "abc"
a = null // compilation error

To allow nulls, we can declare a variable as nullable string, written String?:

var b: String? = "abc"
b = null // ok

5.In Kotlin,== actually checks for equality of values.By convention, an expression like a == b is translated to

   a?.equals(b) ?: (b === null)