Kotlin Cheat Sheet

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For a project I was involved in at short notice, I had to become familiar with the Kotlin programming language. Before starting to code, I wanted to get familiar with the syntax and the language itself. As writing on a paper is the best way to learn (for me), I created the following Cheat sheet. Enjoy!
  1. Basics
    • functions defined with keyword “fun”, followed by the name, optional argument list and optional return type
    • main function is the entry point to a program
    • “Unit“ return type is “void” in Java
      • semicolons are optional
  2. Variables
    • declarable by either “var“ or “val“ keywords
      • val: variable is immutable (readonly)
      • var: variable is mutable (reassignable)
    • Type inference: it is not necessary to specify the type of a variable when it gets an value assigned. If not, type specifying is mandatory.
    • Data types: predefined types like Int, Boolean, Char are available, which behave like classes!
    • Arrays represented by “Array” class
    • Creating with “arrayOf()“ or “Array()” class
  3. Control Flow
    • if-else can be used in assignments: var max = if (a > b) a else b
    • no ternary operator as it is possible to use construction above
    • “when” is the replacement for “switch“. Syntax is simiar, is also usable as expressions (as for if/else)
    • in: to check ranges, e. g. 1.. 7 → println()
    • is: whether a variable is a certain type
    • for: for (value in 1 .. 10) { ... } for (number in numbers) { … }
  1. Null safety
    • all variables are non-nullable. They can be nullable by assigning a question mark to its type

    • Safe call operator: executing member method only when object is not null
    • let Operator: using member method in a lambda expression

    • Elvis operator: fer defing an alternative value if object is null

    • not-null assertion: converts nullable type to non-nullable type and throws an Exception (Bad. Do not use)
    • Collections and Nullable Types: By default, Collections can’t hold null values. Using the question mark lets collection contain null values:

    • Notice: this means that elements within the list can be null, not the list itself. Nullable collections can be created by using the question mark after the type name:

    •  Collections and elements within the collections can be declared together as nullable as well:
  2. Functions
    • general syntax to define a function:
    • Return type and brackets can be ommited if return value is a single expression
    • “Unit“ type: correspondents to “void” in Java.Functions that return nothing have to return “Unit” (Exception: functions without brackets).
    • Default values for parameters: like PHP, Kotlin supports default values for parameters in order to make some of them optional, for example.
    • Specifying parameter names when calling a function: Kotlin allows explicitly to name the parameter when calling a function. You can mix named and unnamed arguments but have to call the unnamed parameters first.
    • variable number of arguments: “vararg” lets you pass a variable number of arguments to a function. If the function has a second non-vararg parameter, it has to be named during function call.
    • 4 types of functions: top level, member, local, internal
      • top-level: functions that are defined in a package and usable everywhere (after importing the package)
      • member: member methods of a class/object
      • local/nested: functions inside functions for more encapsulation and readability.
      • internal: usable within a package
    • infix notation: str1 matches str2 instead of str1.matches(str2). Creating an infix with keyword “infix” before “fun“.
  3. Classes
    • keyword “class“ for defining a class
    • Common base class “Any” (like “Object” in Java)
    • instantiation without “new” (as in Java/PHP)
    • var/val differentiation also available in classes
    • property access with dot operator (class.method())
    • Constructors: two types: primary and secondary
      • Primary Constructor: is part of the class header and declared after class name
      • initialization block: introduced with “init” keyword, used for variable initialization for example
      • using var/val in constructor arguments makes the variables to object properties.
    • Secondary Constructors: defined in the chess (like Java constructors) and must call the primary constructor.
    • Default visibility: public; internal visibility enables access within the same package. Further visibilities: private, protected.
    • Getter/Setter: Kotlin has internal getters and setters for member variables. It is possible to access the class members with the dot notation and Kotlin will internaly call the getter/setter. Custom getters/setters are also possible.
    • Inheritance: by default, all classes are not inheritable unless defined with “open“ before “class“ keyword:

    • parent classes must be initialized by child classes, either through primary or secondary constructor
    • overriding functions: by default all functions are final, they can be available with {open“ keyword.
    • overriding properties:  with “open” keyword in base class and “override” in child
    • Data classes: getters, setters, equal(), hashcode() methods are generated by Kotlin when using a data class.