Kotlin notes for Java/Scala developers

Main features

  • type inference
  • nullable types
  • function types
  • interactive shell
  • online playground
  • java-to-kotlin converter


Language feature

  • smart casts, (type check + cast)
  • fun (function) can be declared on level of file (no need to put it into file), no static utility classes are required. For java callers such functions will be placed as static functions into auto-generated class (named after kotlin file). Class name can be redefined, via @file:JvmName(“StringFunctions”).
  • return type can be skipped for expression-body function
  • string templates/interpolation (${expression} or $variable)
  • val/var – like java’s final/not-final
  • public by default
  • properties (val – only getter, var – getter and setter), round braces can be omitted, even for javabean objects
  • “package” and “import” directives – like Java’s
  • classes, class members, functions and properties can be imported
  • it is possible to have folder hierarchy not like packages hierarchy (unlike Java), but it is good practice to have follow java way in case of mixed project java+kotin
  • good practice to have multiple classes within same file (since classes are small)
  • enum classes
  • semicolon is used only to separate enum constants from enum methods (if defined any)
  • “when” instead of “switch”
  • “is” like “instaceof” but no additional cast is required afterwards (within underlying block)
  • last expression at block its result
  • while/do-while like java’s, “for” like java’s for-each
  • “in” (“!in”) keyword checks if an item is inside some collection
  • closed range – <start>..<end>
  • java’s “try-with-resources” is implemented as library function
  • “”” – triple quoting string – nothing need be escaped
  • local functions
  • Any class (open) – like java’s Object



  • creating collections:
    • setOf
    • hashSetOf
    • arrayListOf
    • hashMapOf (key to val)
  • named parameters (but not for java methods)
  • default parameters
  • @JvmOverloads – makes from kotlin function with default parameters a set of overloaded java functions, for each default parameter
  • extension functions and properties (adding methods to other’s class), can access public methods and properties of class receiver. From java side extension methods seen as static members of class named after kotlin file name. Could not be overridden, called by variable declared type, i.e. statically resolved. Member function is always takes precedence over extension function.
  • varargs – “vararg”, but arrays should be unpacked, “*” – spread operator
  • infix call – “infix” keyword before function definition, like making a pair: “1 to 2”
  • destruction declarations: val (one, two) = 1 to 2
  • “inline” – marked functions are not called as a functions but embedded into caller’s body (can spare time for call and size for object that internally represents a lambda, but increase a size of byte-code). Inline functions called from java as usual functions, with no in-linement. Type information for template parameters are reified (not erased) at runtime for such functions. it is better to use for functions with lambda parameters, in other cases compiler is smart enough, can injects itself:
    • not every lambda (passed as a parameter to in-lined function) can be inlined (if is is not called, but only stored as local variable for instance)
    • to prevent inlining some lambda parameter of in-lined function, “noninline” specification can be used for such parameter
  • “withLock” – multithread protect
  • “use” – alternative to java’s try-with-resources



  • aliasing
  • class, method, property from file or static members of class can be imported
  • * – like java’s



  • “super” – calls parent parent’s method, “super<type>” – to avoid ambiguity in case few parents with the same method, unlike java, it is not mandatory to have it as first method call
  • “open” – class or/and method, means not final,  should be explicitly specified, for both class and method.
  • “final” – like java’s class/method, could not be inherited/overridden, but unlike java, it is by default, if omitted any modifier. Used if opened function was overridden (overwriting keeps opened state) but further overwriting should be prohibited (function should be closed)
  • “abstract” – class/method that should be overwritten, opened by default, methods do not have a body.
  • visibility (public by default):
    • public
    • protected – only visible by subclass, no package-wide like java
    • internal – module-wide visibility (becomes public in java)
    • private – like java, but with exception, that outer class doesnt see private members of inner or nested class.
  • nested class – doesnt depend on outer class, “static” specified at java, but default for kotlin
  • inner class – depend on outer class, default for java, but “inner” specification should be used with kotlin.
  • sealed – defines nested classes of some sealed base class or at the same kotlin file where base sealed class defined. Opened by default. Used to avoid default branch at “when” construct.
  • main constructor is defined outside class body (“init” block and “constructor” keyword together are used if default logic should be customized)
  • class ClassName private constructor() {} – with private constructor
  • constructor(params): super/this(params) {} – secondary constructor
  • visibility of properties can be changed
  • compiler generated (can be overridden, inherited from Any)
    • toString(), like java
    • equals – called while ==, compares content, not references by default, === in koltin for reference equality
    • hashCode – like java’s, equal objects should have equal hash codes, but not vice versa
    • copy – to support immutability
  • data class – actual toString, equals and hashCode are generated on basis of properties declared only at primary constructor.
  • by – kotlin support for decorator design pattern
  • overloading operations (“operator fun” as a member or extension function), no “commutativity” (left<->right):
    • binary: times, div, mod, plus, minus, <op name>Assign (for compound operations), it conflicts with not compound, better use only one, or use val and var parameter to separate them
    • unary: unaryPlus, unaryMinus, not, inc, dec
    • equals – override from Any, not an operator
    • compareTo – overide from Any, not an operator, mapped to <, >, <=, >=, even an object has come from java
    • index operator: get/set
    • range belongings operator: “in” – contains
    • rаnge generation operator: “..” – rangeTo
    • loops iterating: “in” – iterator
    • destructing declaration: componentX()
  • delegated properties: “by” on property level, an access to property is delegated to corresponding property of another object (which implements getValue/setValue methods)
  • lazy property initialization “by lazy <lambda>”
  • storing property values at map, i.e. dynamic properties
  • <class name>::class.java – gets kotlin class for specified java class.



  • singleton – can inherit classes and interfaces
  • companion – defined with a class and can call private class members, its members can be accessed as outer class members
  • anonymous inner object – can implement a several interfaces



  • blocks of code within curvy brackets
  • it – current element
  • syntax : var_name: var_type, … -> body
  • if lambda is a last parameter of function it can be placed after closing round bracket after parameter list
  • if lambda is a single argument, and it is placed outside of parameter list, empty round brackets can be omitted.
  • clojures not final variables
  • reference to class function ClassName/InstanceName::FunctionName, or to top-level function ::FunctionName, or constructor ::ClassName
  • kotlin lambda can be passed to java method with lambda parameter, if no variables captured by lambda – same lambda instance is used all the time (otherwise, each time new lambda is created)
  • java’s SAM != kotlin lambda, SAM constructor, named as corresponding functional interface
  • “with” – parameter can be used implicitly at body of lambda, return result is a last expression of lambda
  • “apply” – called as an extension, always returns an object that it has been called on, which can be used implicitly within a lambda expression, as with “with”. Useful for object initializations.
  • local return from lambda: “return@label”, jumps execution flow after a block of code marked with “label”, function name can be used instead of “label”
  • anonymous functions, it is just  form of lambda expression, “fun (params) {body }” – if return specified, it performs local return (no label required), and as a general rule: “return” returns from closest “fun”.



  • can contain property declaration, abstract methods, non-abstract methods (like Java 8), but no state allowed.
  • override modifier in front of function (like java’s @override) is mandatory



  • same “map”, “filter”, “find”, “any”, “groupBy”, “flatMap” (map+flatten) operations like at java
  • lazy operations: sequences, like Java, but could not be paralleled, can be created from a collection, or be generated (initial value+lambda)
  • create an immutable collection “<collection type>Of” or “<mutable collection type>Of”: list, arrayList, set, hashSet, linkedSet, sortedSet, map, hashMap, linkedMap, sortedMap, arrayOf
  • mutable and immutable collections
  • primitive types arrays: IntArray, CharArray, BooleanArray, ByteArray, etc



  • nullable types – <type_name>?
  • java’s functionality @Nullable and @NotNull
  • if nullable variable is checked it is not a null, it becomes non-nullable within a context of check being performed
  • safe call operator ?. – calls a method or return a null as a result
  • elvis operator ?: – provides default value if null (or do something, like throwing exception)
  • safe cast as? – return null if cast is failed, otherwise keeps  variable casted with a context
  • !! – assertion that checks nullable variable is not null, and returns it
  • “let” function accepts lambda and in case of non-null executes body ad provides “it” with a value of object it is called on, otherwise nothing happens
  • “lateinit” – for non-null properties, that should be left not initialized within constructor. Always “var”.
  • extension function can be called on null values (of nullable types)
  • template parameter is nullable by default, <T: Any> – to specify a template type as non-nullable
  • java’s (platform, means no nullability information provided) types – type_name! – can be used for both: nullable and non-nullable. Kotlin recognizes java’s annotations @Nullable, @NotNull, etc. But in case no annotations are specified, java’s type is treated as “platform” type.



  • by default generic parameter considered as nullable
  • no row type, like java
  • like java, inclduding type boundaries, just “:” instead of java’s “extends”
  • reified, like “inline fun <reified T>” – to preserve type information at runtime, some (all) parameters can be reified, such functions cannot be called from java code;
    • can be used for: “is”, “!is”, “as”, “as?”, “::class”, “::class.java”, as a type argument
    • cannot be used for: create a new instance of T, call T’s companion methods, pass not-reified parameter to function that expects reified parameter
  • variance (specified on class level):
    • co-variance, <out T>, means T can be used only on “out” positions, applies only to public, protected or internal class members.
    • contra-variance, <in  T>, means T can be used only on “in” positions
    • invariant – no relations, T at any position
    • constructor parameters are neither “in” nor “out”
    • iser-side variance specification, like at java boundaries extends/super, when variance is specified on each T occurrence, means at each method
  • type projections – restrictions of user side variance on calling some type’s methods that can broke variance



  • @Deprecated – allows to specify new version of deprecated construction
  • can have a parameters of the following types:
    • primitive types
    • strings
    • enum
    • class references (<class name>::)
    • annotation classes
    • arrays (“arrayOf”)
  • “const” modifier for compile-time constant – can be used at annotations, should be placed at top-level file or object, only primitive types and String are allowed.
  • since kotlin’s property corresponds to java’s field, getter and/or setter, it should be specified what exactly (target) is expected to be decorated with the annotation, like @<target name>:<annotation name>, possible targets:
    • property – not for java
    • field
    • get/set
    • receiver
    • param – constructor’s parameter
    • setparam
    • delegate – delegator instance
    • file – needs to be placed on top of file even before “package” instruction
  • annotations can be applied almost to every code line
  • to customize byte-code generation for java:
    • @JvmName – changes method of field name
    • @JvmStatic – turns object’s method into java static
    • @JvmOverloads – generates overloads methods for java from kotlin method with default values
    • @JvmField turns kotlin property into java’s field (with no getters or setters)
  • declaring annotations: “annotation class <class name>(<parameters list>)”, but no body is allowed
  • meta-annotations – annotation that applied to annotation classes, like @Target (defines retention)



  • java’s – java.lang.reflect
  • kotlin’s – kotlin.reflect
  • “::<parameter>” – gets an reflection instance of parameter


DSL, it is hard, as usually…

  • invoke – callable objects
  • lambdas with receivers



Real Estate marketplace

A friend of mine created his own project, it is in essense a real estate marketplace – http://metr.ua
Assisted with some issues related to Spring, Hibernate, Amason S3 during development.

Main difference from other marketplaces, among other things is that there are no real estate agency fees or comissions – sellers, buyers and tentants can contact each other directly (buyers can save thousands of dollars on fees).
Pretty good design and usability!

Apache Kafka notes

Apache Kafka Java example, topic, producer, consumer:


Apache Kafka and Zookeeper clusters docker:


Purpose of Kafka, mostly:

  • messaging systems
  • streaming data

Message – data unit

Batch – collection of messages flushed to disk together for performance optimisation (after some amount is collected, or some time passed).

Schema – message serialization by producer and deserialization by consumer, popular – avro. Schema can be stored within Kafka separately from actual messages.

Topic – group of partitions (default number of partition and replication factor: 1)

Partition – messages are appended at the end, or deleted from beginning. Message ordering is preserved only within partition.

Producers send messages to Kafka, of no partition key specified, messages evenly distributed over partition within topic specified. If a message need to be written to specific partition, message key should be specified or custom partitioner can be written.

Consumers – consume messages from a a topic, as part of consumer group. Several consumer groups can subscribe for topic. Only one consumer from a group can read messages from specific partition.

Kafka server – one or more brokers.

Controller – one elected broker from cluster, for administration tasks, like assigning partition to brokers and monitoring brokers failures.

Each partition is read/written by single broker, called leader for this partition (for consistency). Partition can be bound to multiple brokers, but all other brokers beside leader for this partition just replicate (duplicate) it for failover reasons.

Messages within partitions of some topic are preserved (retention) for:

  • some time (default is 7 days)
  • partition length (default 1 GB)

MirrorMaker makes cluster duplication between datacenters, for failover reasons.

Broker important settings:

  • id
  • port
  • zookeeper
  • log directory

Topic important settings:

  • number of partition created automatically
  • retention time
  • retention size

Brokers are stateless regarding number of consumed messages, recent offset should be stored and provided by consumer.

Log compaction – relates to messages with same key – most recent message is preserved, while previous can be deleted, as less interested for consumer.

Message compression – group of messages (batch), is compressed:

  • on producer, to minimize network traffic
  • on leader, it is decompressed, offset is assigned for each message and messages get compressed again to minimize storage size and speedup potential cross-cluster replication.

Replication – copying messages from leader to followers, and notifying producer about success/failure:

  • sync – producer is acknowledged by leader about successful write, after all followers write the message to disk.
  • async – producer is acknowledged by leader about successful write, after message is flushed to disk on leader node only, but before all followers write the message to disk.

Writing producers:

  • connects to random broker from cluster, gets information about leaders, and connects second type for sending actual messages to leader broker for topic specified.
  • messages can be sent:
    • synchronously, per message
    • asynchronously, per message or in batches (flushed by count of messages pre-collected, or by time limit for collecting)
    • fire-and-forget

Producer important settings:

  • list of brokers
  • key and value serializers
  • type (sync, async)
  • acks count
  • partitioner class
  • compression codec (gzip, snappy, none)
  • batch size or time to wait for async batching mode

Reading consumers:

  • connects to random broker from cluster, gets information about leaders, and connects second type for reading actual messages to leader broker for topic specified.
  • high and low level API

Consumer groups:

  • name is global across cluster
  • adding new consumer to group causes rebalancing consumers between brokers, what can cause message deliver inconsistency. To mitigate: shutdown all consumers and start them up.

Consumed messages, but not committed, can be consumed once again (by another consumer) after rebalancing, if:

  • consumer which consumed some amount of messages crashed before commit
  • rebalancing happened for another reason, like adding new consumer into group, altering cluster, adding brokers, changing replication factor

Committing offset:

  • auto (default every 5 seconds), or next poll(), commits offset of previous poll()
  • manually, commitSync() or commitAsync(), commits all messages from last poll() called.
  • manually, particular offset (sync or async)

Rebalancing can have special logic assositated with, by implementing ConsumerRebalanceListener interface.

Exiting consumer loop: consumer.wakeup()

High level consumer API:

  • current offset is stored within Zookeeper, it is handled automatically underneath.

Low level (simple) consumer API:

  • offset is stored by consumer itself
  • contacts any broker to find a leader broker for topic of interest

Multithreaded consumers should map: one partition – one thread – one consumer.

Consumer can subscribe for topic or to specific partitions.

Consumer important settings:

  • group id
  • id
  • zookeeper, with timeouts
  • client id (kafka client identification)
  • offset autocomits timeouts
  • initial offset (latest, smalest, ?)
  • time interval to wait for incoming messages before exception

Administration, tools:

  • tool for graceful shutdown of broker (another leader will be selected proactively or shutdown operation failed if there is no new leader, to minimize downtime, milliseconds, and avoid data loss)
  • tool for rebalancing of leads between available brokers
  • tools for populating newly added broker with partitions (or replacing existing partitions from a broker planned for decommission), since after new broker is added, it is empty.
  • tool for increasing number of partition at some topic
  • tool to list all topics, partitions, all other details for specific cluster.

Functional approach specifics

Pure functions – no side effects, nothing else than transforming input parameters into returning value. State is kept outside, it can be passed into function and returned from it.


  • First-class citizen functions
  • High order functions
  • Anonymous functions
  • Closures
  • Currying
  • Lazy evaluation

Referential transparency – function call can be replaced by its returning value

Higher-order-function – function that accepts as an input other function

Partial function application (currying) – transform function with two parameters into function with one parameter, giving the dropped parameter a some value

Functional data structures – are immutable.

Instead of exceptions -> Option, Either, Result – returning values

Laziness – evaluating parameter when it used, but not declared

Memoization – remembering some resource-consuming operation’s result, and returning for later calls (some kind of caching)

Strict functions – all parameters are evaluated before function call, non-strict functions – with lazy parameters

Recursive functions consumes a data, corecursive – produces, both need terminating condition.

Monoid – set of some type, binary operation that takes two instances of the type and returns a new instance of the type, and empty “identity” element


  • computation context for other value
  • container, with two (or three) function defined: unit (constructor) and flatMap OR unit and compose OR unit, map, join

It is always possible to transform impure function into pure one, with two side-effects, one producing the pure function input, other consumes pure function output.

Effect – an object that contains an operation that produces side-effect, it has a method that executes a side-effect

Side-effect – is actually non-functional operation

C++11, BS book extract, part II, chapter 12 + 13

Parts of a Function Declaration

  • [[noreturn]], indicating that the function will not return using the normal call/return mechanism
  • override, indicating that it must be overriding a virtual function from a base class
  • final, indicating that it cannot be overriden in a derived class

That is, a prefix auto indicates that the return type is placed after the argument list. The suffix return type is preceded by −>.
The essential use for a suffix return type comes in function template declarations in which the return type depends on the arguments. For example:
template<class T, class U> auto product(const vector<T>& x, const vector<U>& y) −> decltype(x∗y);
However, the suffix return syntax can be used for any function.

A constexpr function cannot have side effects, so writing to nonlocal objects is not possible.

If there is a possible ambiguity, an initializer_list parameter takes priority.

Unspecified Number of Arguments

  • a variadic template
  • an initializer_list as the argument type
  • Terminate the argument list with the ellipsis ( … )


Automatic Overload Resolution

  • Exact match
  • Match using promotions
  • Match using standard conversions
  • Match using user-defined conversions
  • Match using the ellipsis … in a function declaration


Overloading takes place among the members of an overload set. By default, that means the functions of a single scope; functions declared in different non-namespace scopes do not overload.


The C++ standard library provides provides one of the following guarantees for every library operation:

  • The basic guarantee for all operations: The basic invariants of all objects are maintained, and no resources, such as memory, are leaked.
  • The strong guarantee for key operations: in addition to providing the basic guarantee, either the operation succeeds, or it has no effect.
  • The nothrow guarantee for some operations: in addition to providing the basic guarantee, some operations are guaranteed not to throw an exception.


Both the basic guarantee and the strong guarantee are provided on the condition that

  • user-supplied operations (such as assignments and swap() functions) do not leave container elements in invalid states
  • user-supplied operations do not leak resources
  • destructors do not throw exceptions


double compute(double) noexcept; // may not throw an exception. It terminates unconditionally by invoking std::terminate() if exception is thrown.


The noexcept Operator – template<typename T> void my_fct(T& x) noexcept(Is_pod<T>());

The handler is invoked:

  • [1] If H is the same type as E
  • [2] If H is an unambiguous public base of E
  • [3] If H and E are pointer types and [1] or [2] holds for the types to which they refer
  • [4] If H is a reference and [1] or [2] holds for the type to which H refers


The guiding principles are:

  • Don’t throw an exception while handling an exception.
  • Don’t throw an exception that can’t be caught.


The specific rules for calling terminate() are

• When no suitable handler was found for a thrown exception
• When a noexcept function tries to exit with a throw
• When a destructor invoked during stack unwinding tries to exit with a throw
• When code invoked to propagate an exception (e.g., a copy constructor) tries to exit with a throw
• When someone tries to rethrow ( throw; ) when there is no current exception being handled
• When a destructor for a statically allocated or thread-local object tries to exit with a throw
• When an initializer for a statically allocated or thread-local object tries to exit with a throw
• When a function invoked as an atexit() function tries to exit with a throw


By default, terminate() will call abort(). If that is not acceptable, the user can provide a terminate handler function by a call std::set_terminate() from <exception>

If an exception is not caught on a thread, std::terminate() is called.


Problematic windows updates

wusa.exe /uninstall /kb:2859537
wusa.exe /uninstall /kb:2872339
wusa.exe /uninstall /kb:2882822

Book read roadmap