lessons

Lessons:

• 26-02-2024 - Lesson 01 - Introduction and Modern VMs, e.g. JVM, Node
• 26-02-2024 - Lesson 02 - Metadata, Classpath and Java Type System introduction
• 27-02-2024 - Lesson 03 - Members and static vs instance (non-static)
• 04-03-2024 - Lesson 04 - Boxing and Unboxing
• 04-03-2024 - Lesson 05 - Nested, Inner, Abstract classes, Virtual Methods
• 05-03-2024 - Lesson 06 - Anonymous Types
• 11-03-2024 - Lesson 07 - Kotlin to Java type system
• 11-03-2024 - Lesson 08 - Object declarations
• 12-03-2024 - Lesson 09 - Kotlin Reflection

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Lesson 01 - Introduction and Modern VMs, e.g. JVM, Node

• Bibliography and Lecturing methodology: github and slack.
• Tools: javac, javap, kotlinc, JDK 17 and gradle.
• Program outline in 3 parts:
  1. Java Type System and Reflection;
  2. Metaprogramming and Performance;
  3. Lazy processing.
• Project in 3 parts according to program outline.
• Managed Runtime or Execution Environment or informally virtual machine (VM) or runtime.
• Execution Environment includes:
  • Compiler,
  • Programming languages,
  • Standard libraries,
  • Dependency manager (e.g. gradle, maven)
  • Central Repository (e.g. Maven Central Repository, Nuget, NPM, etc)
• Examples of languages targeting JVM: Java, kotlin, Scala, Clojure.
• Examples of languages targeting Node: JavaScript, TypeScript, Kotlin.
• JVM runs .class files with bytecode
• JVM translates bytecode to machine code (e.g. IA-32, AMD64, etc depending of the CPU)
• In Javascript ecosystem modules are deployed in source code i.e. Javascript.
• Distinguish between Programming Language <versus> VM

Lesson 02 - Metadata, Classpath and Java Type System introduction

• One file .class for each class definition
• Metadata:
  • data that provides information about other data
  • In a .class the metadata provides a description and structure of a Type.
• Using javap -c -p AppKt.class to inspect metadata and bytecode definition of class AppKt
• CLASSPATH
  • e.g. -cp . - local folder
  • e.g. -cp '.:~/JetBrains/IntelliJIdea2022.2/plugins/Kotlin/lib/*'
  • (for windows use ; rather than :)

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• Type System - Set of rules and principles that specify how types are defined and behave.
• Two kinds of types: Primitive and Reference types.
• Classes have Members
• Members may be: Fields or Methods.
• There are NO properties in Java Type System.
• Using javap -p Person.class to inspect metadata
• The fully qualified name of a class includes its package name.
• Constructor is a method with the name <init> returning void.

Lesson 03 - Members and static vs instance (non-static)

• Member access syntax: Receiver.Member.
• The Receiver is the target of the member access and it is a Type (for static members) or an Object (for non-static/instance members).
• JOL - Java Object Layout:
  • java -cp .;jol-cli-0.17-full.jar org.openjdk.jol.Main estimates <classqualifiedname>
  • (linux replace ; by : on -cp (classpath))
• Object header = mark word (used for hash, locks, GC, etc) + class word (class-specific metadata).
• Fields alignment, reorder and padding gap.

Lesson 04 - Boxing and Unboxing

• Static initializer: initializes static fields.
• Immutable values:
  • constant value calculated at compile time, e.g. Kotlin const
  • immutable, yet dynamically initializable
• Boxing and Unboxing.
• Boxing : <Wrapper>.valueOf(value)
  1. Heap allocation
  2. Object header initialization
  3. Copy value
• Unboxing: <Wrapper Instance>.<primitive>Value(), e.g. nr.intValue()

Lesson 05 - Nested, Inner, Abstract classes, Virtual Methods

• Nested classes.
• Inner classes: non-static in Java or inner in Kotlin.
• Abstract classes cannot be directly instantiated.
• Interfaces represent abstract types that cannot be instantiated too.
• Override
• Names collision and Member access ambiguity
• Methods call resolution: Fields, Methods, and Virtual Methods.

Lesson 06 - Anonymous Types

• Anonymous Classes in Java
• Object Expressions in Kotlin
• Resulting types from compilation

Lesson 07 - Kotlin to Java type system

• Homework 2 - virtual and non-virtual methods.
• Kotlin Class Members
• Analyzing Kotin properties in JVM.
• There are NO properties in Java Type System.
• A Kotlin property may generate:
  • Backing field
  • Getter, i.e. function get... -- called with invoke... bytecode.
  • Setter, i.e. function set... (if defined with var).
• Top-level declarations
• Extension functions

Lesson 08 - Object declarations

• Singleton design pattern
• object keyword:
  • private constructor
  • Singleton instance in static INSTANCE field;
• companion object - specific type of object declaration associated with its owner class.

Lesson 09 - Kotlin Reflection

• Reflection object oriented API for metadata
• Kotlin Reflection API (kotlin.reflect): KClass, KCallable ----->* KFunction
• KClass is representing a type in Kotlin.
• KCallable base type of KFunction and KProperty
• KProperty and KMutableProperty
• KCallable ----->* KParameter
• KFunction properties: name, type, parameters and instanceParameter.
• KParameter property kind: INSTANCE versus EXTENSION_RECEIVER
• KParameter property isOptional
• KClass::createInstance()
• KFunction::call()