#Java
Collections
It was designed to meet three goals:

• its implementations for the fundamental collections had to be highly efficient.
• it had to allow different types of collections to work in a similar manner and with interoperability.
• it had to adapt a new collection easily.

AbstractCollection: Implements most of the Collection interface.

Lists
AbstractList: Extends AbstractCollection and implements most of the List interface.
AbstractSequentialList: Extends AbstractList for use by a collection that uses sequential rather than random access of its elements.
LinkedList: Implements a linked list by extending AbstractSequentialList.
ArrayList: Implements a dynamic array by extending AbstractList.

Sets
AbstractSet: Extends AbstractCollection and implements most of the Set interface.
HashSet: Extends AbstractSet for use with a hash table.
LinkedHashSet: Extends HashSet to allow insertion-order iterations.
TreeSet: Implements a set stored in a tree. Extends AbstractSet.

Maps
AbstractMap: Implements most of the Map interface.
HashMap: Extends AbstractMap to use a hash table.
TreeMap: Extends AbstractMap to use a tree.
WeakHashMap: Extends AbstractMap to use a hash table with weak keys.
LinkedHashMap: Extends HashMap to allow insertion-order iterations.
IdentityHashMap: Extends AbstractMap and uses reference equality when comparing documents.

#Java
LinkedList

1. void add(int index, Object element)
Inserts the specified element at the specified position index in this list. Throws IndexOutOfBoundsException if the specified index is out of range (index < 0 || index > size()).

2. boolean add(Object o)
Appends the specified element to the end of this list.

3. boolean addAll(Collection c)
Appends all of the elements in the specified collection to the end of this list, in the order that they are returned by the specified collection's iterator. Throws NullPointerException if the specified collection is null.

4. boolean addAll(int index, Collection c)
Inserts all of the elements in the specified collection into this list, starting at the specified position. Throws NullPointerException if the specified collection is null.

5. void addFirst(Object o)
Inserts the given element at the beginning of this list.

6. void addLast(Object o)
Appends the given element to the end of this list.

7. void clear()
Removes all of the elements from this list.

8. Object clone()
Returns a shallow copy of this LinkedList.

9. boolean contains(Object o)
Returns true if this list contains the specified element. More formally, returns true if and only if this list contains at least one element e such that (o==null ? e==null : o.equals(e)).

10. Object get(int index)
Returns the element at the specified position in this list. Throws IndexOutOfBoundsException if the specified index is out of range (index < 0 || index >= size()).

11. Object getFirst()
Returns the first element in this list. Throws NoSuchElementException if this list is empty.

12. Object getLast()
Returns the last element in this list. Throws NoSuchElementException if this list is empty.

13. int indexOf(Object o)
Returns the index in this list of the first occurrence of the specified element, or -1 if the list does not contain this element.

14. int lastIndexOf(Object o)
Returns the index in this list of the last occurrence of the specified element, or -1 if the list does not contain this element.

15. ListIterator listIterator(int index)
Returns a list-iterator of the elements in this list (in proper sequence), starting at the specified position in the list. Throws IndexOutOfBoundsException if the specified index is out of range (index < 0 || index >= size()).

16. Object remove(int index)
Removes the element at the specified position in this list. Throws NoSuchElementException if this list is empty.

17. boolean remove(Object o)
Removes the first occurrence of the specified element in this list. Throws NoSuchElementException if this list is empty. Throws IndexOutOfBoundsException if the specified index is out of range (index < 0 || index >= size()).

18. Object removeFirst()
Removes and returns the first element from this list. Throws NoSuchElementException if this list is empty.

19. Object removeLast()
Removes and returns the last element from this list. Throws NoSuchElementException if this list is empty.

20. Object set(int index, Object element)
Replaces the element at the specified position in this list with the specified element. Throws IndexOutOfBoundsException if the specified index is out of range (index < 0 || index >= size()).

21. int size()
Returns the number of elements in this list.

22. Object[] toArray()
Returns an array containing all of the elements in this list in the correct order. Throws NullPointerException if the specified array is null.

23. Object[] toArray(Object[] a)
Returns an array containing all of the elements in this list in the correct order; the runtime type of the returned array is that of the specified array.

GoTo
- is a statement that performs a one-way transfer of control to another point of code; in contrast to a function call which returns control.

Its use has declined significantly since the advent of structured programming in the 1960s. Structured programming languages like #Pascal introduced control structures such as: subroutines, loops and multiway branch for clarity and efficiency. These replace equivalent flows written using gotos and ifs.

Language support:
• #C
• #CSharp: also makes case and default statements labels, whose scope is the enclosing switch statement; goto case or goto default is often used to replace explicit "fall-through", which C# disallows.
• #Perl
• #PHP there was no native support for goto until version 5.3
• #Java: goto is a reserved word, but is unusable.
• #Python: does not support it but there are several joke modules that provide it.

The structured program theorem proves that goto is not necessary to write programs.

The diamond problem
- is an ambiguity that arises when two classes B and C inherit from A, and class D inherits from both B and C. If there is a method in A that B and C have overridden, and D does not override it, then which version of the method does D inherit: that of B, or that of C?

For example, in the context of GUI software development, a class Button may inherit from both classes Rectangle (for appearance) and Clickable (for functionality/input handling), and classes Rectangle and Clickable both inherit from the Object class. Now if the equals method is called for a Button object and there is no such method in the Button class but there is an overridden equals method in Rectangle or Clickable (or both), which method should be eventually called?

How different languages deal with it:
• Common #Lisp: by order "...in the order in which parent classes are named in the subclass definition"
• Curl: "and the secondary constructor will be invoked for all other subclasses."
• Eiffel: "Eiffel will automatically join features together, if they have the same name and implementation."
• Go: compile-time error
• #Java: compile-time error
• #OCaml: by order "...are inherited in the same order, with each newly inherited method overriding any existing methods."
• #Perl: by order "...from as an ordered list. The compiler uses the first method it finds..."
• #Python: by order
• #Ruby: by order "...as rightmost depth first resolution."
• #Scala: by order "allows multiple instantiation of traits, which allows for multiple inheritance by adding a distinction between the class hierarchy and the trait hierarchy. A class can only inherit from a single class, but can mix-in as many traits as desired." This approach is the most similar to #Nile's; traits being qualifications.
• Tcl: by order "the order of specification in the class declaration affects the name resolution for members..."

Languages that allow only single inheritance, where a class can only derive from one base class, do not have the diamond problem.

Moreover, languages such as #Ada, Objective-C, #CSharp, #Delphi/Free #Pascal, Java, #Swift and PHP allow multiple-inheritance of interfaces (called protocols in Objective-C and Swift). Interfaces are like abstract base classes that specify method signatures without implementing any behavior.

When several interfaces declare the same method signature, as soon as that method is implemented (defined) anywhere in the inheritance chain, it overrides any implementation of that method in the chain above it (in its superclasses). Hence, at any given level in the inheritance chain, there can be at most one implementation of any method. Thus, single-inheritance method implementation does not exhibit the Diamond Problem even with multiple-inheritance of interfaces.

Swing versus AWT
Every Swing lightweight interface ultimately exists within an AWT heavyweight component because all of the top-level components in Swing (JApplet, JDialog, JFrame, and JWindow) extend an AWT top-level container.

AWT components
• are heavyweight
• are implemented by platform-specific code.
• are rendered and controlled by a native peer component specific to the underlying windowing system.

Swing components
• are lightweight
• are written entirely in Java and therefore are platform-independent.
• do not require allocation of native resources in the OS's windowing toolkit.
• Rendering is provided by Java 2D.
• More powerful and flexible
• More components such as tabbed panel, scroll panes, trees, tables, and lists.
• Customizable look and feel

After Java 6, both Swing and AWT components can co-exist in one GUI without Z-order issues.

#Java

“Fuck both of them, and fuck Java on top of it. But if you must, of course, Swing.”
— #TheSemicolon

Minimum object size is 16 bytes for modern 64-bit JDK since the object has 12-byte header, padded to a multiple of 8 bytes. In 32-bit JDK, the overhead is 8 bytes, padded to a multiple of 4 bytes.

References have a typical size of 4 bytes on 32-bit platforms and on 64-bits platforms with heap boundary less than 32Gb (-Xmx32G), and 8 bytes for this boundary above 32Gb.

#Java

read more

The names of #Java constants are in capital letters; and are never one character long.

#Naming_convention

Data layout
- is how multidimensional arrays are stored in a linear storage such as RAM. It is critical for:
• correctly passing arrays between programs written in different programming languages
• performance when traversing an array because modern CPUs, due to caching, process sequential data more efficiently than non-sequential data
• contiguous access makes it possible to use SIMD instructions that operate on vectors of data

Row-major versus column-major order
The difference between the orders lies in which elements of an array are contiguous in memory. In a row-major order, the consecutive elements of a row reside next to each other, whereas the same holds true for consecutive elements of a column in a column-major order. While the terms allude to the rows and columns of a two-dimensional array, the orders can be generalized to arrays of any dimension.

Transposition
As exchanging the indices of an array is the essence of array transposition, an array stored as row-major but read as column-major (or vice versa) will appear transposed. As actually performing this rearrangement in memory is typically an expensive operation, some systems provide options to specify individual matrices as being stored transposed.

Languages support
• Row-major: #C/C++/Objective-C (for C-style arrays), PL/I, #Pascal, Speakeasy, SAS, and Rasdaman
• Column-major: #Fortran, #MATLAB, GNU Octave, S-Plus, #R, #Julia, and Scilab.
• Neither (for less dense arrays):
• Iliffe vectors: #Java, #Scala, #Swift. #Ruby, #Perl, #PHP, #JavaScript, Visual Basic .NET
• Lists of lists: #Python, Wolfram Language of Wolfram Mathematica
• Tables of tables: #Lua

#Java
The JVM
- is the detailed, formal specification for a 32-bit abstract machine. Abstract implies hardware and OS-independent and thus is interoperable across platform-dependent implementations. It operates on abstracte data types rather the native data types of any ISA:
• 32-bits types: integers, floats, and references are called primitives
• 64-bits types: long and double are supported but consume two units of storage
• smaller types: boolean, byte, short and char are extended to 32-bit ints

It loads code, verifies it, and executes it. It has instructions for:
• Load and store
• Arithmetic
• Type conversion
• Object creation and manipulation
• Operand stack management (push & pop)
• Control transfer (branching)
• Method invocation and return
• Throwing exceptions
• Monitor-based concurrency

The specification includes various dynamic features such as GC and thread management.

JDK
- is an software development kit for Java and consists of:
• a private stand-alone implementation of the JVM such as JRE's HotSpot
• the Java standard library, JCL
• an interpreter/loader (java)
• a compiler (javac)
• an archiver (jar)
• a documentation generator (Javadoc)

HotSpot
- relies on JIT; and divides the memory into generations:
• young generation: heap for short-lived objects
• old generation: heap for long-lived objects
• permanent generation: used for class definitions and associated metadata

JVM languages
Any language that can express a valid class file. The existing languages includes ports from other languages:
• JRuby (#Ruby)
• Jython (#Python)
And entirely new languages that compile to Java bytecode:
• Clojure
• Apache Groovy
• #Scala
• #Kotlin

JVM languages are compatible with each other i.e. libraries of one can be used with programs of another. A new feature will supports dynamically typed languages in the JVM, and is developed within the Da Vinci Machine project whose mission is to extend the JVM so that it supports languages other than Java.

OpenCL (Open Computing Language)
- is a parallelism framework for writing programs that execute across heterogeneous platforms consisting of one host CPU and any number of compute devices. These devices include GPUs, CPUs with SIMD instructions, FPGAs, Movidius Myriad 2, Adapteva epiphany and DSPs.

A compute device is broken down to several compute units, which themselves are broken down to multiple PEs (processing elements). A single function execution can run on any number of PEs in parallel. How a compute device is subdivided into compute units and PEs is up to the vendor.

It defines an API for programs running on the host to launch functions on compute devices and manage device memory. This API is defined for C and C++, as well as third-parties such as #Python, #Java, #Perl, #DotNET, etc. A more recent, higher-level model is SYCL; which is purely based on C++11. Programs in OpenCL are compiled at run-time therefore its applications are portable between various host devices.

A four-level memory hierarchy is defined for the compute device:
• global: shared by all PEs, high access latency __global
• read-only: smaller, low latency, writable only by the host __constant
• local: shared by a group of PEs __local
• per-element private memory: device registers; __private

Not every device needs to implement each level of this hierarchy in hardware. Consistency between the various levels in the hierarchy is relaxed, and only enforced by explicit synchronization constructs, notably barriers. The host provides handles on device memory buffers and functions to transfer data back and forth.

OpenCL C
- is the programming language used to write compute kernels. Though based on C99, it is adapted to fit the device model.

A memory buffer resides in a specific level and its pointer is annotated with a region qualifier: __global, __local, __constant, and __private

Comparison with #C:
• There is no main; functions are marked __kernel to signal that they are entry points, and are to be called from programs running on the host
• There are no function pointers, bit fields or variable-length arrays
• Recursion is forbidden
• stdlib is replaced by a custom set of standard functions, geared toward math programming
• Scalar types such as float and double behave similarly to those of C

Features:
• Vector types available in fixed-lengths of 2, 3, 4, 8 and 16; and for various base types e.g. float4 (4-vector of single-precision floats)
• Operations for vector types
• Synchronization facilities
• Functions to work with work-items and work-groups
• More specialized types incl. 2D and 3D image types

#OpenCL #OpenCL_C

8 steps to set-up a Java project in VS Code using Git, GitHub, and Gradle

This will help you set-up a Java project in VS Code that:
• Has source control using Git
• Has build automation with Gradle
• Benefits from a flexible development environment that is VS Code
• Is cross-platform and high-level yet, statically-typed and robust per Java
• Has collaborative possibilities through GitHub

But first, make sure you:
• Have Git, Gradle, Java, and VS Code installed, and in PATH
• Have the Git, Gradle and Java VS Code extensions installed and enabled
• Have a GitHub account

If you do, follow these steps:
1. Open the command prompt and use cd to navigate to anywhere you want
2. Use mkdir <project-name> to create a new directory, and cd <project-name> to navigate into it
3. Once inside, do gradle init and interact with it to initialize Gradle there
4. Do code . to open that directory in VS Code
5. Press Ctrl+Shift+G ⌨️, stage all changes, and commit your first commit
6. Create a new repo on GitHub, and copy the URL it gives you
7. Do git init to initialize the directory as a Git repo, git remote add origin <github-url> to set, and git remote -v to verify its remote origin [Source]
8. From within VS Code, click on sync on the bottom left corner

You are done!

#tutorial #java #vscode #gradle #github #git #cli #sourcecontrol #buildautomation