University lessons on Java.

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├── 8americain
│   ├── Carte.java
│   ├── CollectionCartes.java
│   ├── Joueur.java
│   ├── Main.java
│   ├── MainJoueur.java
│   ├── Symboles.java
│   ├── TasCartes.java
│   └── Valeurs.java
├── Abstraction
│   ├── Fibonnaci_seq.java
│   ├── Geo_seq.java
│   ├── Numerical_seq.java
│   └── Test_seq.java
├── Collection
│   └── CollectionTest.java
├── Exam2022
│   ├── Boite.java
│   ├── EcrireFichierProduit.java
│   ├── LireFichierProduit.java
│   ├── MainGestionProduit.java
│   ├── Main.java
│   ├── Produit.java
│   └── Stock.csv
├── Exam2022_03
│   ├── EntNat.java
│   ├── ErrConst.java
│   ├── ErrModif.java
│   ├── ErrNat.java
│   └── Main.java
├── Exam2022_06
│   ├── copied.txt
│   ├── IndexNonValideException.java
│   ├── InverserFichier.java
│   ├── Main.java
│   └── MonTableau.java
├── Files
│   ├── exercice.java
│   ├── Ex.java
│   ├── temp_dir
│   │   └── blabla.txt
│   └── test.txt
├── Inheritance
│   ├── Barman.java
│   ├── Cowboy.java
│   ├── Human.java
│   ├── Robber.java
│   ├── Sherif.java
│   ├── Story.java
│   └── Woman.java
├── Map
│   └── MapTest.java
├── Matrix
│   ├── Matrix.java
│   └── MatrixTest.java
├── README.md
├── Streams
│   ├── FiboStream.java
│   ├── Streams
│   │   ├── CartesianProduct.java
│   │   └── Pair.java
│   └── StremsEx.java
├── TabPolynom
│   ├── Polynom.java
│   └── TestPolynom.java
├── Threads
│   ├── A.java
│   ├── E.java
│   ├── Fibonacci
│   │   ├── Fibo.java
│   │   └── Main.java
│   ├── FileSearch
│   │   ├── FileSearch.java
│   │   └── Main.java
│   ├── IntSum
│   │   ├── Main.java
│   │   └── Sum.java
│   ├── MatrixMult
│   │   ├── Main.java
│   │   └── MatrixRes.java
│   ├── TabCount
│   │   ├── Main.java
│   │   └── TabCount.java
│   └── WordCount
│       ├── FileCounter.java
│       ├── FileException.java
│       ├── FileFinder.java
│       ├── FileProcessor.java
│       └── Main.java
└── two_degree
    ├── Test_Eq.java
    └── Two_D_Eq.java

• getState() returns RUNNABLE, BLOCKED, NEW or TERMINATED. ( which are the states that a thread can be in.)
• getPriority() returns the priority of the thread. Priority is an integer between 1 and 10. The higher the priority the sooner the execution will end. (high priority = execute first).
• setPriority() sets the priority to the thread.
• start() starts the thread, calls the run() method of the sepecified instance. This instance either implements java.lang.Runnable, or extends java.lang.Thread.
• join() ou join(ms) waits for the a thread to end. This method can take a timeout in ms. This informs the main thread how long it should wait. At the end of the timeout the main thread continues whether if the other thread is done or not.

Collections has multiple methods :

• add(Elt) : adds a new element to the collection, regardless wether or not the element is already in the collection.
• addAll(Collection) : adds a collection of elements to the "caller" collection.
• remove(index) : removes the element at the index from the collection, returns the removed element.
• remove(Elt) : removes the first occurence of the element from the collection, returns true or false depending on whether if the element is removed or not.
• removeAll(Collection) : removes all elements from the collection if they are contained in the collection given as the argument.
• retainAll(Collection) : removes all elements if they are not present in the collection given as the argument.
• contains(Element) : returns true or false
• containsAll(Collection) : returns true if the "caller" collection contains all elements in the collection given as the argument. Returns false otherwise.
• size() : returns the length as an int.
• clear() : emptys the collection
• interator() : returns an interator of the collection. ( can be used for the foreach )
• toArray() : returns an array containing the elements of the collection,
• toArray(Array) : modifies the array given as the argument with the content of the list. If not enough room, the array will not be filled

Examples of toArray() functions :

    List<String> l = new LinkedList<>();
    l.add("a");
    l.add("b");

    // no need to specify the correct length, toArray() will figure it out
    tab = l.toArray(new String[0]);         
    
    System.out.println("tab :  " + Arrays.toString(tab)); // returns "tab :  [a, b]"
  
    // =============================================================================
    List<String> l = new LinkedList<>();
    l.add("a");
    l.add("a");

    // 1st version
    String[] tab = new String[2];

    // if you pass in a constructed array, make sure that the length matches
    l.toArray(tab);
    System.out.println("tab :  " + Arrays.toString(tab)); // returns "tab :  [a, b]"

    // 2nd version

    String[] tab = new String[1];

    // if the array is already constructed and do not match the needed length
    // if will be filled with a null value; that is it ! 
    l.toArray(tab);
    System.out.println("tab :  " + Arrays.toString(tab)); // returns "tab :  [null]"

• forEach(Consumer) : iterates over the collection and consumes the elements, for example to print the elements.

The Collection interface inherits from the Iterable interface. The collection interface has 3 derived interfaces.

• Set : No order, no duplicates, add(), remove(), contains()... > HashSet and TreeSet are implementing the Set Interface.
• List : Order, duplicates, access an element via its position, add(), remove(), contains(), get()... > ArrayList and LinkedList are implementing the List Interface.
• Queue : FIFO behaviour, add(), remove(), peek() to get but don't remove queue element ... > PriorityQueue and LinkedList are implementing the Queue interface.

Keep in mind that each collection implementation provides its own data structuring, but at the end of the day all they do is CRUD operations. a.k.a: add elements, see if an element is present, remove elements, access/read/update the value of the elements

Map Interface comes with a bunch of methods. The idea behind Map interface is to pair a key with an Object. It can be done withe these methods :

• put(key, value) pairs the value with the given key. If a value already exists for the given key, it will be updated with the new value.
• get(key) gets the object with the given key. Returns null if there are no value for the given key.
• containsKey(key) Returns wether or not a given key is present, even if it's linked to a null object.
• containsValue(value) Returns wether or not a given value is present. containsValue(null) will be true only if null has been inserted into the map.
• size() Returns the number of pairs in the map.
• values() returns an array of the values present in the Map.
• keySet() returns a Set of the keys (no repetitions and no order).

Many Classes implements Map Interface :

• HashMap > stores the keys in a Hash Table. Linear complexity.
• TreeMap > stores the keys in a tree structure and needs a comparator to do it. Logarithmic complexity.
• LinkedHashMap > stores the keys in a LinkedList structure. Linear complexity.

When using an interface like Map, Collection... we are using a generic interface. Generic types needs to be specified explicitly. For example :

Map<Integer, String> map = new TreeMap<>(); // ==== will create a map with Integers as keys and Strings as values
List<String> l = new ArrayList<>(); // ============ will create a list of Strings
List<List<Integer>> list_l = new LinkedList<>(); // will create a list of List of Integer, ie a 2D mutable Array

We can't create a list or a Map of int, float, double... they are primitive types and they are not supported for this use. The solution is boxing. instead of using an int, we will use an Integer. Today, java is doing Auto-boxing. Inside an Integer list we can put an int. Java will automatically construct an Integer object theat holds the int value. (Boxes it in) It's the same process for double and Double, float and Float...

The diamond operator <> is used to infer the type of the data :

Map<Integer, String> map = new HashMap<Integer, String>();  // long and too much informations
Map<Integer, String> map = new HashMap<>(); // =============== short and easier to read

Java allows its users to specify the content of a generic class : for example if we declare a List l as : List<? extends T> l; ; l will be able to hold only the objects that inherit from T The oposite is : List<? super T> l; ; l will be able to hold only the objects from which T inherits.

An iterator is an object used to iterate over a collection. It is basically a pointer that points to an object inside the collection. Warning : An iterator must be used as soon as it's created. If the collection is modified between creation and use of the iterator, it can throw an exception. Here are the Iterator methods :

• hasNext() returns wether or not the current element has a next element.
• next() points to the next element and returns its value. Throws an exception if the next element does not exists.
• remove() removes the element that is actually pointed out by the iterator. next() must have been called at least once.

Example of use :

    List<String> l = new ArrayList<>();
   
    l.add("a"); l.add("b"); l.add("c"); l.add("d"); l.add("a");
    
    Iterator<String> it = l.iterator();
    
    while(it.hasNext())
        System.out.printf("it : %s\t", it.next());
        System.out.println("");

Iterators can be used to iterate over an array or an HashMap but it's not optimal.

## Collections Class Do not mismatch with Collection interface The Collections class is a polymorphic class that implements lots of algorithms, as sort() and reverse(). It operates on Collections.

The List class extends the Collections Class. It adds add(index), remove(index, Elt), addAll(index, Collection) and so on... These are the classes that extends the List Class :

1. ArrayList > better for access
2. LinkedList > better for add and remove
3. Stack > particular use
4. Vector > particular use
5. CopyOnWriteArrayList > thread safe

Here are 4 usual ways to iterate over a List of the Collections class :

for(int i = 0; i < l.size() ; ++i) System.out.println(l.get(i));

Iterator<String> it = l.iterator();
while(it.hasNext()) System.out.println(it.next());

l.forEach(s -> System.out.println(s));

l.forEach(System.out::println);

l.sort((a, b) -> a - b);

l.sort(Comparator.naturalOrder());
l.sort(Comparator.reverseOrder());

Collections.sort(l);

l = l.stream().sorted(Comparator.naturalOrder()).collect(Collectors.toList());

Most of the files classes are contained in the java.io package.

• InputStream > bytes streams Abstract
• OutputStream > bytes streams Abstract
• Reader > Unicode char streams Abstract
• Writer > Unicode char streams Abstract
• File > File and directory manipulation
• StremTokenizer > lexical analysis of an entry stream
• BufferedReader > Adds a buffer to read a character stream

read() and write() are the two main methods of streams Classes.

3 steps of usage with streams :

1. Open
2. Use
3. Close

With streams, you can read and write from multiple sources / destinations :

• Files
• Socket (network communication)
• Images
• communication between two executions files
• bytes arrays
• strings
• URL...

• public InputStream () Constructor.
• abstract int read () Returns the byte (0-255) in the input stream or -1 if it's the end of the input stream.
• public int read (byte[] buff) Fills up the buffer with the bytes from the input stream.
• public int read (byte[] buff, start, nb) Fills up nb of the buffer slots from start with the bytes from the input stream.
• public long skip (n) Passes n bytes in the input stream.
• public int available () Returns how many bytes are available for reading.
• public void close () Closes the InputStream.
• synchronized void mark (bytes_limit) Places a mark to maybe go back in the future. bytes_limit stands for the number of bytes read after the mark can be forgotten.
• synchronized void reset () Places the stream at the last mark.
• public boolean markSupported () Indicates if the stream supports mark notion (used to go backwards).

• public OutputStream () Constructor.
• abstract void write (byte) Writes the given byte into the stream.
• public abstract int write (byte[] buff) Writes the buff array into the stream.
• public abstract int write (byte[] buff, start, len) Writes len slots the byte array into the stream from start.
• public void close () Closes the OutputStream.

System.out.println("about to write in file " + filename + " type 'STOP' to end :");
try{
    Scanner sc = new Scanner(System.in); // scanner used to read user input
    FileWriter fw = new FileWriter(filename); // writing stream, the file will be created or replaced if it already exists
    String buff = "";
    while(!(buff = sc.next()).contains("STOP") && buff != null)
        fw.append(buff + "\n"); // write the buffer into the stream
    fw.flush(); // commit changes
    fw.close(); // close the stream
    sc.close(); // close the scanner
}catch(IOException e){
    e.printStackTrace();
}

try{
    FileReader fr = new FileReader(filename); // reading stream
    Scanner sc = new Scanner(fr);   // scanner to read from file
    while(sc.hasNextLine()) // read while next line exists
        System.out.println(sc.nextLine()); // prints a line and go to the next
    sc.close(); // close the scanner
    fr.close(); // close the reading stream
}catch(IOException e){
    e.printStackTrace();
}

String filename = "random_name.txt";
File myfile = new File(filename); // file to test
System.out.println(filename + " exists : " + test.exists()); 

File current = new File(".");
File[] files_array = file.listFiles(); // returns an array of files
Arrays.stream(files_array).forEach(System.out::println); // prints the array

It is possible to know wether a file is a directory or not using the method isDirectory().

• Programmatically, If dest file already exists it will replace its content.

try{
    Scanner read = new Scanner(new FileReader(src)); // reader
    FileWriter fw = new FileWriter(dest); // ========== writer

    while(read.hasNextLine()) // ============ while scanner has a next line
        fw.append(read.nextLine() + "\n"); // write every lines
    fw.close(); read.close(); // ============ close ressources (closes also the FileReader explicitly)
}catch(IOException e){e.printStackTrace();}

• Using java.nio.file.Files, If dest file already exists it will throw an exception.

try{
    Files.copy(new File(src).toPath(), new File(dest).toPath()); // toPath() returns a path object needed for the copy operation of Files
}catch(IOException e){e.printStackTrace();}