Le Resume Ultime
de Java

🧸 ELI5

A window is like a school desk. Buttons, labels, text fields, and tables are little objects you place on the desk. A layout manager is the teacher saying: "you sit north, you sit center, you sit in rows."

📌 Exam rules

• AWT: Frame, Panel, Button, TextArea, FileDialog.
• Swing: JFrame, JButton, JTable, JScrollPane.
• Layouts: FlowLayout flows like words, BorderLayout has North/South/East/West/Center, GridLayout makes boxes.
• Events: a listener waits for the click and runs code.

📁 File

new File("notes.txt") does not read the file. It creates a Java object that says "maybe there is a thing called notes.txt here."

• exists(): is it really there?
• length(): how big?
• delete(): remove it.
• renameTo(): change its name/path.

🚪 Streams

A stream is a tiny door where bytes or characters pass one by one.

• FileInputStream: read bytes.
• FileReader: read characters.
• BufferedReader: read more comfortably.
• PrintWriter: write formatted text.

🔥 Exceptions

Files are messy. They can be missing, locked, forbidden, or broken. Java forces you to handle checked exceptions.

• FileNotFoundException: cannot find/open.
• IOException: read/write problem.
• finally: old way to close.

🧬 Inheritance

If class Dog extends Animal, a Dog can reuse Animal behavior. That is an IS-A relationship.

• override: child replaces parent method behavior.
• overload: same method name, different parameter list.
• final method: cannot be overridden.
• static method: hidden, not truly overridden.

⚙️ Blocks order

Static blocks run when the class wakes up. Instance blocks run every time you build a new object, before the constructor finishes.

🧹 ELI5

You built a toy robot. If nobody has its remote anymore, the robot is lost. Java may clean it from memory later.

Robot r = new Robot();
r = null;       // no remote points to the robot now
// robot is eligible for GC

⚠️ About finalize()

The SCJP PDF mentions finalize() as code that may run before deletion. That is legacy knowledge.

• You cannot know when GC will run.
• You cannot trust finalize() for closing files or sockets.
• Modern Java deprecated finalization for removal; see OpenJDK JEP 421.

🪨 Immutable String

Imagine a word carved on a stone. You cannot erase it. If you want a new word, Java gives you a new stone.

String x = "Java";
x.concat(" Rules!");
System.out.println(x);  // Java

x = x.concat(" Rules!");
System.out.println(x);  // Java Rules!

🧵 Builder vs Buffer

When you change text many times, use a mutable builder.

🚨 Use for internal promises

Good assertion: after sorting, the list should be ordered. Bad assertion: checking if the user typed a password. User mistakes need real exceptions or validation, not assertions.

int age = readAgeFromTrustedTestData();
assert age >= 0 : "age cannot be negative";

▶️ Enable / disable

Assertions are disabled by default, exactly as the SCJP PDF says.

java -ea com.insat.TestClass   // enable assertions
java -da com.insat.TestClass   // disable assertions

📸 Save object

The class must implement Serializable. Then ObjectOutputStream can write the object.

class Employee implements Serializable {
    String name;
    transient String secret; // not saved
}

ObjectOutputStream out = null;
try {
    out = new ObjectOutputStream(new FileOutputStream("employees.ser"));
    out.writeObject(new Employee());
} catch (IOException e) {
    System.out.println(e);
} finally {
    try {
        if (out != null) out.close();
    } catch (IOException e) {
        System.out.println(e);
    }
}

📦 Restore object

ObjectInputStream reads the saved bytes and returns an object. You cast it back to the real type.

ObjectInputStream in = null;
try {
    in = new ObjectInputStream(new FileInputStream("employees.ser"));
    Employee e = (Employee) in.readObject();
} catch (IOException e) {
    System.out.println(e);
} catch (ClassNotFoundException e) {
    System.out.println(e);
} finally {
    try {
        if (in != null) in.close();
    } catch (IOException e) {
        System.out.println(e);
    }
}

🔍 Regex search

A regex is a tiny search language. It can find pieces that match a pattern.

Pattern p = Pattern.compile("ab");
Matcher m = p.matcher("abaaaba");

while (m.find()) {
    System.out.print(m.start() + " ");
}
// output: 0 4

✂️ Tokenizing

If input is "Ali;20;INSAT", tokens are Ali, 20, and INSAT.

String line = "Ali;20;INSAT";
String[] tokens = line.split(";");
System.out.println(tokens[0]); // Ali

🛍️ The big families

🧠 Course answer

Collection is the root interface for groups like List and Set. Collections is a utility class with static methods like sort(), reverse(), and synchronizedList().

Collection c = new ArrayList(); // broad, old style
List names = new ArrayList();   // ordered bag
Set ids = new HashSet();        // unique bag
Map byId = new HashMap();       // key -> value bag

🏃 Start vs run

This is non-negotiable: call start() to create a new worker. Calling run() is just a normal method call on the same worker.

Thread t = new Thread(new Runnable() {
    public void run() {
        System.out.println("work");
    }
});
t.start(); // correct: new thread
t.run();   // not parallel: just calls the method

🚦 Shared data danger

If two workers touch the same toy box at the same time, they can make a mess. That is a race condition. Use synchronization or better design.

• synchronized protects one critical section.
• StringBuffer is synchronized; StringBuilder is not.
• Many threads are useful, but too many threads can slow everything.

📞 Client steps

1. Create Socket(host, port).
2. Open output stream to send.
3. Open input stream to receive.
4. Close the socket when finished.

🏢 Server steps

1. Create ServerSocket(port).
2. Call accept() to wait for a client.
3. Create streams from the accepted socket.
4. Often start a new thread per client.

📌 Model classes for 2024 notes

Use this when the question says Notes.obj, DS, Examen, and Moyennes.obj.

class StudentNote implements Serializable {
    int id;
    String nom;
    String prenom;
    double ds;
    double examen;

    StudentNote(int id, String nom, String prenom, double ds, double examen) {
        this.id = id;
        this.nom = nom;
        this.prenom = prenom;
        this.ds = ds;
        this.examen = examen;
    }

    double moyenne() {
        return ds * 0.3 + examen * 0.7;
    }
}

class StudentAverage implements Serializable {
    int id;
    String nom;
    String prenom;
    double moyenne;

    StudentAverage(StudentNote n) {
        id = n.id;
        nom = n.nom;
        prenom = n.prenom;
        moyenne = n.moyenne();
    }
}

🧑‍💼 Model class for employee exams

2017 and 2018 both ask employee files and sorting/filtering. Equality by function code is strange, but the 2018 exam explicitly asks it.

class Employee implements Serializable {
    int id;
    String nom;
    String prenom;
    int codeFonction;
    String fonction;
    int salaire;

    public boolean equals(Object o) {
        if (!(o instanceof Employee)) return false;
        Employee e = (Employee) o;
        return codeFonction == e.codeFonction;
    }

    public int hashCode() {
        return codeFonction;
    }
}

AddToNotes.java pattern

List<StudentNote> notes = loadNotes("Notes.obj");
BufferedReader br = new BufferedReader(new FileReader("SomeNotes.txt"));
String line;
while ((line = br.readLine()) != null) {
    String[] p = line.split(":");
    notes.add(new StudentNote(
        Integer.parseInt(p[0].trim()),
        p[1].trim(),
        p[2].trim(),
        Double.parseDouble(p[3].trim()),
        Double.parseDouble(p[4].trim())
    ));
}
br.close();
saveNotes("Notes.obj", notes);

Moyennes + CatMoyennes pattern

List<StudentNote> notes = loadNotes("Notes.obj");
List<StudentAverage> avgs = new ArrayList<StudentAverage>();

for (int i = 0; i < notes.size(); i++) {
    avgs.add(new StudentAverage(notes.get(i)));
}

Collections.sort(avgs, new Comparator<StudentAverage>() {
    public int compare(StudentAverage a, StudentAverage b) {
        return Double.compare(b.moyenne, a.moyenne);
    }
});

// then write avgs to Moyennes.obj or print them