JavaGenericsRTF2 | Mohamed Sofiene Barka | RT2.2

Intro

• Writing a Linked Lists class? what would it hold? Integers, Chars, Strings...?
• Unless you write a Class for each type in Java and hope that programmer doesn't need a custom Linked List for his own made up class, you're Okay! or just use Generics..
• Generics are a Way to Work with IDK-TYPES and use them as Aliases to whatever type the programmer will use
• It were added to Java 5.0 in 2004

How

• when creating : Define an Alea to whatever Type, and use it as THAT TYPE xD
• when use : You will Let the constructor know about the Types used else it won't make any sense LOL
• : you'll use such way, you can use mutiple Aleases : for a Map for example : <KeyType, ValType>

class ClassName <TypeAlea> {
  public ClassName(TypeAlea ParamOfThatType){}
} 
// somewhere else..
ClassName<TypeRef like Integer, String, any Object> x = new ClassName<TypeRef>(params);

Code would make Sense..

• Lets make a Number Class, it can take any kind of numbers (Integers, Real Numbers, Complexe -Let's Suppose we've done one-..)

This Won't work because it expects an Integer, how would I make a Number out of 4.2

class Number {
  private int x;
  public Number(int x) {
    this.x = x;
  }
  public int get() {return x;}
}

class Main {
  public static void main(String[] args) {
    Number x = new Number(4.2);
    System.out.println(x.get());
  }
}

Sike

class Number <T> {
  private T x;
  public Number(T x) {
    this.x = x;
  }
  public T get() {return x;}
}

class Main {
  public static void main(String[] args) {
    Number<Float> x = new Number<Float>(4.2f);
    System.out.println(x.get());
  }
}

even with custom classes, didn't change anything with the Number Class..

class Complex {
  private int x, y;
  public Complex(int x, int y) {
    this.x = x;
    this.y = y;
  }
  public String toString() {
    return x + ( y > 0 ? "+" : "-") + "j" + Math.abs(y); 
  }
}

class Number <T> {
  private T x;
  public Number(T x) {
    this.x = x;
  }
  public T get() {return x;}
}

class Main {
  public static void main(String[] args) {
    Number<Complex> x = new Number<Complex>( new Complex(1, -7));
    System.out.println(x.get());
  }
}

Generic type should be Reference as It looks, you can't use int, use Integer, same for Float and Double Strings and Objects are references already, so no problem with them.. That's why C++ is Better!

Complex can contain whatever Numbers too.. but implementing that was painful.. nvm

More Coding.. Key-Value Pair ig!? Let's make a Small Redis Clone..

No Refernce when Defining, Allowing All Types!

// K for Keys, V for Values..
class Redis <K, V> {
  private K key;
  private V value;
  public Redis(K key, V value) {
    this.key = key;
    this.value = value;
  }
  public void set(V value) {
    this.value = value;
  }
  public V get(K key) {
    if (this.key == key) return value;
    throw new Error("Undefined");
  }
  public String toString() {
    return key + ": " + value + "\n"; 
  }
}

class Main {
  
  public static void main(String[] args) {
    Redis x = new Redis("id", 15665);
    // didn't know you can do that xD
    x.set("bruh");

    System.out.println(x);
  }
}

Now you should use whatevr you specified when Creating the Object

class Redis <K, V> {
  private K key;
  private V value;
  public Redis(K key, V value) {
    this.key = key;
    this.value = value;
  }
  public void set(V value) {
    this.value = value;
  }
  public V get(K key) {
    if (this.key == key) return value;
    throw new Error("Undefined");
  }
  public String toString() {
    return key + ": " + value + "\n"; 
  }
}

class Main {
  
  public static void main(String[] args) {
    Redis <String, Integer> x = new Redis <String, Integer>("id", 15665);
    // It doesnt Work now
    // x.set("bruh");

    x.set(11447);
    System.out.println(x);
  }
}