Exercice1.java

/**
 * @author Lukasz Piliszczuk <lukasz.pili AT gmail.com>
 */
public class Main {

    public static void main(String... args) {

        Character character1 = new Character();
        character1.hp = 100;

        Character character2 = character1;
        character2.hp = 300;

        System.out.println(character1.hp); // 300
        System.out.println(character2.hp); // 300
    }
}

class Character {
    int hp;
}

Exercice2.java

/**
 * @author Lukasz Piliszczuk <lukasz.pili AT gmail.com>
 */
public class Main {

    public static void main(String... args) {

        Character character1 = new Character();
        character1.hp = 100;
        setTo300(character1);

        System.out.println(character1.hp); // 300
    }
    
    public static void setTo300(Character character) {
        character.hp = 300;
    }
}

class Character {
    int hp;
}

Exercice3.java

/**
 * @author Lukasz Piliszczuk <lukasz.pili AT gmail.com>
 */
public class Main {

    public static void main(String... args) {

        Character character1 = new Character();
        character1.hp = 100;
        setTo300(character1);

        System.out.println(character1.hp);
    }
    
    public static void setTo300(Character character) {
        character = new Character();
        character.hp = 300;
    }
}

class Character {
    int hp;
}

exercice4.java

/**
 * @author Lukasz Piliszczuk <lukasz.pili AT gmail.com>
 */
public class Main {

    public static void main(String... args) {

        int i = 1000, j = 1000;
        System.out.println(i == j); // true

        Integer v = 1000;
        Integer w = 1000;

        System.out.println(v == w); // false
        System.out.println(v.equals(w)); // true

        v = 500; // new Integer(500)
        w = 500; // new Integer(500)

        System.out.println(v == w); // false

        v = 127; // new Integer(100)
        w = 127; // w = v

        System.out.println(v == w); // true

        System.out.println(v.equals(w));

    }
}