Comparisons

public class Main {
    public static void main(String[] args) {
        int a = 50;
        int b = a;
        b = 30;
        System.out.println(a + " " + b); // 50 30

        TestClass t = new TestClass(); // there isn't any value in x - there is an address
        TestClass t2 = t;
        t2.x = 20;
        System.out.println(t.x + " " + t2.x); // 20 20

        String y = "John";
        String z = y; // the same as "String z = new String(y)", so the value of y won't change
        z = "George";
        System.out.println(y + " " + z); // John George

        // Comparing strings
        String s1 = "John";
        String s2 = "John";
        if (Objects.equals(s1, s2)) // while == compares memory addresses, equals() compares content
            System.out.println("Equal");
        // s1.equals(s2) would work the same, however if s1 is null, it would throw a NullPointerException (an error)
    }
}

class TestClass {
    int x = 10;
}
                                    

Object class

With the Object class, we can create instances of different classes (polymorphism), but to use the methods of these classes, we have to use downcasting (as shown in the example below). All classes in Java inherit from the Object class directly or indirectly.

public class Main {
    public static void main(String[] args) {
        Object x = new Coordinates(2, 3);
        System.out.println(((Coordinates)x).getX());
        System.out.println(x.getClass()); // getting the name of the class the object belongs to
    }
}

class Coordinates {
    private int x;
    private int y;

    Coordinates(int x, int y) {
        this.x = x;
        this.y = y;
    }

    int getX() {return x;}
    int getY() {return y;}
}