Object Class in Java

Every class you write has a parent, even when you don't write extends. If no parent is named, the compiler makes the class extend java.lang.Object. So Object sits at the top of every class hierarchy, and every object in Java, whether it's a String, an array or your own Student, inherits its methods. Knowing those few methods well is the foundation for this whole module.

The Root of Every Class

public class Root {
    static class Student {
        String name;
        Student(String name) { this.name = name; }
    }
 
    static void describe(Object o) {
        System.out.println(o.getClass().getSimpleName() + ": " + o);
    }
 
    public static void main(String[] args) {
        Student s = new Student("Navika");
        System.out.println(s.getClass().getSuperclass().getName());
 
        Object any = s;
        System.out.println(any.equals(s));
        System.out.println(any.equals(new Student("Navika")));
 
        describe("Srihitha");
        describe(42);
        describe(3.5);
    }
}

java.lang.Object
true
false
String: Srihitha
Integer: 42
Double: 3.5

How it works

1. Student has no extends, yet its superclass is java.lang.Object.
2. An Object variable can hold any object: Object any = s;.
3. equals is inherited from Object. Its default compares references, so a second Student with the same name isn't "equal". The next lessons fix that.
4. describe(Object o) accepts a String, an Integer (42 is autoboxed) and a Double: one method for every type.

Object's Methods

That's 11 methods in all (counting the three wait overloads). wait and notify are covered with multithreading later in the course; the rest each get a lesson in this chapter.

Common Mistakes

Thinking equals compares contents by default. It compares references until you override it.

Overriding equals without hashCode. Hash-based collections then break (see "Object Equality and Hash Codes in Java").

Using Object parameters everywhere. Generics (covered later in the course) give type safety; Object loses it.

Interview Corner

Practice

1. Easy: Print getClass().getName() and getClass().getSuperclass().getName() for a String, an Integer and an int[].
2. Harder: Write static void printAll(Object... items) that prints each item's simple class name and value, and call it with five different kinds of values.

Key Takeaways