Object, the Cosmic Superclass

Object: the cosmic superclass

In case we you do not define a superclass in a class header, that class will still extend the Object class. As a result of the fact that subtype/supertype relations are transative, this means every class in Java is a subclass of Object. Therefore, Object is sometimes referred to as the cosmic superclass, since it is the superclass of everything.

A number of methods is defined in the Object class. Some of those are outside the scope of this course, such as clone(), finalize(), getClass(), notify(), notifyAll(), and wait(). That leaves us with three methods that we will cover: toString(), equals(), and hashCode(). Here, we will only cover the first. The other two will be covered next week. Although you have used the equals method in the past, we will cover it in more depth than we did before.

toString() method

The toString() method is used to obtain a String that describes the object, and is automatically called when we print an arbitrary object, like System.out.println(anyObject);. The default behavior defined in the Object class is that the JVM prints the name of the class, the @ symbol and something which resembles (but is not exactly) the identity of the object. When you print objects of your own classes, you typically see something like this: ClassName@23df3521. This is due to the default implementation of toString() method in the Object class. However, since toString() is just a method, it is very useful to override it if we want to print objects of the class we are creating. The System.out.println method, or concatenation to a String with an expression like "My object" + myObj makes sure thetoString() method is called automatically to convert the to a String automatically.

public class EmptyClass { }