The Object Class

All classes have a parent class (have a super class) except one. The class at the top of the Java class hierarchy is called Object. If a class definition does not extend a parent class then it automatically has Object as a parent class.

If a child class extends a parent class, then the parent class either extends its parent or automatically extends Object. Ultimately all classes have Object as an ancestor.

This means that all classes in Java share some common characteristics. Those characteristics are defined in Object . For example, all classes have a toString() method because the class Object defines that method so all classes get it by inheritance. Of course, usually when you write a class you override the toString() method.

Constructors for our classes have not mentioned Object . According to the rule, the compiler automatically does this:

  // constructor
  public Video( String ttl, int lngth )
  {
    super();     // use the super class's constuctor
    title = ttl; length = lngth; avail = true; 
  }

This is correct. Video automatically has Object as a parent class. The compiler automatically does this:

class Video extends Object
{
. . .
}