Java Class Declarations

In order to write object-oriented code using Java, you need to be able to declare and use classes. Classes are declared using the following syntax:

class ClassName {
 // Class body
}

This declares a class named ClassName that is a subclass of Object. To declare a class that is a subclass of another class (named SuperClass) you use the extends clause:

class ClassName extends SuperClass {
 // Class body
}

An interface defines methods that are provided by those classes that implement the interface. To specify that a class implements one or more interfaces, you use the implements clause:

Given the following code, which statements are true?

public interface Automobile { String describe(); }
class FourWheeler implements Automobile{
  String name;
  public String describe(){ return " 4 Wheeler " + name; }
}
class TwoWheeler extends FourWheeler{
   String name;
   public String describe(){ return " 2 Wheeler " + name; }
}

Select 3 options:

1. An instance of TwoWheeler is also an instance of FourWheeler.
2. An instance of TwoWheeler is a valid instance of Automobile.
3. The use of inheritance is not justified here because a TwoWheeler is not really a FourWheeler.
4. The code will compile only if name is removed from TwoWheeler.
5. The code will fail to compile.

Answer: a,b,c,
Explanation: The use of inheritance in this code is not justifiable, since conceptually, a TwoWheeler is-not-a FourWheeler. I posted a question in the forum with respect to this question and the admin responded.

class ClassName 
extends SuperClass implements
   Interface1, Interface2 {
// Class body
}

By stating that ClassName implements Interface1 and Interface2 we require ClassName to provide an implementation of all the methods defined in Interface1 and Interface2.
A class may specify the optional modifiers public, abstract, and final. These are placed before the class declaration. A public class may be accessed outside the package in which it is declared. An abstract class is a class that is incomplete in the sense that it may declare one or more abstract methods^[1]. A final class identifies a class that may not be extended by a subclass. A class may not be both final and abstract. The following declares a public and final class:

The following class is a final class that cannot be extended.

public final class ClassName {
 // Class body
}

The class body declares members^{[2] (field variables and methods), constructors, and initializers. Class members may also be inner classes or inner interfaces. Class members and initializers are covered in Module 5.}

[1] Abstract method: A method whose implementation is deferred to a subclass.

[2] Member: An element of a class or interface, such as a field variable, method, or inner class. An element of a package, such as a class, interface, or subpackage.