7.8 Ap0080: self-assessment, classes, constructors, and accessor  (Page 6/8)

The discussion for Question 4 explained that if you define any constructor in a new class, you must define all constructors that will ever beneeded for that class. When you define one or more constructors, the default noarg constructor is no longer provided by the system on your behalf.

Question 4 illustrated a simple manifestation of a problem arising from the failure to define a noarg constructor that would be needed later. The reason that it was needed later was that the programmer attempted to explicitlyuse the non-existent noarg constructor to create an instance of the class.

A more subtle problem

The problem in this program is more subtle. Unless you (or the programmer of the superclasses) specifically write code to cause the system to behave otherwise, each time you instantiate an object of a class, the systemautomatically calls the noarg constructor on superclasses of that class up to and includingthe class named Object . If one or more of those superclasses don't have a noarg constructor, unless the author of the subclass constructor has taken this intoaccount, the program will fail to compile.

Calling a non-existing noarg constructor

This program attempts to instantiate an object of a class named Subclass , which extends a class named Superclass . By default, when attempting to instantiate the object, the system will attempt to call a noarg constructor defined in Superclass .

Superclass has no noarg constructor

The Superclass class defines a parameterized constructor that requires a single incoming parameter of type int . However, it does not also define a noarg constructor. Because the parameterized constructor is defined, the default noarg constructor does not exist. As a result, JDK 1.3 produces the following compiler error:

Back to Question 5

Answer 4

A. Compiler Error

Explanation 4

Constructors

Java uses the following kinds of constructors:

• Those that take arguments, often referred to as parameterized constructors , which typically perform initialization on the new object using parameter values.
• Those that don't take arguments, often referred to as default or noarg constructors, which perform default initialization on the new object.
• Those that don't take arguments but perform initialization on the new object in ways that differ from the default initialization.

Constructor definition is optional

You are not required to define a constructor when you define a new class. If you don't define a constructor for your new class, a default constructor will beprovided on your behalf. This constructor requires no argument, and it is typically used in conjunction with the new operator to create an instance of theclass using statements such as the following:

The default constructor

The default constructor typically does the following:

• Calls the noarg constructor of the superclass
• Assists in the process of allocating and organizing memory for the new object
• Initializes all instance variables of the new object with the following four default values:
  • numeric = 0,
  • boolean = false,
  • char = all zero bits
  • reference = null