Default and No-Argument Constructors

Every class must have at least one constructor. As you learned in Section 3.7, if you do not provide any constructors in a class's declaration, the compiler creates a default constructor that takes no arguments when it is invoked. The default constructor initializes the instance variables to the initial values specified in their declarations or to their default values (zero for primitive numeric types, false for boolean values and null for references). In Section 9.4.1, you will learn that the default constructor performs another task in addition to initializing instance variables to their default value.

If your class declares constructors, the compiler will not create a default constructor for your class. In this case, to specify the default initialization for objects of your class, you must declare a no-argument constructoras in lines 1215 of Fig. 8.5. Like a default constructor, a no-argument constructor is invoked with empty parentheses. Note that the Time2 no-argument constructor explicitly initializes a Time2 object by passing to the three-argument constructor 0 for each parameter. Since 0 is the default value for int instance variables, the no-argument constructor in this example could actually be declared with an empty body. In this case, each instance variable would receive its default value when the no-argument constructor is called. If we omit the no-argument constructor, clients of this class would not be able to create a Time2 object with the expression new Time2().

Common Programming Error 8.5

If a class has constructors, but none of the public constructors are no-argument constructors, and a program attempts to call a no-argument constructor to initialize an object of the class, a compilation error occurs. A constructor can be called with no arguments only if the class does not have any constructors (in which case the default constructor is called) or if the class has a public no-argument constructor.

Software Engineering Observation 8.6

Java allows other methods of the class besides its constructors to have the same name as the class and to specify return types. Such methods are not constructors and will not be called when an object of the class is instantiated. Java determines which methods are constructors by locating the methods that have the same name as the class and do not specify a return type.