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Class Constructors
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The new operator creates an instance of a class as in, for example, ... The statement declares a variable of the Test type and creates an instance of the class with the new operator. The argument of new must correspond to a special method in the class called a constructor. The constructor signature looks similar to that of a regular method. It has a name that matches the class name and holds a list of arguments in parenthesis. However, a constructor has no return type. Instead, an instance of the class type itself returns. Constructors are useful for initializing values and invoking any methods needed for initialization. The code here shows a simple class with a constructor and one method.
The above code for the constructor , shows that in the process of creating an instance of the class, an inital value for the instance variable is passed as an argument in the constructor. Java does not actually require an explicit constructor in the class description. If you do not include a constructor, the Java compiler will create a default constructor in the byte code with an empty argument. This default constructor is equivalent to the explicit Test(){}
In the discussion of data fields, we noted that the data can receive explicit initial values or default values. You might wonder when does this initialization actually occur? The java compiler, in fact, puts the initialization of the data into the byte code for the constructor. So, for instance, the above default constructor is equivalent to the one shown below where a constructor explicitly sets the the int variable to 0.
Similarly, for a class with several types of data, the initialization of the default or explicit values occurs in the constructor:
You can define multiple constructors to provide optional ways to create and initialize instances of the class. (We will futher discuss overloading of constructors and methods later in this chapter.)
Note that if a class includes one or more explicit
constructors, the java
compiler does not create the default constructor. So for
the class shown above, the code will generate a compiler error stating that no such constructor exists. Latest update: Oct. 17, 2004 |
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