3.7 Constructors
The main purpose of constructors is to set the initial state of an object, when the object is created by using the new operator.
The following simplified syntax is the canonical declaration of a constructor:
access_modifier class_name
(
formal_parameter_list
)
throws_clause
// Constructor header
{ // Constructor body
local_variable_declarations
statements
}
Constructor declarations are very much like method declarations. However, the following restrictions on constructors should be noted:
- Modifiers other than an access modifier are not permitted in the constructor header. For more on access modifiers for constructors, see §6.5, p. 345.
- Constructors cannot return a value, and therefore, do not specify a return type, not even void, in the constructor header. But their declaration can use the return statement (without the return value) in the constructor body (§4.13, p. 184).
- The constructor name must be the same as the class name.
Class names and method names exist in different namespaces. Thus there are no name conflicts in Example 3.5, where a method declared at (2) has the same name as the constructor declared at (1). The method Name() at (2) is also breaking the convention of starting a method name with a lowercase character. A method must always specify a return type, whereas a constructor does not. However, using such naming schemes is strongly discouraged.
A constructor that has no parameters, like the one at (1) in Example 3.5, is called a no-argument constructor.
Example 3.5 Namespaces
public class Name {
Name() { // (1) No-argument constructor
System.out.println(“Constructor”);
}
void Name() { // (2) Instance method
System.out.println(“Method”);
}
public static void main(String[] args) {
new Name().Name(); // (3) Constructor call followed by method call
}
}
Output from the program:
Constructor
Method
The Default Constructor
If a class does not specify any constructors, then a default constructor is generated for the class by the compiler. The default constructor is equivalent to the following implementation:
class_name
() { super(); } // No parameters. Calls superclass constructor.
A default constructor is a no-argument constructor. The only action taken by the default constructor is to call the superclass constructor. This ensures that the inherited state of the object is initialized properly (§5.3, p. 209). In addition, all instance variables in the object are set to the default value of their type, barring those that are initialized by an initialization expression in their declaration.
In the following code, the class Light does not specify any constructors:
class Light {
// Fields:
int noOfWatts; // Wattage
boolean indicator; // On or off
String location; // Placement
// No constructors
//…
}
class Greenhouse {
// …
Light oneLight = new Light(); // (1) Call to default constructor
}
In this code, the following default constructor is called when a Light object is created by the object creation expression at (1):
Light() { super(); }
Creating an object using the new operator with the default constructor, as at (1), will initialize the fields of the object to their default values (i.e., the fields noOfWatts, indicator, and location in a Light object will be initialized to 0, false, and null, respectively).
A class can choose to provide its own constructors, rather than relying on the default constructor. In the following example, the class Light provides a no-argument constructor at (1).
class Light {
// …
Light() { // (1) No-argument constructor
noOfWatts = 50;
indicator = true;
location = “X”;
}
//…
}
class Greenhouse {
// …
Light extraLight = new Light(); // (2) Call to no-argument constructor
}
The no-argument constructor ensures that any object created with the object creation expression new Light(), as at (2), will have its fields noOfWatts, indicator, and location initialized to 50, true, and “X”, respectively.
If a class defines any constructor, the default constructor is not generated. If such a class requires a no-argument constructor, it must provide its own implementation, as in the preceding example. In the next example, the class Light does not provide a no-argument constructor, but rather includes a non-zero argument constructor at (1). It is called at (2) when an object of the class Light is created with the new operator. Any attempt to call the default constructor will be flagged as a compile-time error, as shown at (3).
class Light {
// …
// Only non-zero argument constructor:
Light(int noOfWatts, boolean indicator, String location) { // (1)
this.noOfWatts = noOfWatts;
this.indicator = indicator;
this.location = location;
}
//…
}
class Greenhouse {
// …
Light moreLight = new Light(100, true, “Greenhouse”);// (2) OK
Light firstLight = new Light(); // (3) Compile-time error
}