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Overloading is when
the same method or operator can be used on many different types of data.
For instance the + sign is used to add ints as well as concatenate
strings. The plus sign behaves differently depending on the type of its
arguments. Therefore the plus sign is inherently overloaded.
Methods can be overloaded as well. System.out.println()
can print a double, a float, an int,
a long, or a String. You don't do anything
different depending on the type of number you want the value of.
Overloading takes care of it.
Programmer-defined classes can overload methods as well.
To do this simply write two methods with the same name but different
argument lists. For instance last week you saw several different versions
of the Car constructor, one that took three arguments and one
that took two arguments, and one that took no arguments. You can use all
of these in a single class, though here I only use two because there
really aren't any good default values for licensePlate and maxSpeed.
On the other hand, 0 is a perfectly reasonable default value for speed.
public class Car {
private String licensePlate; // e.g. "New York A456 324"
private double speed; // kilometers per hour
private double maxSpeed; // kilometers per hour
// constructors
public Car(String licensePlate, double maxSpeed) {
this.licensePlate = licensePlate;
this.speed = 0.0;
if (maxSpeed >= 0.0) {
this.maxSpeed = maxSpeed;
}
else {
maxSpeed = 0.0;
}
}
public Car(String licensePlate, double speed, double maxSpeed) {
this.licensePlate = licensePlate;
if (maxSpeed >= 0.0) {
this.maxSpeed = maxSpeed;
}
else {
maxSpeed = 0.0;
}
if (speed < 0.0) {
speed = 0.0;
}
if (speed <= maxSpeed) {
this.speed = speed;
}
else {
this.speed = maxSpeed;
}
}
// other methods...
}
Normally a single identifier refers to exactly one method
or constructor. When as above, one identifier refers to more than one
method or constructor, the method is said to be overloaded. You
could argue that this should be called identifier overloading rather than
method overloading since it's the identifier that refers to more than one
method, not the method that refers to more than one identifier. However in
common usage this is called method overloading.
Which method an identifier refers to depends on the
signature. The signature is the number, type, and order of the arguments
passed to a method. The signature of the first constructor in the above
program is Car(String, double). The signature of the second
method is Car(String, double, double). Thus the first version
of the Car() constructor is called when there is one String
argument followed by one double argument and the second version is used
when there is one String argument followed by two double arguments.
If there are no arguments to the constructor, or two or
three arguments that aren't the right type in the right order, then the
compiler generates an error because it doesn't have a method whose
signature matches the requested method call. For example
Error: Method Car(double) not found in class Car.
Car.java line 17
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