Objects, Classes, and Interfaces |
When you write your method, you declare the number and type of the arguments required by that method. You declare the type and name for each argument in the method signature. For example, the following is a method that computes the monthly payments for a home loan based on the amount of the loan, the interest rate, the length of the loan (the number of periods), and the future value of the loan (presumably the future value of the loan is zero because at the end of the loan, you've paid it off):This method takes four arguments: the loan amount, the interest rate, the future value and the number of periods. The first three are double-precision floating point numbers, and the fourth is an integer.double computePayment(double loanAmt, double rate, double futureValue, int numPeriods) { double I, partial1, denominator, answer; I = rate / 100.0; partial1 = Math.pow((1 + I), (0.0 - numPeriods)); denominator = (1 - partial1) / I; answer = ((-1 * loanAmt) / denominator) - ((futureValue * partial1) / denominator); return answer; }As with this method, the set of arguments to any method is a comma-delimited list of variable declarations where each variable declaration is a type/name pair:
type name
As you can see from the body of the
computePayment()
method, you simply use the argument name to refer to the argument's value.Argument Types
In Java, you can pass an argument of any valid Java data type into a method. This includes primitive data types such as doubles, floats and integers as you saw in thecomputePayment()
method, and reference data types such as objects and arrays. Here's an example of a constructor that accepts an array as an argument. In this example, the constructor initializes a new Polygon object from a list of Points (Point is a class in the java.awt package that represents a x, y coordinate):Unlike some other languages, you cannot pass methods into Java methods. But you could pass an object into a method and then invoke the object's methods.Polygon polygonFrom(Point[] listOfPoints) { . . . }Argument Names
When you declare an argument to a Java method, you provide a name for that argument. This name is used within the method body to refer to the item.A method argument can have the same name as one of the class's member variables. If this is the case, then the argument is said to hide the member variable. Arguments that hide member variables are often used in constructors to initialize a class. For example, take the following Circle class and its constructor:
The Circle class has three member variablesclass Circle { int x, y, radius; public Circle(int x, int y, int radius) { . . . } }x
,y
andradius
. In addition, the constructor for the Circle class accepts three arguments each of which shares its name with the member variable for which the argument provides an initial value.The argument names hide the member variables. So using
x
,y
orradius
within the body of the constructor refers to the argument, not to the member variable. To access the member variable, you must reference it throughthis
--the current object:Names of method arguments cannot be the same as another argument name for the same method, the name of any variable local to the method, or the name of any parameter to aclass Circle { int x, y, radius; public Circle(int x, int y, int radius) { this.x = x; this.y = y; this.radius = radius; } }catch()
clause within the same method.Pass by Value
In Java methods, arguments are passed by value. When invoked, the method receives the value of the variable passed in. When the argument is of primitive type, pass-by-value means that the method cannot change its value. When the argument is of reference type, pass-by-value means that the method cannot change the object reference, but can invoke the object's methods and modify the accessible variables within the object.This is often the source of confusion--a programmer writes a method that attempts to modify the value of one its arguments and the method doesn't work as expected. Let's look at such method and then investigate how to change it so that it does what the programmer originally intended.
Consider this series of Java statements which attempts to retrieve the current color of a Pen object in a graphics application:
At the time when the. . . int r = -1, g = -1, b = -1; pen.getRGBColor(r, g, b); System.out.println("red = " + r + ", green = " + g + ", blue = " + b); . . .getRGBColor()
method is called, the variablesr
,g
, andb
all have the value -1. The caller is expecting thegetRGBColor()
method to pass back the red, green and blue values of the current color in ther
,g
, andb
variables.However, the Java runtime passes the variables' values (
-1
) into thegetRGBColor()
method; not a reference to ther
,g
, andb
variables. So you could visualize the call togetRGBColor()
like this:getRGBColor(-1, -1, -1)
.When control passes into the
getRGBColor()
method, the arguments come into scope (get allocated) and are initialized to the value passed into the method:Soclass Pen { int redValue, greenValue, blueValue; void getRGBColor(int red, int green, int blue) { // red, green, and blue have been created and their values are -1 . . . } }getRGBColor()
gets access to the values ofr
,g
, andb
in the caller through its argumentsred
,green
, andblue
, respectively. The method gets its own copy of the values to use within the scope of the method. Any changes made to those local copies are not reflected in the original variables from the caller.Now, let's look at the implementation of
getRGBColor()
within the Pen class that the method signature above implies:This method will not work as intended. When control gets to theclass Pen { int redValue, greenValue, blueValue; . . . // this method does not work as intended void getRGBColor(int red, int green, int blue) { red = redValue; green = greenValue; blue = blueValue; } }println()
statement in the following code, which was shown previously,getRGBColor()
's arguments,red
,green
, andblue
, no longer exist. Therefore the assignments made to them within the method had no effect;r
,g
, andb
are all still equal to-1
.Passing variables by value affords the programmer some safety: Methods cannot unintentionally modify a variable that is outside of its scope. However, you often want a method to be able to modify one or more of its arguments. The. . . int r = -1, g = -1, b = -1; pen.getRGBColor(r, g, b); System.out.println("red = " + r + ", green = " + g + ", blue = " + b); . . .getRGBColor()
method is a case in point. The caller wants the method to return three values through its arguments. However, the method cannot modify its arguments, and, furthermore, a method can only return one value through its return value. So, how can a method return more than one value, or have an effect (modify some value) outside of its scope?For a method to modify an argument, it must be of a reference type such as an object or array. Objects and arrays are also passed by value, but the value of an object is a reference. So the effect is that arguments of reference types are passed in by reference. Hence the name. A reference to an object is the address of the object in memory. Now, the argument in the method is referring to the same memory location as the caller.
Let's rewrite the
getRGBColor()
method so that it actually does what you want. First, you must introduce a new object, RGBColor, that can hold the red, green and blue values of a color in RGB space:Now, we can rewriteclass RGBColor { public int red, green, blue; }getRGBColor()
so that it accepts an RGBColor object as an argument. ThegetRGBColor()
method returns the current color of the pen by setting thered
,green
andblue
member variables of its RGBColor argument:And finally, let's rewrite the calling sequence:class Pen { int redValue, greenValue, blueValue; void getRGBColor(RGBColor aColor) { aColor.red = redValue; aColor.green = greenValue; aColor.blue = blueValue; } }The modifications made to the RGBColor object within the. . . RGBColor penColor = new RGBColor(); pen.getRGBColor(penColor); System.out.println("red = " + penColor.red + ", green = " + penColor.green + ", blue = " + penColor.blue); . . .getRGBColor()
method affect the object created in the calling sequence because the namespenColor
(in the calling sequence) andaColor
(in thegetRGBColor()
method) refer to the same object.
Objects, Classes, and Interfaces |