Objects, Classes, and Interfaces

Passing Information into a Method

Similar to functions in other programming languages, Java methods accept input from the caller through its argments. Arguments provide information to the method from outside the scope of the method.

When you write your method, you determine 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).

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;
}

This method takes four arguments: double loanAmt, double rate, double futureValue, and int numPeriods. The first three are double-precision floating point numbers, and the fourth is an integer. 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 method, you simply use the argument name to refer to its value.

Argument Types

In Java, you can pass an argument of any valid Java data type into a method. This includes simple data types such as doubles, floats and integers as you saw in the computePayment() method above, and complex data types such as objects and arrays. Here's an example of a method that accepts an array as an argument. In this example, the method creates a new Polygon object from a list of x, y points.

Polygon polygonFrom(Point listOfPoints[]) {
    . . .
}

Unlike some other languages, you cannot pass methods into Java methods.

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. As with other names in Java, an argument name must be a legal Java identifier.

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:

class Circle {
    int x, y, radius;
    public Circle(int x, int y, int radius) {
        . . .
    }
}

The Circle class has three member variables x, y and radius. 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. Thus using x, y or radius within the body of the constructor refers to the argument, not to the member variable. To access the member variable, you must reference it through this--the current object:

class Circle { 
    int x, y, radius; 
    public Circle(int x, int y, int radius) {  
        this.x = x;
        this.y = y;
        this.radius = radius;
    } 
}

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 a catch() clause within the same method.

Pass by Value

In Java methods, arguments of simple data types are passed by value. When invoked, the method receives the value of the variable passed in, not a reference to the variable itself. For example, consider this series of Java statements which attempts to retrieve the current color of a Pen object in a graphics application:

. . .
int r = -1, g = -1, b = -1;
pen.getRGBColor(r, g, b);
System.out.println("red = " + r + ", green = " + g + ", blue = " + b);
. . .

At the time when the getRGBColor() method is called, the variables r, g, and b all have the value -1. The caller is expecting the getRGBColor() method to pass back the red, green and blue values of the current color in the r, g, and b variables.

However, the Java runtime passes the variables' values (-1) into the getRGBColor() method; not a reference to the r, g, and b variables. Thus the call to getRGBColor() actually looks like this: getRGBColor(-1, -1, -1).

When control passes into the getRGBColor() method, new, local variables are created with the names of the parameters provided in the method signature and are initialized to the value passed into the method.

class Pen {
    int redValue, greenValue, blueValue;
    void getRGBColor(int red, int green, int blue) {
        . . .
    }
}

So, getRGBColor() gets access to the values of r, g, and b in the caller through its local variables red, green, and blue, respectively. The method gets a new copy of the values to use locally. Any changes made to those local variables 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:

class Pen {
    int redValue, greenValue, blueValue;
    void getRGBColor(int red, int green, int blue) {
	red = redValue;
	green = greenValue;
	blue = blueValue;
    }
}

This method will not work. When control gets to the println() statement in this code snippet, getRGBColor()'s local variables red, green, and blue no longer exist. Therefore the assignments made to those variables had no effect; r, g, and b are all still equal to -1.

. . .
int r = -1, g = -1, b = -1;
pen.getRGBColor(r, g, b);
System.out.println("red = " + r + ", green = " + g + ", blue = " + b);
. . .

Passing variables by value affords the programmer some safety. Methods cannot unintentially 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 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.

To allow a method to modify a argument, you must pass in an object. Objects in Java are also passed by value, however, the value of an object is a reference. So, the effect is that the object is passed in by reference. When passing an argument by reference, the method gets a reference to the object. A reference to an object is the address of the object in memory. Now, the local variable within the method is referring to the same memory location as the variable within the caller.

So, let's rewrite the getRGBColor() method so that it actually works. First, you must introduce a new object, RGBColor, that can hold the red, green and blue values of a color in RGB space.

class RGBColor {
    public int red, green, blue;
}

Now, we can rewrite getRGBColor() so that it accepts an RGBColor object as an argument. The getRGBColor() method returns the current color of the pen by setting the red, green and blue member variables of its RGBColor argument.

class Pen {
    int redValue, greenValue, blueValue;
    void getRGBColor(RGBColor aColor) {
	aColor.red = redValue;
	aColor.green = greenValue;
	aColor.blue = blueValue;
    }
}

And finally, let's rewrite the calling sequence:

. . .
RGBColor penColor = new RGBColor();
pen.getRGBColor(penColor);
System.out.println("red = " + penColor.red + ", green = " + penColor.green + ", blue = " + penColor.blue);
. . .

The modifications made to the RGBColor object within the getRGBColor() method affect the object created in the calling sequence, because the names penColor (in the calling sequence) and aColor (in the getRGBColor() method) refer to the same object.

The data type wrapper classes (Float, Integer, and so on) provided in the java.lang package are particularly useful for returning a single value whose type is one of Java's built-in simple types through a method's arguments.

Objects, Classes, and Interfaces