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Think Java by Chris Mayfield, Allen B. Downey

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Chapter 6. Value Methods

Some of the methods we have used, like the Math methods, return values. But all the methods we have written so far have been void; that is, they don’t return values. In this chapter, we’ll write methods that return values, which we call value methods.

Return Values

When you invoke a void method, the invocation is usually on a line all by itself. For example, here is the countup method from “Recursive Methods”:

public static void countup(int n) {
    if (n == 0) {
        System.out.println("Blastoff!");
    } else {
        countup(n - 1);
        System.out.println(n);
    }
}

And here is how it is invoked:

countup(3);
System.out.println("Have a nice day.");

On the other hand, when you invoke a value method, you have to do something with the return value. We usually assign it to a variable or use it as part of an expression, like this:

double error = Math.abs(expected - actual);
double height = radius * Math.sin(angle);

Compared to void methods, value methods differ in two ways:

  • They declare the type of the return value (the return type);

  • They use at least one return statement to provide a return value.

Here’s an example: calculateArea takes a double as a parameter and returns the area of a circle with that radius:

public static double calculateArea(double radius) {
    double result = Math.PI * radius * radius;
    return result;
}

As usual, this method is public and static. But in the place where we are used to seeing void, we see double, which means that the return value from this method is a double ...

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