Invoking methods with a return value

If it makes sense for a method to accept parameters (input), there are times when it needs to return a value to the code that invoked it (output). Again, here the notion of output is related to the data flow out of a method, not data being displayed to the screen. A method that does not return any value is said to return void, which is a keyword in Java. We’ve seen this keyword in the main method implementation: public static void main(String [] args). In fact, we’ve been actually using methods with return values. Remember the Scanner class, which we used to obtain input from the user. To create a Scanner object, we have the statement:

Scanner input = new Scanner(System.in);

Once we’ve created the Scanner object, we can use various methods to read values of different types. The one method we’ve seen is nextInt, which accepts no arguments, but returns the integer value read from the keyboard. To store the value read, we use the assignment operator as follows:

int num = input.nextInt();

In this case, the nextInt method retrieves the keyboard input and returns it as an integer value, which we store in the variable num.

Another method defined in the Scanner class allows us to read a double value from the keyboard. A Java statement to invoke the method would be:

double x = input.nextDouble();

Just as we need to know the number and types of the parameters for a method, we also need to know the type of the return value in order for us to invoke it properly. That information is available to us in the method header. In essence, we need to look at how we go about creating methods, not just how to invoke them. We’ll start with implementing methods with no parameters and then move to ones with parameters and a return value.

It may at times make sense not to store the value returned from a method. Here’s a code segment that first creates a 50x100 Board object and then invokes the method getCols to obtain the number of columns. The value returned is displayed without storing it in a separate variable.

Board b = new Board(50, 100);

System.out.print(“Number of columns: “ + b.getCols());