Chapter 4  Void methods

So far we’ve only written short programs that have a single class and a single method (main). In this chapter, we’ll show you how to organize longer programs into multiple methods and classes. We’ll also present the Math class, which provides methods for common mathematical operations.

4.1  Math methods

In mathematics, you have probably seen functions like sin and log, and you have learned to evaluate expressions like sin(π/2) and log(1/x). First, you evaluate the expression in parentheses, which is called the argument of the function. Then you can evaluate the function itself, maybe by punching it into a calculator.

This process can be applied repeatedly to evaluate more complex expressions like log(1/sin(π/2)). First we evaluate the argument of the innermost function, then evaluate the function itself, and so on.

The Java library includes a Math class that provides common mathematical operations. Math is in the java.lang package, so you don’t have to import it. You can use, or invoke, Math methods like this:

double root = Math.sqrt(17.0); double angle = 1.5; double height = Math.sin(angle);

The first line sets root to the square root of 17. The third line finds the sine of 1.5 (the value of angle).

Arguments of the trigonometric functions – sin, cos, and tan – should be in radians. To convert from degrees to radians, you can divide by 180 and multiply by π. Conveniently, the Math class provides a constant double named PI that contains an approximation of π:

double degrees = 90; double angle = degrees / 180.0 * Math.PI;

Notice that PI is in capital letters. Java does not recognize Pi, pi, or pie. Also, PI is the name of a variable, not a method, so it doesn’t have parentheses. The same is true for the constant Math.E, which approximates Euler’s number.

Converting to and from radians is a common operation, so the Math class provides methods that do it for you.

double radians = Math.toRadians(180.0); double degrees = Math.toDegrees(Math.PI);

Another useful method is round, which rounds a floating-point value to the nearest integer and returns a long. A long is like an int, but bigger. More specifically, an int uses 32 bits; the largest value it can hold is 231−1, which is about 2 billion. A long uses 64 bits, so the largest value is 263−1, which is about 9 quintillion.

long x = Math.round(Math.PI * 20.0);

The result is 63 (rounded up from 62.8319).

Take a minute to read the documentation for these and other methods in the Math class. The easiest way to find documentation for Java classes is to do a web search for “Java” and the name of the class.

4.2  Composition revisited

Just as with mathematical functions, Java methods can be composed. That means you can use one expression as part of another. For example, you can use any expression as an argument to a method:

double x = Math.cos(angle + Math.PI / 2.0);

This statement divides Math.PI by two, adds the result to angle, and computes the cosine of the sum. You can also take the result of one method and pass it as an argument to another:

double x = Math.exp(Math.log(10.0));

In Java, the log method always uses base e. So this statement finds the log base e of 10, and then raises e to that power. The result gets assigned to x.

Some math methods take more than one argument. For example, Math.pow takes two arguments and raises the first to the power of the second. This line of code assigns the value 1024.0 to the variable x:

double x = Math.pow(2.0, 10.0);

When using Math methods, it is a common error to forget the Math. For example, if you try to invoke pow(2.0, 10.0), you get an error message like:

File: Test.java [line: 5] Error: cannot find symbol symbol: method pow(double,double) location: class Test

The message “cannot find symbol” is confusing, but the last line provides a useful hint. The compiler is looking for pow in the same class where it is used, which is Test. If you don’t specify a class name, the compiler looks in the current class.

4.3  Adding new methods

You have probably guessed by now that you can define more than one method in a class. Here’s an example:

The name of the class is NewLine. By convention, class names begin with a capital letter. NewLine contains two methods, newLine and main. Remember that Java is case-sensitive, so NewLine and newLine are not the same.

Method names should begin with a lowercase letter and use “camel case”, which is a cute name for jammingWordsTogetherLikeThis. You can use any name you want for methods, except main or any of the Java keywords.

newLine and main are public, which means they can be invoked from other classes. They are both static, but we can’t explain what that means yet. And they are both void, which means that they don’t yield a result (unlike the Math methods, for example).

The parentheses after the method name contain a list of variables, called parameters, where the method stores its arguments. main has a single parameter, called args, which has type String[]. That means that whoever invokes main must provide an array of strings (we’ll get to arrays in a later chapter).

Since newLine has no parameters, it requires no arguments, as shown when it is invoked in main. And because newLine is in the same class as main, we don’t have to specify the class name.

The output of this program is:

First line. Second line.

Notice the extra space between the lines. If we wanted more space between them, we could invoke the same method repeatedly:

public static void main(String[] args) { System.out.println("First line."); newLine(); newLine(); newLine(); System.out.println("Second line."); }

Or we could write a new method that displays three blank lines:

public static void threeLine() { newLine(); newLine(); newLine(); } public static void main(String[] args) { System.out.println("First line."); threeLine(); System.out.println("Second line."); }

You can invoke the same method more than once, and you can have one method invoke another. In this example, main invokes threeLine, and threeLine invokes newLine.

Beginners often wonder why it is worth the trouble to create new methods. There are many reasons, but this example demonstrates a few of them:

  • Creating a new method gives you an opportunity to give a name to a group of statements, which makes code easier to read and understand.
  • Introducing new methods can make a program smaller by eliminating repetitive code. For example, to display nine consecutive new lines, you could invoke threeLine three times.
  • A common problem solving technique is to break tasks down into sub-problems. Methods allow you to focus on each sub-problem in isolation, and then compose them into a complete solution.

4.4  Flow of execution

Pulling together the code from the previous section, the complete program looks like this:

When you look at a class definition that contains several methods, it is tempting to read it from top to bottom. But that is likely to be confusing, because that is not the flow of execution of the program.

Execution always begins at the first statement of main, regardless of where it is in the source file. Statements are executed one at a time, in order, until you reach a method invocation, which you can think of as a detour. Instead of going to the next statement, you jump to the first line of the invoked method, execute all the statements there, and then come back and pick up exactly where you left off.

That sounds simple enough, but remember that one method can invoke another one. In the middle of main, we go off to execute the statements in threeLine. While we are executing threeLine, we go off to execute newLine. Then newLine invokes println, which causes yet another detour.

Fortunately, Java is good at keeping track of which methods are running. So when println completes, it picks up where it left off in newLine; when newLine completes, it goes back to threeLine, and when threeLine completes, it gets back to main.

In summary, when you read a program, don’t read from top to bottom. Instead, follow the flow of execution.

4.5  Parameters and arguments

Some of the methods we have used require arguments, which are the values you provide when you invoke the method. For example, to find the sine of a number, you have to provide the number, so sin takes a double as an argument. To display a message, you have to provide the message, so println takes a String.

When you use a method, you provide the arguments. When you write a method, you name the parameters. The parameter list indicates what arguments are required. The following class shows an example:

printTwice has a parameter named s with type String. When we invoke printTwice, we have to provide an argument with type String.

Before the method executes, the argument gets assigned to the parameter. In this example, the argument "Don't make me say this twice!" gets assigned to the parameter s.

This process is called parameter passing because the value gets passed from outside the method to the inside. An argument can be any kind of expression, so if you have a String variable, you can use it as an argument:

String argument = "Never say never."; printTwice(argument);

The value you provide as an argument must have the same type as the parameter. For example, if you try:

printTwice(17); // syntax error

You will get an error message like this:

File: Test.java [line: 10] Error: method printTwice in class Test cannot be applied to given types; required: java.lang.String found: int reason: actual argument int cannot be converted to java.lang.String by method invocation conversion

Sometimes Java can convert an argument from one type to another automatically. For example, Math.sqrt requires a double, but if you invoke Math.sqrt(25), the integer value 25 is automatically converted to the floating-point value 25.0. But in the case of printTwice, Java can’t (or won’t) convert the integer 17 to a String.

Parameters and other variables only exist inside their own methods. Inside main, there is no such thing as s. If you try to use it there, you’ll get a compiler error. Similarly, inside printTwice there is no such thing as argument. That variable belongs to main.

Because variables only exist inside the methods where they are defined, they are often called local variables.

4.6  Multiple parameters

Here is an example of a method that takes two parameters:

public static void printTime(int hour, int minute) { System.out.print(hour); System.out.print(":"); System.out.println(minute); }

In the parameter list, it may be tempting to write:

public static void printTime(int hour, minute) { ...

But that format (without the second int) is only legal for variable declarations. In parameter lists, you need to specify the type of each variable separately.

To invoke this method, we have to provide two integers as arguments:

int hour = 11; int minute = 59; printTime(hour, minute);

A common error is to declare the types of the arguments, like this:

int hour = 11; int minute = 59; printTime(int hour, int minute); // syntax error

That’s a syntax error; the compiler sees int hour and int minute as variable declarations, not expressions. You wouldn’t declare the types of the arguments if they were simply integers:

printTime(int 11, int 59); // syntax error

4.7  Stack diagrams

Pulling together the code fragments from the previous section, here is a complete class definition:

printTime has two parameters, named hour and minute. And main has two variables, also named hour and minute. Although they have the same names, these variables are not the same. hour in printTime and hour in main refer to different storage locations, and they can have different values.

For example, you could invoke printTime like this:

int hour = 11; int minute = 59; printTime(hour + 1, 0);

Before the method is invoked, Java evaluates the arguments; in this example, the results are 12 and 0. Then it assigns those values to the parameters. Inside printTime, the value of hour is 12, not 11, and the value of minute is 0, not 59. Furthermore, if printTime modifies one of its parameters, that change has no effect on the variables in main.

One way to keep track of everything is to draw a stack diagram, which is a state diagram (see Section 2.3) that shows method invocations. For each method there is a box called a frame that contains the method’s parameters and variables. The name of the method appears outside the frame; the variables and parameters appear inside.

Figure 4.1: Stack diagram for PrintTime.