26-Jun-15 Methods

About methods A method is a named group of declarations and statements If a method is in the same class, you execute those declarations and statements by calling the method If the method belongs to some other object, you execute it by sending a message to that object When you call the method, you can give it parameters (information, such as numbers, going into the method) A method typically has a return value (a single piece of information coming out of the method) Declarations and statements parameter return value

Example method Inside a CokeMachine class, you might have this method: public void insertCoin(int coin) { // “coin” is a parameter if (coin == 5 || coin == 10 || coin == 25) { amountEntered = amountEntered + coin; } } Inside the CokeMachine class, you might have code like this: int dime = 10; insertCoin(dime); // this is a call to the insertCoin method In some other class, you might have code like this: CokeMachine cheater = new CokeMachine(); CokeMachine honestAbe = new CokeMachine(); int dime = 10; honestAbe.insertCoin(dime); // this is a message to honestAbe

Why methods? Methods help you break up a complex problem into simpler subproblems, which you can solve separately Methods can use other methods, but should not depend on the “inner workings” of those other methods--just what they do, not how they do it Methods allow you to give names to parts of your computation, thus making your program more readable Readability is essential in good programming Proper choice of method names is important Verbs are usually best, since methods “do something” If you have anything the least bit complicated to compute, you should write (and debug) it in only one place This is an application of the DRY principle (“Don’t Repeat Yourself”) Methods are very good for putting code in only one place

Defining a method A method has the syntax: access return-type method-name ( parameters ) { method-variables code } The access ( public, protected, or private ) is optional Example: private boolean isAdult(int age) { int magicAge = 21; return age >= magicAge; } Example: double average(int a, int b) { return (a + b) / 2.0; }

Returning a result from a method If a method is to return a result, it must specify the type of the result: boolean isAdult ( … You must use a return statement to exit the method with a result of the correct type: return age >= magicAge ;

Returning no result from a method The keyword void is used to indicate that a method doesn’t return a value The return statement must not specify a value Example: void printAge(String name, int age) { System.out.println(name + " is " + age + " years old."); return; }  There are two ways to return from a void method:  Execute a return statement  Reach the closing brace of the method

Instance variables A variable declared within a class (but not within a method) is an instance variable Every object (“instance”) of that class has its own variable with that name Example: Every CokeMachine has its own amountEntered Instance variables are “visible”--available for use--everywhere within the class in which they are defined Instance variables should almost always be marked private, so that other classes cannot “see” them Example: You don’t want a Customer object to be able to change the amountEntered in a CokeMachine without inserting coins Every instance variable should say something meaningful about its object

Scope of variables Parameters to a method are visible throughout the method Parameters are discarded when the method returns Variables declared within a block (between { and } ) are visible from where they are declared up to the end of the block These variables are discarded when the block is exited Example: public double average(double x, double y, double z) { // x, y, and z are visible throughout this method double count = 3.0; // count is visible from here forward double avg = (x + y + z) / count; // avg is visible from here forward return avg; // the value of avg is returned, but the variable itself, // and the variable count, are discarded } Note that count and avg could be instance variables, but since they don’t say anything about the object they are in, this would be very poor style

Methods and static methods Java has two kinds of methods: static methods and non-static methods (also know as instance methods) However, before we can talk about what it means to be static, we have to learn a lot more about classes and objects Most methods you write should not, and will not be static Every Java program has a public static void main(String[ ] args) method This starts us in a “static context” To “escape from static”, I recommend starting every program in a certain way, as shown on the next slide

Escaping from static class MyClass { public static void main(String[] args) { new MyClass().run(); } void run() { // Your real code begins here } } You can replace the names MyClass and run with names of your choice, but notice that each name occurs in two places, and they have to match up

Documenting a method Javadoc comments start with /** and end with */, and are put immediately before the method The text is written as if the method description began with the words “This method...” The comment ends with the following name description (one for each parameter) The description tells what information the named parameter provides You don’t need to mention the parameter type—Javadoc knows that If any instance variables are used, their use should be described References to the object should be with “this”– this Tree, this JPanel, description A description of the value that is returned (omit if void exception cause (one for each Exception) Tells what condition will cause this Exception to be thrown HTML markup may be used within the Javadoc comment To generate the actual documentation from the comments (in Eclipse), do: Project  Generate Javadoc...

Example Javadoc documentation public class Pair { private static final long serialVersionUID = 1L; private Student student1; private Student student2;... /** * Returns one of the two students in this pair. The order of students * is irrelevant and should not be depended upon. * which 1 or 2. One of the students in this pair. */ public Student getStudent(int which) { if (which == 1) return student1; else if (which == 2) return student2; else error("There is no student " + which); return null; }... }

The End Though this be madness, yet there is method in it. --Shakespeare, Hamlet