MethodsAbout methodsExample methodWhy methods?Defining a methodReturning a result from a methodReturning no result from a methodInstance variablesScope of variablesMethods and static methodsEscaping from staticDocumenting a methodExample Javadoc documentationThe EndJan 14, 2019MethodsAbout 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)Declarationsand statementsparameterparameterparameterreturn valueExample 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 honestAbeWhy 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 placeDefining 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 methodInstance 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 objectScope 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 v alue 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 styleMethods 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 slideEscaping 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 upDocumenting 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 tags:@param 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, etc.@return descriptionA description of the value that is returned (omit if void)@throws 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 {