Methods: Documentation, Reference Parameters, Modularization 2

1. Methods: Documentation, Reference Parameters, Modularization 2 CS0007: Introduction to Computer Programming

2. Review • Methods are… • a collection of statements that perform a specific task. • Why do we have methods? • to break problems in smaller, more manageable, subproblems. • Instead of writing one long method that contains all of the statements necessary to solve a problem, writing several small methods that solve each specific part of the problemis called… • Divide and Conquer • If you use Divide and Conquer to design your programs you are using… • Modular Design

3. Review • A method is only executed when… • called. • An argument is… • data passed to a method during a method call. • The local variables in methods that take values from arguments are called… • parameters • void methods are… • methods that do not return a value. • Value returning methods return a value to where? • where it was called.

4. Documenting a Method • When documenting a method, you should include three things: • A short description of what the method does • The names of the parameters and what they are used for • A description of what the method returns • Example: MethodDoc.java • You will be expected to include this kind of comments for the methods you write • You can use special tags in your documentation comments to produce better documentation in JavaDocs: • @param– parameters • @return – return description • Example: MethodJavaDoc.java

5. Value Parameters publicstaticvoidmethod1() { intx = 23; method2(x); System.out.println(x); } publicstaticvoidmethod2(inty) { y = 11; System.out.println(y); } • What will this display to the screen if method1 is called? • Answer: 11then 23 • The value of x does not change despite the fact that it is passed as an argument to a parameter whose value does change. • This is because all arguments that are primitive types are passed by value (value parameter) • When a variable is passed by value, only a copy of the variables value is passed to the corresponding parameter.

6. Reference Parameters publicstaticvoidmethod1() { DecimalFormatformatter = newDecimalFormat("#.0"); method2(formatter); System.out.println(formatter.format(3.5)); } publicstaticvoidmethod2(DecimalFormatinFormatter) { inFormatter.setMaximumFractionDigits(2); System.out.println(inFormatter.format(3.5)); } • What will this display to the screen if method1 is called? • Answer: 3.50 then 3.50 • The value of formatterchanges because it is used as an argument whose corresponding parameter’s (inFormatter) value is changed. • This is because all arguments that are reference variables are passed by reference (reference parameters). • When an argument is passed by reference, the reference to the object that the argument is pointing to is sent to the corresponding parameter.

7. Reference Parameters publicstaticvoid method1() { String x = "Heim"; method2(x); System.out.println(x); } publicstaticvoidmethod2(String y) { y = "Eric"; System.out.println(y); } • What will this display to the screen if method1 is called? • Answer: Eric then Heim • Wait…isn’t x a reference variable, so shouldn’t it change when y does? • Answer: No, because Strings are immutable in Java. • If an object is immutable, it means it cannot be changed.

8. Command Line Arguments • Remember the main method header? publicstaticvoidmain(String[] args) • Notice it has a string array parameter args…what is that? • Answer: args accepts command-line arguments. • Command line arguments are arguments that can be passed to a program when running it from the command line. • Java Program Here I am • “Here”, “I”, and “Am”become the three elements in the argsarray. • Example: CommandLineArgs.java • Notice, main can take a variable number of arguments. • How? • Answer: It accepts an array of any length! • If you allow a parameter to be an array, you can allow the method to take a variable number of arguments!

9. Modular Design • Again, the goal of modular design is to break a problem down into smaller subproblems. • Then, when you go to program a solution to the problem, you can create a method that solve each subproblem • For instance, say we wanted to create a program that would do the following: • Take in a user’s desired function: • Find the area of a triangle given the lengths of the sides. • Find the area of a rectangle given the length and width. • Find the area of a regular pentagon given the length of a side. • Depending on the input, take in the appropriate input in order to perform the function (coordinates, lengths of sides) • Compute the area. • Display the area. • We can write a program in one long method, but that could be overwhelming and hard to read… • OR we could break it down into modules for us to program one at a time. • GeometryCalculator.java