CIS 234: Java Methods

CIS 234: Java Methods Dr. Ralph D. Westfall April, 2010

What is a Method? • a block of code that does something • has a name and parentheses after name • curly brackets enclose statement(s) in body • usually has header declarations, and oftenhas arguments inside the parentheses public static voidmain (String [] args) { System.out.println("Hi!"); }

Declarations - type of access • modifiers before method name • public – any other class can use method • main method must be public • private – only methods in same class can use it • can leave out the access modifier • defaults to friendly – can be used by other classes in same "package" (a package is somewhat like a directory)

Declarations - static • static means unchanging • all objects created from this class will use this same method • there will not be additional copies of a static class method • if don't use word static, each object created from this class will have its own instance (copy) of method

Declarations – void or [type] • return value type • void means doesn't return any data • if not void, need to identify the type of data the method will return public static long cubeIt(int x) // int arg { // returns a long integer return x * x * x; }

Arguments (args) • the parentheses often enclose "argument(s)" (data) that the method uses, identified by data type public void say(String what) // argument { System.out.println(what); // used here } // from Hello, world! Person class code

Arguments - 2 • Java is "strongly typed," so the data type of every variable must be declared somewhere in a Java class • argument data types are declared within method's parentheses public static double raised(int salary, double pct) { return (100 + pct) / 100.0) * salary; }

Calling a Method • from within its class • method name([arguments, if has any]) ; say("Hi!"); // literal argument • from another object or class • [object].[method name]([arguments]) ; myObject.cube(num); // variable argument myObject.raised(empSalary, 5); //both types Math.sqrt(4); // literal in a class method

Calling a Method - 2 • arguments must be in right order static double raise(double salary, double pct) ...... raise (wages, increase) ; // correct order raise(7, 50000) ; // logical error • argument names don't need to match myObject.raise(pay, increase) ; // declared in method as // (double salary, double pct)

"Signature" • a method's "signature" is a pattern: • number, order, and types of arguments • can have 0 [called with [name]( );] to many • an "overloaded" method has more than one signature • e.g., can be called with different patterns of arguments: (); (int x); (int x, double y); (int x, char y);

Types of Java Programs • applications • class that can run without any other programs (often used with other classes) • class(es) that doesn't run by itself, but is used by another application class • applet • class runs inside a web browser (the browser actually is a program) // [PgDn]

Types of Java Programs - 2 • servlet • runs on a server and makes it possible to generate web pages with database content • programmer can create a Java Server Page (JSP), which then gets compiled into a servlet (Blackboard pages are servlets) • servlets are part of the Java Enterprise Edition (JDK + Java EE bundle)

Main Method • one class in a Java application must have a main( ) method • but a class used by an application doesn't need a main( ) method • main method must be declared as: public static void

Main Method - 2 • main method runs first when application starts • it can call methods in its own class, and/or use methods in other classes • methods it calls in its own class must be static • variables that are not inside any method in the class must also be static

Main Method - 3 • main method has String[ ] argument(s) • means that you can "pass" one or more words as inputs into class when you run it public class MyProgram { public static void main (String[ ] args) [in DOS after class has been compiled] C:\>[path] java MyProgram Le 19 // 2 args

Using Extra Methods in a Class • saves typing (or pasting) • can use a separate method, rather than keep repeating same block of code in main method • makes code easier to read and maintain • smaller programs • code is organized into blocks //Notes Page

Exercise • following pattern in DemoMethods.java, fill in the blanks in the handout to: • declare 3 numbers and one String as member variables (above main method), using meaningful variable names • assign values to three of these variables in the loadData method (leave one of the numeric variables without a value

Exercise (part 2) • following pattern in DemoMethods.java, write the following 4 methods: • print a literal value with say( method • add 2 numbers together and store them in a member variable (above main method) • print this calculated member variable • multiply two numbers and print result with some identifying text

CIS 234: Java Methods