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CHAPTER 3 METHODS Introduction • A method is also called as a function or an operation. • A method is a block of statements that is executed to perform specific task when it is called. • To execute a method, we must call it. • There are 2 types of method: User defined method Pre-defined method User defined method • User can create a method using the following format: Modifier ReturnValueType MethodName (formal parameter list) { Body of a method; return statement; } • Example: public static int square(int x) Modifier • The modifier in the function header is optional. • The purpose is to specify the way the method can be accessed. • There are different kinds of modifier such as: Modifier The ReturnValueType specifies the data type of value that the method can return. If the ReturnValueType is void, this means that the method does not return any value. If the ReturnValueType is other than void, then, the method must have a return statement. • The method may have list of parameters to be used. However, if the is no parameters, then, the parameters should be written as ‘void’ or left blank. The parameters listed at method header are called formal parameter. Writing Method Header Below are examples of how function header can be written from function description. • Method square( ) is a public static method that takes an integer parameter and returns an integer value public static int square(int x) • Method mult( ) is a public method takes 3 parameters of type float and returns a floating point value public float mult( float a, float b, float c) Writing Method Header • method Total( ) is a public method that takes 2 integer parameter and does not return any value public void Total(int x, int y) • method DisplayValue( ) is a public static method does not accept any parameter and does not return any value public static void DisplayValue( ) Notice that there is no semicolon at the end of function header. Calling a Method Calling a public static method Since a static method is free from object, the method can be executed directly without the use of object. Calling a non-static method Any non-static method must be accessed via object of the class. This means the object must be declared first prior to calling the method. Passing Parameters • When calling a method that accepts parameters, we must pass the actual parameters in the same order, same number and compatible type as formal parameters. • Formal parameters are parameters listed at method header. • Actual parameters are parameters listed at method call. Method Overloading • Overloading methods refer to more than one method that shares the same name but different parameter list defined in a class. • Example: #Module page 50 Predefined Methods • Pre-defined method refers to the methods in Java API. • The method that are present in the Java Class Libraries that we can use. • Using Java Predefined Classes, Included in the Java SDK are more than 2,000 classes that can be used to add functionality to your programs APIs (the complete list is published on www.java.sun.com. Predefined Methods • Here are some classes that we can use: The String Class //we use some of it Using System.out //we use this already Formatting Output The Math Class The Wrapper Classes //we use it already...not all Dialog Boxes //we use it in input/output dialog System.in object //we use it in input/output Console Input Using the Scanner Class Predefined Methods • Classes are grouped in packages according to functionality here are some packages examples: Predefined Methods • Classes in java.lang are automatically available to use. • Classes in other packages need to be "imported" using this syntax: import package.ClassName; or import package.*; Example: import java.utils.*; JOptionPane • JOptionPane makes it easy to pop up a standard dialog box that prompts users for a value or informs them of something. • While the JOptionPane class may appear complex because of the large number of methods, almost all uses of this class are one-line calls to one of the static showXxxDialog. • Example: showMessageDialog ( ) showInputDialog ( ) JOptionPane String class • String class contains methods that can be used to manipulate string. • A string refers to a sequence of characters enclosed in double quotes. • Example: #Module page 56 The Wrapper Class • A wrapper class is a class that surrounds a relatively simple item in order to add functionality to the simple item. • fWraps the value of primitive data type into an object which is useful when methods require an object argument and also useful for converting Strings to an int or double. The Wrapper Class • Here are wrapper classes for some of the Java primitive types: Wrapper Class Primitive Type Integer int Long long Float float Double double Character char • Note that the wrapper class names are the same as the primitive names except for the uppercase first letter. • The wrapper classes are defined in the java.lang package. The Java compiler automatically imports all the classes in the java.lang package, so there's no need to import the wrapper classes explicitly. The Wrapper Class • With GUI programs, all numeric output is string based. • So to display a number, you need to convert the number to a string prior to calling the GUI display method. • All numeric input is string based, too. • So to read a number in a GUI program, you first read the input as a string and then convert the string to a number. The Wrapper Class Here are string conversion methods provided by the numeric wrapper classes: The Wrapper Class • Type of Wrapper class The Math Class • All methods of math class are static. • Unlike String class, the math class methods are accessible without object. (#Module page 62) Let’s Try ! What are the two types of method? Give 3 examples of pre-defines method.