Chapter 6: User-defined Methods

STATIC METHODS

Objectives

• public static int max(int a, int b){
• if (a > b) {
• return a;
• }
• else {
• return b;
• }

}

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Method

• A method is a named block of code which only runs when it is called.
• Methods are used to perform certain actions, and they are also known as functions.
• A method must be defined within a class.
• A method definition consists of the new method's name and a block of statements(method body)
• Ex: public static void display(){ // method body }
• A method call is an invocation of a method's name, causing the method's body statements to execute.
• A method only executes when it is called

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Method Definition/Declaration Syntax

• modifiers returnType methodName(Parameters List){
• method body
• }

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Creating a Method

• public class Methods {

/*

• * Methods are created within a class.
• * Look at the example below.
• */
• public static void methodName() {
• /*
• * Method body goes here. This is a list of
• * statements that you would like your method to do.
• */
• }
• }

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Method Declaration (Refer to the previous example)

• public static: These are the modifiers in the method
• void: This is the return type for the method
• methodName: This is the Method Name
• Since the () are empty: There is no parameters in this method
• {}:  Everything enclosed between braces is part of the method body

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Modifiers

• Modifiers define where the method can be accessed from in an application.
• 4 access modifiers available in Java
• public: accessible in all classes in your application.
• protected: accessible within the class in which it is defined and in its subclass(es)
• private: accessible only within the class in which it is defined.
• default (declared/defined without using any modifier) : accessible within same class and package within which its class is defined.

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Method Calling and Method Calling Syntax

• A method does not run until it is called
• A method can called multiple times

​

Method Call Syntax:

methodName(values for the parameters);

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Example of Method Declaration and Method Call

• public class MethodExample {
• /*
• * Recall that methods are created with a class.
• * The following method prints "My First Method" when called.
• * It takes no parameters and returns no value.
• */
• public static void display() {
• System.out.println("My First Method");
• }

​

• public static void main(String[] args) {
• display(); // Method call
• }
• }

​

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Information to Remember when Writing Methods

• What is the name of your method? Choose a name that describes what the method does
• Does your method take any parameters?
• If you answer is no, leave the parenthesis black.
• If yes, follow the following steps to create your parameters
• How many parameters does the method take?
• What are the data types of the parameters
• What are the names of the parameters
• What is the returnType of your method.
• If your method returns no value, the return type is void
• If the method return a value, we put the data type of the value that we expect to be returned by the method

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A Tutorial Video on Methods (Play the Video)

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Scope of variable/method definitions

• The name of a defined variable or method item is only visible to part of a program, known as the item's scope
• A variable declared in a method has the scope limited to that method.
• A variable declared in a method cannot be accessed and used outside of the method
• We can be able to declare variables outside of any method but within a class.
• A variable declared within a class but outside any method is called a class member variable or field.
• A variable declare anywhere within a method is referred to as a local variable
• We will discuss class variables in chapter 8

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�Method Parameters and arguments

• A parameter is a method input specified in a method definition.
• Parameters act as variables inside the method.
• Parameters are placed after the method name, inside the parentheses.
• Parameters provide a way of getting information from outside the method to be used in the method body
• A method can have no parameters
• You can also be able to define multiple parameter separated by commas
• An argument is a value provided to a method's parameter during a method call

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Parameterized Method Example

• public class MethodExample {
• /*
• * When called, this method prints an integer value stored in a parameter.
• * It takes one integer parameter and returns no value
• */
• public static void displayNum(int num) {
• System.out.println("The number is " + num);
• }

​

• public static void main(String[] args) {
• displayNum(20); // Method call
• displayNum(30); // Method call
• }
• }

​

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Example: Writing a method

• Write a program with a method named printTotal that accepts two integers as an argument and prints the sum of the two integers. Call this method from main(). (See solution in the next Slide)

​

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Example: Writing a method

• public class MethodExample {
• /*

* This method takes two parameters are prints the sum of the parameters.

* It returns no value

• */
• public static void printTotal(int num1, int num2) {
• int total = num1 + num2;
• System.out.println(total);
• }
• public static void main(String[] args) {
• printTotal(20, 30); // prints 50
• printTotal(60, 60); // prints 120
• }
• }

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A Video Tutorial on Variable Scope (Play the Video)

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Why Use Methods?

• To reuse code: define the code once and use it many times.
• Decomposing a program into methods can greatly aid program readability, helping yield an initially correct program, and easing future maintenance.
• Avoid writing redundant code

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Return

• If you remember, we said parameters provide a way of getting values into a method.
• We can also be able to get a value out from a method using the return statement
• To return a value from a method we use the keyword return.
• Return is used to exit a method with or without a value from a method
• The data value of the values that gets returned must match returnType of a method
• A method can only return one item, not two or more.
• A method that does not return any value has void as the return type

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Return Statement Syntax and Semantics

• Syntax:

Return <expression>

​

​

A value we get from a return statement can be stored in a variables

• Semantics
• Evaluate the <expression>
• Take the result from step 1 to wherever the method was called from and exit from the method

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Calling Methods in Expressions

• A method call evaluates to the returned value. Therefore a method call can appear within an expression or can printed
• A method with a void return type cannot be used within an expression

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Example: Writing a method that returns a value

• Write a program with a method named getTotal that accepts two integers as an argument and returns the sum of the two integers. Call this method from main().

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Example:

public class MethodExample {

• /*
• * This method takes two parameters are return the sum of the parameters.
• */

public static int getTotal(int num1, int num2) {

• int total = num1 + num2;
• return total;
• }
• public static void main(String[] args) {
• System.out.println( getTotal(20, 30)) ; // prints 50, the value returned from the method call
• int sum = getTotal(60, 60); //The method call evaluates 120, which is kept in variable sum
• System.out.println(sum); // prints 120, the value in the variable sum
• }
• }

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Calling methods from methods

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 Method Name Overloading

• Method name overloading or just method overloading is a situation where two or more methods in the same class have the same name but differing in the number or types of parameters
• We determine which method to call based on the argument types
• We allowed to define more than two same-named methods as long as each has distinct parameter types
• A method's return type does not influence overloading

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 Method name overloading

• method name overloading or just method overloading is a situation where two or more methods in the same class have the same name but differing in the number or types of parameters

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 �Parameter Error Checking�

• Method name overloading or just method overloading is a situation where two or more methods in the same class have the same name but differing in the number or types of parameters

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