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Chapter 7

User-Defined Methods

Java Programming from Thomson Course Tech, adopted by kclukJava Programming from Thomson Course Tech, adopted by kcluk

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Predefined Classes

• Methods already written and provided by Java

• Organized as a collection of classes (class libraries)

• To use: import package

• Method type: data type of value returned by method

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class Math (Package: java.lang)

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class Math (Package: java.lang)

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class Character (Package: java.lang)

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class Character (Package: java.lang)

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class Character (Package: java.lang)

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Syntax: Value-Returning Method

modifier(s) returnType methodName(formal parameter list){ //body of method}

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User-Defined Methods

• Value-returning methods– Used in expressions– Calculate and return a value– Can save value for later calculation or print value

• Modifiers: public, private, protected, static, abstract, final

• returnType: type of value that the method calculates and returns (using return statement)

• methodName: Java identifier; name of method

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Syntax

Syntax: Formal Parameter List

The syntax of the formal parameter list is:

(dataType identifier, dataType identifier,…)

Method Call

The syntax to call a value-returning method is:

methodName(actual parameter list)

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Syntax

Syntax: return StatementThe return statement has the following syntax:

return expr;

Syntax: Actual Parameter List

The syntax of the actual parameter list is:

(expression or variable, expression or variable, …)

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Equivalent Method Definitionspublic static double larger(double x, double y){ double max; if(x >= y) max = x; else max = y; return max;}

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Equivalent Method Definitions

public static double larger(double x, double y){ if(x >= y) return x; else return y;}

Activities, ref to Supple:

Download and run the class AdditionExample.

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Flow of Execution

• Execution always begins with the first statement in the method main

• User-defined methods execute only when called • Call to method transfers control from caller to

called method• In method call statement, specify only actual

parameters, not data type or method type• Control goes back to caller when method exits

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Programming Example: Largest Number

• Input: Set of 10 numbers• Output: Largest of 10 numbers• Solution:

– Get numbers one at a time

– Method largest number: returns the larger of 2 numbers

– For loop: calls method largest number on each number received and compares to current largest number

pls refer to book pg 332-334

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Solution: Largest Number

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Sample Run: Largest Number

Sample Run: (When you execute this program, enter all numbers in the same line.)Enter 10 numbers in the same line.10.5 56.34 73.3 42 22 67 88.55 26 62 11The largest number is 88.55

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Void Methods

• Similar in structure to value-returning methods

• No method type

• Call to method is always stand-alone statement

• Can use return statement to exit method early

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Void Methods: Syntax

Method DefinitionThe general form (syntax) of a void method without parameters is as follows:

modifier(s) void methodName(){ statements}

Method Call (Within the Class)

The method call has the following syntax:

methodName();

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Void Methods with Parameters: Syntax

Method DefinitionThe definition of a void method with parameters has the

following syntax:modifier(s) void methodName(formal parameter list){

statements}

Formal Parameter ListThe formal parameter list has the following syntax:

(dataType variable, dataType variable, …)

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Void Methods with Parameters: Syntax

Method CallThe method call has the following syntax: methodName(actual parameter list);

Actual Parameter ListThe actual parameter list has the following syntax: (expression or variable, expression or variable, …)

Activities, ref to Supple:

Download and run the class AdditionExample2

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Primitive Data Type Variables as Parameters

• A formal parameter receives a copy of its corresponding actual parameter

• If a formal parameter is a variable of a primitive data type– Value of actual parameter is directly stored– Cannot pass information outside the method– Provides only a one-way link between actual

parameters and formal parameters

Download & run class PrimitivePass

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Reference Variables as Parameters:

Created by new operater • If a formal parameter is a reference variable

– Copies value of corresponding actual parameter– Value of actual parameter is address of object

where actual data is stored– Both formal and actual parameter refer to

same object

Download & run class ReferencePass

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Uses of Reference Variables as Parameters

• Adv:

• Can return more than one value from a method

• Can change the value of the actual object

• When passing address, would save memory space and time, relative to copying large amount of data

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Reference Variables as Parameters: type String

Download & run Supplement 7C: prog A & B

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Scope of an Identifier Within a Class

• Scope (of an identifier): refers to those parts of a program where the identifier is accessible

• Local variables: variables declared within a method (or block)

• Within a class – Any method can call any other method – Exception: static method cannot call a nonstatic

method

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Scope Rules• Identifier declared within a block is

accessible– Only within the block from the point at which it

is declared until the end of the block – By those blocks nested within that block if

the nested block does not have an identifier with the same name as the identifier in the outside block

*Outside block: block that encloses nested block

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Scope Rules: Demonstrated

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Scope Rules: Demonstratedpublic class ScopeRules{ static final double rate = 10.50; static int z; static double t;

public static void main(String[] args) { int num; double x, z; char ch;

//... } public static void one(int x, char y) { //... }

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public static int w; public static void two(int one, int z) { char ch; int a;

//block three { int x = 12; //... }//end block three //... }}

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Scope Rules: Demonstrated

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Method Overloading: An Introduction

• Method overloading: more than one method can have the same name

• Overloading Rules– Every method must have a different number of formal

parameters OR

– If the number of formal parameters is the same, then the data type of the formal parameter (in the order listed), must differ in at least one position

– Types of parameters determine which method executes

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Programming Example: Data Comparison

• Input: Data from 2 different files• Data format: Course Number followed by scores• Output: Course Number, Group Number, Course

Average• Solution:

– Read from more than one file, write output to file– Generate bar graphs– User-defined methods and re-use (calculateAverage

and printResult)– Parameter passing

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Programming Example: Data Comparison

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Chapter Summary

• Predefined methods

• User-defined methods– Value-returning methods– Void methods– Formal parameters– Actual parameters

• Flow of Execution

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Chapter Summary

• Primitive data type variables as parameters– One-way link between actual parameters and

formal parameters (limitations caused)

• Reference variables as parameters – Can pass one or more variables from a method– Can change value of actual parameter

• Scope of an identifier within a class• Method overloading