chapter 7: user-defined methods j ava p rogramming: from problem analysis to program design, from...

Chapter 7: User-Defined Chapter 7: User-Defined MethodsMethods

JJavaava PProgramming:rogramming:

From Problem Analysis to Program From Problem Analysis to Program

Design,Design, Second EditionSecond Edition

Java Programming: From Problem Analysis to Program Design, Second Edition 2

Chapter Objectives

Understand how methods are used in Java programming.

Learn about standard (predefined) methods and discover how to use them in a program.

Learn about user-defined methods. Examine value-returning methods, including

actual and formal parameters.


Chapter Objectives

Explore how to construct and use a value-returning, user-defined method in a program.

Learn how to construct and use user-defined void methods in a program.

Explore variables as parameters. Learn about the scope of an identifier. Become aware of method overloading.


Predefined Classes

Methods already written and provided by Java.

Organized as a collection of classes (class libraries).

To use, import package. Method type: The data type of the value

returned by the method.


Predefined Classes


class Character (Package: java.lang)


Syntax of Value-Returning Method

modifier(s) returnType methodName (formal parameter list){ statements}


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 the value that the method calculates and returns (using return statement).

methodName: Java identifier; name of method.


Syntax

Syntax of formal parameter list:

dataType identifier, dataType identifier,...

Syntax to call a value-returning method:

methodName(actual parameter list)


Syntax

Syntax of the actual parameter list:

expression or variable, expression or variable, ...

Syntax of the return statement:

return expr;


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


Equivalent Method Definitionspublic static double larger(double x, double

y){ if (x >= y) return x; else return y;}


Equivalent Method Definitions

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


Programming Example: Palindrome Number

Palindrome: An integer or string that reads the same forwards and backwards.

Input: Integer or string. Output: Boolean message indicating whether

integer string is a palindrome.


Solution: isPalindrome Method

public static boolean isPalindrome(String str){ int len = str.length(); int i, j; j = len - 1;

for (i = 0; i <= (len - 1) / 2; i++) { if (str.charAt(i) != str.charAt(j)) return false; j--; } return true; }


Sample Runs: Palindrome Number


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 the method call statement, specify only actual

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


Programming Example: Largest Number

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

Get numbers one at a time. Method larger number: Returns the larger of 2 numbers. For loop: Calls method larger number on each number

received and compares to current largest number.


Solution: Largest Numberstatic Scanner console = new Scanner(System.in);

public static void main(String[] args){ double num; double max; int count; System.out.println("Enter 10 numbers."); num = console.nextDouble(); max = num; for (count = 1; count < 10; count++) { num = console.nextDouble(); max = larger(max, num); } System.out.println("The largest number is " + max); }


Sample Run: Largest Number

Sample Run:Enter 10 numbers:10.5 56.34 73.3 42 22 67 88.55 26 62 11The largest number is 88.55


Void Methods

Similar in structure to value-returning methods.

Call to method is always stand-alone statement.

Can use return statement to exit method early.


Void Methods: Syntax

Method definition:The 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();


Void Methods with Parameters: Syntax

Method definition:The definition of a void method with parameters has the following syntax:

modifier(s) void methodName (formal parameter list)

{ statements

}

Formal parameter list:The formal parameter list has the following syntax:

dataType variable, dataType variable, ...


Void Methods with Parameters: Syntax

Method call:The method call has the following syntax:

methodName(actual parameter list);

Actual parameter list:The actual parameter list has the following syntax:

expression or variable, expression or variable, ...


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.


Reference Variables as Parameters

If a formal parameter is a reference variable: Copies value of corresponding actual parameter. Value of actual parameter is address of the

object where actual data is stored. Both formal and actual parameters refer to same

object.


Uses of Reference Variables as Parameters

Can return more than one value from a method.

Can change the value of the actual object. When passing an address, saves memory

space and time, relative to copying large amount of data.


Reference Variables as Parameters: type String


Scope of an Identifier within a Class Local identifier: An identifier that is declared within a method

or block and that is visible only within that method or block. Java does not allow the nesting of methods. That is, you

cannot include the definition of one method in the body of another method.

Within a method or a block, an identifier must be declared before it can be used. Note that a block is a set of statements enclosed within braces.

A method’s definition can contain several blocks. The body of a loop or an if statement also forms a block.

Within a class, outside of every method definition (and block), an identifier can be declared anywhere.


Scope of an Identifier within a Class Within a method, an identifier that is used to name a variable in

the outer block of the method cannot be used to name any other variable in an inner block of the method. For example, in the following method definition, the second declaration of the variable x is illegal:

public static void illegalIdentifierDeclaration(){ int x; //block { double x; //illegal declaration, //x is already declared ... }}


Scope Rules Scope rules of an identifier that is declared within a class

and accessed within a method (block) of the class. An identifier, say X, that is declared within a method (block) is

accessible: Only within the block from the point at which it is declared until the

end of the block. By those blocks that are nested within that block.

Suppose X is an identifier that is declared within a class and outside of every method’s definition (block). If X is declared without the reserved word static (such as a named

constant or a method name), then it cannot be accessed in a static method.

If X is declared with the reserved word static (such as a named constant or a method name), then it can be accessed within a method (block) provided the method (block) does not have any other identifier named X.


Example 7-12public 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) { //... }

Scope Rules


public static int w; public static void two(int one, int z) { char ch; int a; //block three { int x = 12; //... } //end block three //... }}

Scope Rules


Scope Rules: Demonstrated


Method Overloading: An Introduction

Method overloading: More than one method can have the same name.

Two methods are said to have different formal parameter lists: If both methods have a different number of

formal parameters. If the number of formal parameters is the same in

both methods, the data type of the formal parameters in the order you list must differ in at least one position.


Method Overloading

public void methodOne(int x)

public void methodTwo(int x, double y)

public void methodThree(double y, int x)

public int methodFour(char ch, int x,

double y)

public int methodFive(char ch, int x,

String name)

These methods all have different formal parameter lists.


Method Overloading

public void methodSix(int x, double y,

char ch)

public void methodSeven(int one, double u,

char firstCh)

The methods methodSix and methodSeven both have three formal parameters, and the data type of the corresponding parameters is the same.

These methods all have the same formal parameter lists.


Method Overloading

Method overloading: Creating several methods within a class with the same name.

The signature of a method consists of the method name and its formal parameter list. Two methods have different signatures if they have either different names or different formal parameter lists. (Note that the signature of a method does not include the return type of the method.)


Method Overloading

The following method headings correctly overload the method methodXYZ:

public void methodXYZ()

public void methodXYZ(int x, double y)

public void methodXYZ(double one, int y)

public void methodXYZ(int x, double y,

char ch)


Method Overloading

public void methodABC(int x, double y)public int methodABC(int x, double y)

Both these method headings have the same name and same formal parameter list.

These method headings to overload the method methodABC are incorrect.

In this case, the compiler will generate a syntax error. (Notice that the return types of these method headings are different.)


Programming Example: Data Comparison

Input: Data from two 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.


Sample Output

Course No Group No Course Average CSC 1 83.71 2 80.82

ENG 1 82.00 2 78.20

HIS 1 77.69 2 84.15

MTH 1 83.57 2 84.29

PHY 1 83.22 2 82.60

Avg for group 1: 82.04Avg for group 2: 82.01



Chapter Summary

Pre-defined methods User-defined methods:

Value-returning methods Void methods Formal parameters Actual parameters

Flow of execution


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

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