chapter 3: developing class methods object-oriented program development using java: a class-centered...

Chapter 3: Developing Class

Methods

Object-Oriented Program Development Using Java:

A Class-Centered Approach

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Method Development: Algorithms

Algorithm Step-by-step set of instructions Describes how data are to be processed to

produce desired result Must clearly understand difference between

algorithmic and intuitive commands Computers do not understand intuitive

commands Coding the algorithm

Writing in programming language

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Method Development: Algorithms (continued)

Pseudocode

English-like phrases used to describe algorithm

Formulas

Mathematical equations are used

Flowcharts

Diagrams that employ symbols are used

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Using Pseudocode Most commonly used method for

developing algorithms

Short English phrases

Example: Input three numbers into computer

Calculate average by adding numbers and dividing the sum by 3

Display average

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Application: Swapping Values Swapping:

Exchanging data that are either stored or referenced by two variables

Commonly seen in programming Algorithm:

Store first variable’s value into temporary location

Store second variable’s value into first variable

Store temporary value into second variable

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Application: Swapping Values (continued)

How should two variables be made available to a method that will swap their values?

Procedure: Encapsulate two variables within a single

object Use class method to implement swap

algorithm Switch values in instance variables

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static and final Variables

So far, we’ve seen local variable ( declared inside method)

instance variable ( declared inside class but outside method)

Instance variable – term used because each instance of the class has its own version of the variable.

Each object has its own memory place so it can have a distinct value for that class.

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Static variablestatic variable (or class variable) Created only once for each class Shared by all objects created from class – only one copy

of a static variable for all objects of the class Changing the value of a static variable in one object

changes it for all the other objects Declared in same way as instance variables

Except static reserved word is used in declaration

Example static declaration: private static int numEmployees = 125;

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Static or Class Method

Invoked through the class name – don’t have to instantiate an object of the class to invoke the method. e.g. all method of the Math class are

static methods so Math.sqrt(16); or Math.pow(5,3);

These methods perform basic calculations based on values passed as parameters. No object state to maintain in these situations.

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Static Methods (cont.)

Java main method must be static so that main can be executed without instantiating an object from the class containing main.

Static methods do not operate in the context of a particular object.

Can only reference static variables

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Summary of static methods or class methods

Provide means of accessing static variables in absence of any specific object

Restricted to using:

Static variables

Other static methods

Additional values and objects that are passed as arguments into method

Include static reserved word in method’s header

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static Methods (continued)

Can also be used for constructing general-purpose methods

Perform functions using only data passed to them as arguments at time of call

i.e. main() method

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Scope

Section of program within which an identifier is valid

Defines portion of class where variable can be used

Scopes in Java:

Local

Class

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Scope (continued)

Local scope:

Variables declared within a method

All parameters of a method

Class scope:

All instance and static variables declared in a class’s variable declaration section

All methods contained within a class’s method definition section

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Visibility Determines whether a member can be

accessed from outside a class in which it is declared

Modifiers: Public Protected Private

Visibility rules

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Values and Identities

Identity

Every object has a unique identity

Literal values do not have identities

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final Variables

final variables Have initial value that cannot

subsequently be changed

Also called: Named constants

Symbolic constants

Use final reserved word

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final Variables (continued) Example:

public final static int NUMEMPS = 100;

Blank final: No initial value is provided

Value initialized later in program

final float DENSITY;

Java convention: Use uppercase letters for identifiers

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Placement of Statements A variable must be declared before it

can be used Proper placement:

public class className

{

// declaration section

final static variable declarations

static variable declarations

instance variable declarations

continued …

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Placement of Statements Proper placement:

// methods section - example of one method

method header line(parameter list)

{

final variable declarations

variable declarations

object declarations

other Java statements

}

}

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Common Programming Errors Attempting to pass incorrect data types Declaring the same variable locally within

both calling and called methods and assuming the change in one variable affects the other variable

Forgetting to include the data type of a method’s parameters within the header line

Attempting to alter an object’s private variables by passing an object into a non-class method as an argument

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Summary

A method is called by: Giving its name Passing any data to it in parentheses following

the name A method’s return type declares the data type of

the value returned by the method Methods can directly return at most a single

value A called method cannot alter either a primitive

data type argument’s value or a reference variable’s value

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Summary (continued)

Algorithm: Step-by-step procedure Describes how a single computation or task is to be

performed Scope:

Section of a program within which an identifier is valid

Static variables can be accessed without being referenced as a data member of a specific object