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Java Programming

Fifth Edition

Chapter 4More Object Concepts

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Java Programming, Fifth Edition 2

Objectives

Learn about blocks and scope

Overload a method

Learn about ambiguity

Send arguments to constructors

Overload constructors

Learn about the this reference

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Objectives (continued)

Use static variables

Use constant fields

Use automatically imported, prewritten constants

and methods

Use the explicitly imported prewritten classGregorianCalendar

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Objectives (continued)

Learn about composition

Learn about nested and inner classes

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Understanding Blocks and Scope

Blocks

Use opening and closing curly braces

Can exist entirely within another block or entirely

outside and separate from another block

Cannot overlap

Types

Outside or outer blocks

Inside or inner blocks

Nested

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(continued)


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(continued) Scope

Portion of program within which you can refer to a

variable

Comes into scope

Variable comes into existence

Goes out of scope

Variable ceases to exist

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(continued) Redeclare variable

Cannot declare same variable name more than once

within block

Illegal action

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(continued)


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(continued) Override

Variables name within method in which it is declared

Takes precedence over any other variable with same

name in another method

Locally declared variables

Always mask or hide other variables with same name

elsewhere in class

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(continued)


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(continued)


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Overloading a Method

Overloading

Using one term to indicate diverse meanings

Writing multiple methods with same name but with

different arguments

Compiler understands meaning based on arguments

used with method

Convenience for programmers to use one

reasonable name For tasks that are functionally identical

Except for argument types

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Overloading a Method (continued)


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Learning About Ambiguity

If application contains just one version of method

Call method using parameter of correct data type

Or one that can be promoted to correct data type

Ambiguous situation

When methods overloaded

May create situation in which compiler cannot

determine which method to use

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Learning About Ambiguity (continued)

Overload methods

Correctly provide different argument lists for

methods with same name

Illegal methods

Methods with identical names that have identical

argument lists but different return types

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Learning About Ambiguity (continued)


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Sending Arguments to Constructors

Java automatically provides constructor method

When class-created default constructors do not

require parameters

Write your own constructor method

Ensures that fields within classes are initialized to

appropriate default values

Constructors can receive parameters

Used for initialization purposes

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Sending Arguments to Constructors

(continued) Write constructor for class

No longer receive automatically written default

constructor

If classs only constructor requires parameter

Must provide argument for every object of class

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Overloading Constructors

Use constructor parameters

To initialize field values

Or any other purpose

If constructor parameter lists differ

No ambiguity about which constructor method to call

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Overloading Constructors (continued)


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Learning About the this Reference

Instantiate object from class

Memory reserved for each instance field in class

Not necessary to store separate copy of each

variable and method for each instantiation of class

In Java

One copy of each method in class stored

All instantiated objects can use one copy

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(continued) Reference

Objects memory address

Implicit

Automatically understood without actually being

written

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(continued) this reference

Reference to object

Passed to any objects nonstatic class method

Reserved word in Java

Dont need to use this reference in methods you

write

In most situations

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(continued)


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(continued) this reference (continued)

Implicitly received by instance methods

Use to make classes work correctly

When used with field name in class method

Reference to class field

Instead of to local variable declared within method

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Using the this Reference to Make

Overloaded Constructors

More Efficient Avoid repetition within constructors

Constructor calls other constructor

this() More efficient and less error-prone


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Using static Variables

Class methods

Do not have this reference

Have no object associated with them

Class variables

Shared by every instantiation of class

Only one copy ofstatic class variable per class

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Using Constant Fields

Create named constants using keyword final

Make its value unalterable after construction

Can be set in class constructor

After construction cannot change final fields value


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Using Constant Fields (continued)


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Using Automatically Imported,

Prewritten Constants and Methods Many classes commonly used by wide variety of

programmers

Java contains nearly 500 classes

Package or library of classes

Folder provides convenient grouping for classes

Many contain classes available only if explicitly

named within program

Some classes available automatically

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Prewritten Constants and Methods

(continued) Fundamental or basic classes

Implicitly imported into every Java program

java.lang package

Only automatically imported, named package

Optional classes

Must be explicitly named

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Prewritten Constants and Methods

(continued) java.lang.Math class

Contains constants and methods used to perform

common mathematical functions

No need to create instance

Imported automatically

Cannot instantiate objects of type Math

Constructor forMath class private

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Using an Explicitly Imported

Prewritten Class and Its Methods Use prewritten classes

Use entire path with class name

Import class

Import package that contains class

To use import statements

Place before any executing statement in Java file

Prior to import statement

Use blank line or comment line but nothing else

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Prewritten Class and Its Methods

(continued) Wildcard symbol

Alternative to importing class

Import entire package of classes

Use asterisk

Can be replaced by any set of characters

Represents all classes in package

No disadvantage to importing extra classes

Importing each class by name can be form of

documentation

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Prewritten Class and Its Methods

(continued) GregorianCalendar class

Seven constructors

Default constructor creates calendar object

containing current date and time in default locale

(time zone)

Can also specify date, time, and time zone

get() method

Access data fields


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Understanding Composition

Composition

Describes relationship between classes when object

of one class data field is within another class

Called has-a relationship Because one class has an instance of another

Remember to supply values for contained object if

it has no default constructor


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Understanding Composition

(continued)


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A Brief Look at Nested and Inner

Classes

Nested classes

Class within another class

Stored together in one file

Nested class types

static member

Nonstatic member

Local classes

Anonymous classes


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You Do It

Demonstrating scope

Overloading methods

Creating a constructor that requires an argument

Using an explicitly imported prewritten class

Creating an interactive application with a timer

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Dont Do It

Dont try to use a variable that is out of scope

Dont assume that a constant is still a constant

when passed to a methods parameter

Dont overload methods by giving them differentreturn types

Dont think that default constructor means only the

automatically supplied version

Dont forget to write a default constructor for a

class that has other constructors


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Summary

Variables scope

Portion of program in which you can reference

variable

Block Code between a pair of curly braces

Overloading

Writing multiple methods with same name but

different argument lists

Store separate copies of data fields for each object

But just one copy of each method

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Summary (continued)

static class variables

Shared by every instantiation of a class

Literal constants never change

Prewritten classes

Stored in packages

import statement

Notifies Java program that class names refer to

those within imported class

Documents

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