Java Collections Framework - Object-Oriented Programming and Data Structures - Lecture Sl, Lecture notes of Object Oriented Programming

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Generic

Types

and

the

Java

Collections

Framework

Generic

Types

in

Java

-^

When

using

a

collection

(e.g.,

LinkedList,

HashSet,

HashMap

we

generally

have

a

single

type

T

of

elements

that

we

store

in

it

(e.g.,

Integer,

String

-^

Before

Java

when

extracting

an

element,

had

to

cast

it

to

T

before

we

could

invoke

T's

methods

-^

Compiler

could

not

check

that

the

cast

was

correct

at

compile

‐time,

since

it

didn't

know

what

T

was

-^

Inconvenient

and

unsafe,

could

fail

at

runtime

^

Generics in Java provide away to communicate T, thetype of elements in acollection, to the compiler

^

Compiler can check that you haveused the collection consistently

^

Result: safer and more-efficientcode

Another

Example

Map

grades

new

HashMap();

grades.put("John",

new

Integer(67));

grades.put("Jane",

new

Integer(88));

grades.put("Fred",

new

Integer(72));

Integer

x

(Integer)grades.get("John");

sum

=^

sum

x.intValue();

Map<String,

Integer> grades

new

HashMap<String,

Integer>();

grades.put("John",

new

Integer(67));

grades.put("Jane",

new

Integer(88));

grades.put("Fred",

new

Integer(72));

Integer

x^

grades.get("John");

sum

sum

x.intValue();

old new

Type

Casting

In

effect,

Java

inserts

the

correct

cast

automatically,

based

on

the

declared

type

In

this

example,

grades.get("John")

is

automatically

cast

to

Integer

Map<String,

Integer> grades

=^

new

HashMap<String,

Integer>();

grades.put("John",

new

Integer(67));

grades.put("Jane",

new

Integer(88));

grades.put("Fred",

new

Integer(72));

Integer

x

grades.get("John");

sum

=^

sum

x.intValue();

Using

Generic

Types

is

read,

“of

T”

For

example:

Stack

is

read,

“Stack

of

Integer”

The

type

annotation

informs

the

compiler

that

all

extractions

from

this

collection

should

be

automatically

cast

to

T

Specify

type

in

declaration,

can

be

checked

at

compile

time

Can

eliminate

explicit

casts

Advantage

of

Generics

Declaring

Collection c

tells

us

something

about

the

variable

c

(i.e.,

c

holds

only

Strings)



This

is

true

wherever

c

is

used



The

compiler

checks

this

and

won’t

compile

code

that

violates

this

Without

use

of

generic

types,

explicit

casting

must

be

used



A

cast

tells

us

something

the

programmer

thinks

is

true

at

a

single

point

in

the

code



The

Java

virtual

machine

checks

whether

the

programmer

is

right

only

at

runtime

Programming

with

Generic

Types

-^

To

use

the

interface

List

,^ supply

an

actual

type

argument,

e.g.,

List

-^

All

occurrences

of

the

formal

type

parameter

(

E

in

this

case)

are

replaced

by

the

actual

type

argument

(

Integer

in

this

case)

public

interface

List

{^

E

is

a

type

variable

void

add(E

x);

Iterator

iterator();

} public

interface

Iterator

E

next(); boolean

hasNext();

void

remove();

Wildcards

void

printCollection(Collection

c)

Iterator

i

c.iterator();

while

(i.hasNext())

System.out.println(i.next()); } } void

printCollection(Collection