CSE 2231 - Arrayweb.cse.ohio-state.edu/software/2231/web-sw2/extras/slides/08.Array.pdfJava arrays,...

Array

8 February 2019 OSU CSE 1

Array

• The Array component family allows you to manipulate arrays in a way that overcomes surprising limitations of built-in Java arrays, but retains the time/space performance of built-in arrays– Another generic type like Sequence and Set– A best practice alternative from the OSU

CSE components to the built-in Java array (more like built-in arrays than Sequence is)


• Suppose you want to declare an array of type T, where T is a generic parameter, e.g.:T[] table = new T[100];

• Oops; this won’t compile!– The reason is complicated, having to do with

backward compatibility when Java generics were introduced and the inability to call a constructor for type T, among other oddities


Problems With Built-in Arrays

• Or, suppose you want to declare an array of type Queue<T>, e.g.:Queue<T>[] table = new Queue<T>[100];

• Oops; this won’t compile either!– Yet this is exactly the kind of thing you want to

do when creating a hashtable: the buckets are collections of a parameterized type


Problems With Built-in Arrays

Workarounds

• There are various “workarounds” that can be used, but they are ugly and generate warnings that you should generally heed rather than ignore

• These problems do not arise with the Array component family, so it can (and should) be used in many cases where you might be inclined to try built-in Java arrays


The Intuition

• The rather simple idea is to provide a generic interface Array<T> that provides the same functionality as built-in arrays, but where the entries may be of any type T

• The data representation used in Array1L<T> is a built-in array– But this data representation is hidden from the

client, along with one of the workarounds mentioned earlier


Interfaces and Classes

ArrayKernel

extends

Standard

extends


Array

implements

Array1L

Iterable

extends



ArrayKernel

extends

Standard

extends

Array

implements

Array1L

Iterable

extends

ArrayKernel has contracts for four

methods:setEntryentry

mayBeExaminedlength



ArrayKernel

extends

Standard

extends

Array

implements

Array1L

Iterable

extendsArray has contracts for

two methods:replaceEntry

exchangeEntries



ArrayKernel

extends

Standard

extends

Array

implements

Array1L

Iterable

extends

There is really an abstract class as usual in the chain

here, but it is not shown because these slides

describe the client view, and a client needs only interface Array and class Array1L.

Mathematical ModelARRAY_MODEL is (

entries: string of T,

examinableIndices: finite set of integer)exemplar a

constraintfor all i: integer

where (i is in a.examinableIndices)(0 <= i and i < |a.entries|)

type ArrayKernel is modeled by ARRAY_MODEL


Mathematical ModelARRAY_MODEL is (

entries: string of T,

examinableIndices: finite set of integer)exemplar a

constraintfor all i: integer

where (i is in a.examinableIndices)(0 <= i and i < |a.entries|)

type ArrayKernel is modeled by ARRAY_MODEL


The set examinableIndiceskeeps track of the indices in the Array that have been “initialized” by a call to setEntry, because

only these indices may be examined by a call to entry.

Constructor from int

• There is no no-argument constructor, but there is a constructor with one parameter int n

• Requires:n >= 0

• Ensures:|this.entries| = n andthis.examinableIndices = {}


Example


Code State

Array<Integer> ai =new Array1L<>(3);

Example


Code State


ai = (<?, ?, ?>,{})

Example


Code State


ai = (<?, ?, ?>,{})

All we know from the constructor contract is that the length of thisstring of integeris 3; we do not know its

actual value.

setEntryvoid setEntry(int i, T x)• Sets the entry at index i of this to x.• Aliases: reference x• Updates: this• Requires:

0 <= i and i < |this.entries|

• Ensures:this.entries = #this.entries[0, i) * <x> *

#this.entries[i+1, |#this.entries|) andthis.examinableIndices = #this.examinableIndices

union {i}


Example


Code Stateai = (<?, ?, ?>,

{})z = 70

ai.setEntry(1, z);

Example



{})z = 70

ai.setEntry(1, z);

ai = (<?, 70, ?>,{1})

z = 70

Example



{})z = 70

ai.setEntry(1, z);

ai = (<?, 70, ?>,{1})

z = 70

Note the alias created here, which you cannot see in the tracing table; you should be able to draw the appropriate diagram showing it.

Another Example


Code Stateai = (<?, -8, ?>,

{1})z = 70

ai.setEntry(1, z);

Another Example


Code Stateai = (<?, -8, ?>,

{1})z = 70

ai.setEntry(1, z);

ai = (<?, 70, ?>,{1})

z = 70

entry

T entry (int i)

• Reports the entry at index i of this.• Aliases: reference returned by entry• Requires:i is in this.examinableIndices

• Ensures:<entry> = this.entries[i, i+1)


entry

T entry(int i)

• Reports the entry at position i of this.• Aliases: reference returned by entryAt• Requires:i is in this.examinableIndices

• Ensures:<entry> = this.entries[i, i+1)


Note that this implies the index i is “within bounds”, i.e.:

0 <= i andi < |this.entries|

Example


Code Stateai = (<?, 70, -3>,

{1, 2})z = –584

z = ai.entry(1);

Example


Code Stateai = (<?, 70, -3>,

{1, 2})z = –584

z = ai.entry(1);

ai = (<?, 70, -3>,{1, 2})

z = 70

mayBeExamined

boolean mayBeExamined(int i)

• Reports whether the entry at index i of this may be examined using entry.

• Ensures:mayBeExamined =

i is in this.examinableIndices


Example


Code State

ai = (<?, 70, -3>,{1, 2})

boolean ok =ai.mayBeExamined(1);

Example


Code State

ai = (<?, 70, -3>,{1, 2})

boolean ok =ai.mayBeExamined(1);

ai = (<?, 70, -3>,{1, 2})

ok = true

length

int length()

• Reports the length of this.• Ensures:length = |this.entries|


replaceEntryT replaceEntry(int i, T x)• Replaces the entry at index i of this with x, and

returns the old entry at that index.• Aliases: reference x• Updates: this.entries• Requires:

i is in this.examinableIndices

• Ensures:this.entries = #this.entries[0, i) * <x> *

#this.entries[i+1, |#this|) and<replaceEntry> = #this.entries[i, i+1)


Example


Code Stateai = (<?, 70, -3>,

{1, 2})z = 58w = 94

w = ai.replaceEntry(2, z);

Example


Code Stateai = (<?, 70, -3>,

{1, 2})z = 58w = 94


ai = (<?, 70, 58>,{1, 2})

z = 58w = -3

Another Example


Code Stateai = (<?, 70, -3>,

{1, 2})z = 58

z = ai.replaceEntry(2, z);

Another Example


Code Stateai = (<?, 70, -3>,

{1, 2})z = 58


ai = (<?, 70, 58>,{1, 2})

z = -3

Another Example


Code Stateai = (<?, 70, -3>,

{1, 2})z = 58


ai = (<?, 70, 58>,{1, 2})

z = -3

This use of the method avoids creating an alias: it

swaps z with the entry previously at index 2.

exchangeEntriesvoid exchangeEntries(int i, int j)• Exchanges entries at indices i and j of this.• Updates: this.entries• Requires:

{i, j} is subset of this.examinableIndices

• Ensures:this.entries = [#this.entries with entries

at indices i and j exchanged]


Example


Code State

ai = (<?, 70, -3>,{1, 2})

ai.exchangeEntries(1, 2);

Example


Code State

ai = (<?, 70, -3>,{1, 2})

ai.exchangeEntries(1, 2);

ai = (<?, -3, 70>,{1, 2})

iterator

Iterator<T> iterator()

• Returns an iterator over a set of elements of type T.

• Ensures:~this.seen * ~this.unseen =

[string of entries of this.entries,in order by increasing index, where

index is in this.examinableIndices]


Resources

• OSU CSE Components API: Array– http://cse.osu.edu/software/common/doc/


http://cse.osu.edu/software/common/doc/

CSE 2231 - Arrayweb.cse.ohio-state.edu/software/2231/web-sw2/extras/slides/08.Array.pdfJava arrays,...

Documents

Transcript of CSE 2231 - Arrayweb.cse.ohio-state.edu/software/2231/web-sw2/extras/slides/08.Array.pdfJava arrays,...