Review of objects and variables in Java

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Jemimah Conley
7 years ago
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1 CMSC 331 Principles of progrmming lnguges Reiew of ojects nd riles in J Vriles nd Ojects wht hppens when you run this? String = ; System.out.println (); wht is? string literl tht elutes to String oject wht is? rile whose lue is n oject reference wht is String =? declrtion nd n ssignment in one 2 Method Clls wht hppens when you run this? String = ; String =.touppercse (); System.out.println (); wht is touppercse? method of clss String type is String -> String declred s pulic String touppercse () wht is.touppercse ()? method cll on the oject does it chnge? no, it cretes new string 3 Null References wht hppens when you run this? String = null; System.out.println (); null wht hppens when you run this? String = null; String =.touppercse (); System.out.println (); it throws NullPointerException why? ecuse method cll must he suject 4 10/23/2003 1

2 CMSC 331 Principles of progrmming lnguges Shring & Equlity wht hppens when you run this? String = ; String = ; System.out.println (); is tht the sme s this? String = ; String = ; System.out.println (); yes, ecuse String is immutle. There is no wy to distinguish these cses nd, in fct, J irtul mchine my produce upper or lower stte in this cse. 5 Mutle Continers wht hppens when you run this? String = ;.ddelement (); System.out.println (.lstelement ()); wht hppens when you run this? String = ; String = ;.ddelement (); System.out.println (.lstelement ());.ddelement (); System.out.println (.lstelement ()); 6 Alising wht out this? Vector q = ; String = ;.ddelement (); System.out.println (q.lstelement ()); why? ecuse nd q re lised: they re nmes for the sme oject wht if we now do this? if ( == q) System.out.println ( sme oject ); if (.equls (q)) System.out.println ( sme lue ); sme oject sme lue q Alising occurs when seerl different identifiers refer to the sme oject. The term is ery generl nd is used in mny contexts. 7 Alising nd Immutles wht does this do? String = ; String = ;.touppercse (); System.out.println (); it prints why? ecuse strings re immutle The ojects creted y the touppercse method is eentully GCed (grge collected.) FOO 8 10/23/2003 2

3 CMSC 331 Principles of progrmming lnguges Polymorphism wht does this do? Vector e = new Vector () e.ddelement (e); e.ddelement (); System.out.println ( (.lstelement ()).lstelement ()); it prints wht kind of method is ddelement? polymorphic one type is Vector, Oject -> oid declred s pulic oid ddelement (Oject o) On Polymorphism First identified y Christopher Strchey (1967) nd deeloped y Hindley nd Milner, llowing types such s list of nything. E.g. in Hskell we cn define function which opertes on list of ojects of ny type ( is type rile). length :: [] -> Int Polymorphic typing llows strong type checking s well s generic functions. ML in 1976 ws the first lnguge with polymorphic typing. Ad-hoc polymorphism (k oerloding) is the ility to use the sme syntx for ojects of different types, e.g. "+" for ddition of rels nd integers. In OOP, the term is used to descrie riles which my refer t run-time to ojects of different clsses Reference Loops Cn one een dd to itself?.ddelement();.ddelement (); System.out.println (.lstelement ()) yes, try it! nd this?.ddelement (5); no, 5 is primitie lue, not n oject A Pir of Methods Some types wht re the types of ddelement, lstelement? ddelement : Vector, Oject -> oid lstelement : Vector -> Oject A puzzle how re x nd e relted fter this?.ddelement (e); x =.lstelement (); they denote the sme oject cn the compiler infer tht? no! not een tht x nd e he the sme clss /23/2003 3

4 CMSC 331 Principles of progrmming lnguges Downcsts Wht does this do? String = ;.ddelement (); String =.lstelement (); System.out.println (); Compiler rejects it:.lstelement doesn t return String! wht does this do? String = ;.ddelement (); String =.lstelement (); System.out.println (); it prints 13 Upcsting nd Downcsting Suppose we he oject O of clss C1 with superclss C2 In J, upcsting is utomtic ut downcsting must e explicit. Upcsting: treting O s C2 Downcsting: treting O s C1 C2 C1 O 14 Vrile & Oject Clsses Wht does this do? String = ;.ddelement (); Oject o =.lstelement (); System.out.println (o.getclss ()); It prints j.lng.string Wht s going on here? getclss returns n oject representing clss o.getclss () is the clss o hs t runtime System.out.println prints string representtion, i.e., the nme 15 Some Key Concepts Vriles & ojects riles hold oject references (or primitie lues like 5) null is specil oject reference Shring, equlity & mutility distinct ojects cn he the sme lue stte is held in lue of instnce riles n oject cn e mutle (stte my chnge) or immutle two riles cn point to the sme oject; chnging one ffects the other 16 10/23/2003 4

5 CMSC 331 Principles of progrmming lnguges Some Key Concepts Methods non-sttic method hs suject or trget oject tht is the instnce it is clled on Sttic methods re not ssocited with instnce ojects ut with clsses my e polymorphic, i.e., work on seerl types of oject Compile-time & runtime types n oject hs type t runtime: the clss of its constructor rile hs declred, compile-time type or clss runtime clss is suclss of compile-time clss 17 10/23/2003 5

Object Semantics. 6.170 Lecture 2

Object Semantics. 6.170 Lecture 2 Object Semntics 6.170 Lecture 2 The objectives of this lecture re to: to help you become fmilir with the bsic runtime mechnism common to ll object-oriented lnguges (but with prticulr focus on Jv): vribles,