B.3 Data Classes

B Java Distilled

\(\newcommand{\footnotename}{footnote}\) \(\def \LWRfootnote {1}\) \(\newcommand {\footnote }[2][\LWRfootnote ]{{}^{\mathrm {#1}}}\) \(\newcommand {\footnotemark }[1][\LWRfootnote ]{{}^{\mathrm {#1}}}\) \(\let \LWRorighspace \hspace \) \(\renewcommand {\hspace }{\ifstar \LWRorighspace \LWRorighspace }\) \(\newcommand {\TextOrMath }[2]{#2}\) \(\newcommand {\mathnormal }[1]{{#1}}\) \(\newcommand \ensuremath [1]{#1}\) \(\newcommand {\LWRframebox }[2][]{\fbox {#2}} \newcommand {\framebox }[1][]{\LWRframebox } \) \(\newcommand {\setlength }[2]{}\) \(\newcommand {\addtolength }[2]{}\) \(\newcommand {\setcounter }[2]{}\) \(\newcommand {\addtocounter }[2]{}\) \(\newcommand {\arabic }[1]{}\) \(\newcommand {\number }[1]{}\) \(\newcommand {\noalign }[1]{\text {#1}\notag \\}\) \(\newcommand {\cline }[1]{}\) \(\newcommand {\directlua }[1]{\text {(directlua)}}\) \(\newcommand {\luatexdirectlua }[1]{\text {(directlua)}}\) \(\newcommand {\protect }{}\) \(\def \LWRabsorbnumber #1 {}\) \(\def \LWRabsorbquotenumber "#1 {}\) \(\newcommand {\LWRabsorboption }[1][]{}\) \(\newcommand {\LWRabsorbtwooptions }[1][]{\LWRabsorboption }\) \(\def \mathchar {\ifnextchar "\LWRabsorbquotenumber \LWRabsorbnumber }\) \(\def \mathcode #1={\mathchar }\) \(\let \delcode \mathcode \) \(\let \delimiter \mathchar \) \(\def \oe {\unicode {x0153}}\) \(\def \OE {\unicode {x0152}}\) \(\def \ae {\unicode {x00E6}}\) \(\def \AE {\unicode {x00C6}}\) \(\def \aa {\unicode {x00E5}}\) \(\def \AA {\unicode {x00C5}}\) \(\def \o {\unicode {x00F8}}\) \(\def \O {\unicode {x00D8}}\) \(\def \l {\unicode {x0142}}\) \(\def \L {\unicode {x0141}}\) \(\def \ss {\unicode {x00DF}}\) \(\def \SS {\unicode {x1E9E}}\) \(\def \dag {\unicode {x2020}}\) \(\def \ddag {\unicode {x2021}}\) \(\def \P {\unicode {x00B6}}\) \(\def \copyright {\unicode {x00A9}}\) \(\def \pounds {\unicode {x00A3}}\) \(\let \LWRref \ref \) \(\renewcommand {\ref }{\ifstar \LWRref \LWRref }\) \( \newcommand {\multicolumn }[3]{#3}\) \(\require {textcomp}\) \(\newcommand {\intertext }[1]{\text {#1}\notag \\}\) \(\let \Hat \hat \) \(\let \Check \check \) \(\let \Tilde \tilde \) \(\let \Acute \acute \) \(\let \Grave \grave \) \(\let \Dot \dot \) \(\let \Ddot \ddot \) \(\let \Breve \breve \) \(\let \Bar \bar \) \(\let \Vec \vec \) \(\renewcommand {\vec }{\boldsymbol }\) \(\newcommand {\Edge }{\ensuremath {\,\textemdash \,}}\) \(\newcommand \Const [1]{\text {\textsf {#1}}}\) \(\DeclareMathOperator {\lerp }{lerp}\) \(\DeclareMathOperator {\bitlen }{bitlen}\) \(\DeclareMathOperator {\sign }{sign}\) \(\newcommand {\I }{\mathrm {i}}\) \(\newcommand \AND {\mathbin {\&}}\) \(\newcommand \OR {\mathbin {|}}\) \(\newcommand \XOR {\mathbin {{}^{\wedge }}}\) \(\newcommand \shl {\ll }\) \(\newcommand \shr {\ggg }\) \(\newcommand \asr {\gg }\) \(\newcommand \NOT {\ensuremath {\mathord {\sim }}}\) \(\newcommand {\isep }{\mathrel {{.}\,{.}}}\) \(\newcommand {\Id }[1]{\mathit {#1}}\) \(\newcommand {\const }[1]{\mathsf {#1}}\) \(\newcommand {\algorithmname }[1]{\text {\textsc {#1}}}\) \(\newcommand {\bits }[1]{\text {#1}}\) \(\newcommand {\hexa }[1]{\mathtt {0x#1}}\) \(\newcommand {\num }[1]{#1}\) \(\newcommand {\qed }{\quad \square }\) \(\newcommand {\idiv }[2]{\lfloor #1/#2\rfloor }\) \(\newcommand \attribdot {\ensuremath {\mkern 1.5mu.\mkern 1.5mu}}\) \(\newcommand \attribxr [2]{#1\attribdot \text {#2}}\) \(\newcommand \attribir [2]{\Id {#1}\attribdot \text {#2}}\) \(\newcommand \attribii [2]{\Id {#1}\attribdot \Id {#2}}\) \(\newcommand \textsc [1]{#1}\) \(\require {colortbl}\) \(\let \LWRorigcolumncolor \columncolor \) \(\renewcommand {\columncolor }[2][named]{\LWRorigcolumncolor [#1]{#2}\LWRabsorbtwooptions }\) \(\let \LWRorigrowcolor \rowcolor \) \(\renewcommand {\rowcolor }[2][named]{\LWRorigrowcolor [#1]{#2}\LWRabsorbtwooptions }\) \(\let \LWRorigcellcolor \cellcolor \) \(\renewcommand {\cellcolor }[2][named]{\LWRorigcellcolor [#1]{#2}\LWRabsorbtwooptions }\) \(\newcommand {\tcbset }[1]{}\) \(\newcommand {\tcbsetforeverylayer }[1]{}\) \(\newcommand {\tcbox }[2][]{\boxed {\text {#2}}}\) \(\newcommand {\tcboxfit }[2][]{\boxed {#2}}\) \(\newcommand {\tcblower }{}\) \(\newcommand {\tcbline }{}\) \(\newcommand {\tcbtitle }{}\) \(\newcommand {\tcbsubtitle [2][]{\mathrm {#2}}}\) \(\newcommand {\tcboxmath }[2][]{\boxed {#2}}\) \(\newcommand {\tcbhighmath }[2][]{\boxed {#2}}\) \(\newcommand {\toprule }[1][]{\hline }\) \(\let \midrule \toprule \) \(\let \bottomrule \toprule \) \(\def \LWRbooktabscmidruleparen (#1)#2{}\) \(\newcommand {\LWRbooktabscmidrulenoparen }[1]{}\) \(\newcommand {\cmidrule }[1][]{\ifnextchar (\LWRbooktabscmidruleparen \LWRbooktabscmidrulenoparen }\) \(\newcommand {\morecmidrules }{}\) \(\newcommand {\specialrule }[3]{\hline }\) \(\newcommand {\addlinespace }[1][]{}\) \(\newcommand {\LWRsubmultirow }[2][]{#2}\) \(\newcommand {\LWRmultirow }[2][]{\LWRsubmultirow }\) \(\newcommand {\multirow }[2][]{\LWRmultirow }\) \(\newcommand {\mrowcell }{}\) \(\newcommand {\mcolrowcell }{}\) \(\newcommand {\STneed }[1]{}\) \(\newcommand {\LWRldelimtwo }[1][]{\text {#1}~\LWRbigdelim }\) \(\newcommand {\LWRldelimone }[2][]{\LWRldelimtwo }\) \(\def \ldelim #1#2{\def \LWRbigdelim {#1}\LWRldelimone }\) \(\newcommand {\LWRrdelimtwo }[1][]{\LWRbigdelim ~\text {#1}}\) \(\newcommand {\LWRrdelimone }[2][]{\LWRrdelimtwo }\) \(\def \rdelim #1#2{\def \LWRbigdelim {#1}\LWRrdelimone }\) \(\let \symnormal \mathit \) \(\let \symliteral \mathrm \) \(\let \symbb \mathbb \) \(\let \symbbit \mathbb \) \(\let \symcal \mathcal \) \(\let \symscr \mathscr \) \(\let \symfrak \mathfrak \) \(\let \symsfup \mathsf \) \(\let \symsfit \mathit \) \(\let \symbfsf \mathbf \) \(\let \symbfup \mathbf \) \(\newcommand {\symbfit }[1]{\boldsymbol {#1}}\) \(\let \symbfcal \mathcal \) \(\let \symbfscr \mathscr \) \(\let \symbffrak \mathfrak \) \(\let \symbfsfup \mathbf \) \(\newcommand {\symbfsfit }[1]{\boldsymbol {#1}}\) \(\let \symup \mathrm \) \(\let \symbf \mathbf \) \(\let \symit \mathit \) \(\let \symsf \symsfit \) \(\let \symtt \mathtt \) \(\let \symbffrac \mathbffrac \) \(\newcommand {\mathfence }[1]{\mathord {#1}}\) \(\newcommand {\mathover }[1]{#1}\) \(\newcommand {\mathunder }[1]{#1}\) \(\newcommand {\mathaccent }[1]{#1}\) \(\newcommand {\mathbotaccent }[1]{#1}\) \(\newcommand {\mathalpha }[1]{\mathord {#1}}\) \(\def\Alpha{\unicode{x1D6E2}}\) \(\def\Beta{\unicode{x1D6E3}}\) \(\def\Gamma{\unicode{x1D6E4}}\) \(\def\Digamma{\mathit{\unicode{x03DC}}}\) \(\def\Delta{\unicode{x1D6E5}}\) \(\def\Epsilon{\unicode{x1D6E6}}\) \(\def\Zeta{\unicode{x1D6E7}}\) \(\def\Eta{\unicode{x1D6E8}}\) \(\def\Theta{\unicode{x1D6E9}}\) \(\def\Vartheta{\unicode{x1D6F3}}\) \(\def\Iota{\unicode{x1D6EA}}\) \(\def\Kappa{\unicode{x1D6EB}}\) \(\def\Lambda{\unicode{x1D6EC}}\) \(\def\Mu{\unicode{x1D6ED}}\) \(\def\Nu{\unicode{x1D6EE}}\) \(\def\Xi{\unicode{x1D6EF}}\) \(\def\Omicron{\unicode{x1D6F0}}\) \(\def\Pi{\unicode{x1D6F1}}\) \(\def\Rho{\unicode{x1D6F2}}\) \(\def\Sigma{\unicode{x1D6F4}}\) \(\def\Tau{\unicode{x1D6F5}}\) \(\def\Upsilon{\unicode{x1D6F6}}\) \(\def\Phi{\unicode{x1D6F7}}\) \(\def\Chi{\unicode{x1D6F8}}\) \(\def\Psi{\unicode{x1D6F9}}\) \(\def\Omega{\unicode{x1D6FA}}\) \(\def\alpha{\unicode{x1D6FC}}\) \(\def\beta{\unicode{x1D6FD}}\) \(\def\varbeta{\unicode{x03D0}}\) \(\def\gamma{\unicode{x1D6FE}}\) \(\def\digamma{\mathit{\unicode{x03DD}}}\) \(\def\delta{\unicode{x1D6FF}}\) \(\def\epsilon{\unicode{x1D716}}\) \(\def\varepsilon{\unicode{x1D700}}\) \(\def\zeta{\unicode{x1D701}}\) \(\def\eta{\unicode{x1D702}}\) \(\def\theta{\unicode{x1D703}}\) \(\def\vartheta{\unicode{x1D717}}\) \(\def\iota{\unicode{x1D704}}\) \(\def\kappa{\unicode{x1D705}}\) \(\def\varkappa{\unicode{x1D718}}\) \(\def\lambda{\unicode{x1D706}}\) \(\def\mu{\unicode{x1D707}}\) \(\def\nu{\unicode{x1D708}}\) \(\def\xi{\unicode{x1D709}}\) \(\def\omicron{\unicode{x1D70A}}\) \(\def\pi{\unicode{x1D70B}}\) \(\def\varpi{\unicode{x1D71B}}\) \(\def\rho{\unicode{x1D70C}}\) \(\def\varrho{\unicode{x1D71A}}\) \(\def\sigma{\unicode{x1D70E}}\) \(\def\varsigma{\unicode{x1D70D}}\) \(\def\tau{\unicode{x1D70F}}\) \(\def\upsilon{\unicode{x1D710}}\) \(\def\phi{\unicode{x1D719}}\) \(\def\varphi{\unicode{x1D711}}\) \(\def\chi{\unicode{x1D712}}\) \(\def\psi{\unicode{x1D713}}\) \(\def\omega{\unicode{x1D714}}\) \(\def\upAlpha{\unicode{x0391}}\) \(\def\upBeta{\unicode{x0392}}\) \(\def\upGamma{\unicode{x0393}}\) \(\def\upDigamma{\unicode{x03DC}}\) \(\def\upDelta{\unicode{x0394}}\) \(\def\upEpsilon{\unicode{x0395}}\) \(\def\upZeta{\unicode{x0396}}\) \(\def\upEta{\unicode{x0397}}\) \(\def\upTheta{\unicode{x0398}}\) \(\def\upVartheta{\unicode{x03F4}}\) \(\def\upIota{\unicode{x0399}}\) \(\def\upKappa{\unicode{x039A}}\) \(\def\upLambda{\unicode{x039B}}\) \(\def\upMu{\unicode{x039C}}\) \(\def\upNu{\unicode{x039D}}\) \(\def\upXi{\unicode{x039E}}\) \(\def\upOmicron{\unicode{x039F}}\) \(\def\upPi{\unicode{x03A0}}\) \(\def\upVarpi{\unicode{x03D6}}\) \(\def\upRho{\unicode{x03A1}}\) \(\def\upSigma{\unicode{x03A3}}\) \(\def\upTau{\unicode{x03A4}}\) \(\def\upUpsilon{\unicode{x03A5}}\) \(\def\upPhi{\unicode{x03A6}}\) \(\def\upChi{\unicode{x03A7}}\) \(\def\upPsi{\unicode{x03A8}}\) \(\def\upOmega{\unicode{x03A9}}\) \(\def\itAlpha{\unicode{x1D6E2}}\) \(\def\itBeta{\unicode{x1D6E3}}\) \(\def\itGamma{\unicode{x1D6E4}}\) \(\def\itDigamma{\mathit{\unicode{x03DC}}}\) \(\def\itDelta{\unicode{x1D6E5}}\) \(\def\itEpsilon{\unicode{x1D6E6}}\) \(\def\itZeta{\unicode{x1D6E7}}\) \(\def\itEta{\unicode{x1D6E8}}\) \(\def\itTheta{\unicode{x1D6E9}}\) \(\def\itVartheta{\unicode{x1D6F3}}\) \(\def\itIota{\unicode{x1D6EA}}\) \(\def\itKappa{\unicode{x1D6EB}}\) \(\def\itLambda{\unicode{x1D6EC}}\) \(\def\itMu{\unicode{x1D6ED}}\) \(\def\itNu{\unicode{x1D6EE}}\) \(\def\itXi{\unicode{x1D6EF}}\) \(\def\itOmicron{\unicode{x1D6F0}}\) \(\def\itPi{\unicode{x1D6F1}}\) \(\def\itRho{\unicode{x1D6F2}}\) \(\def\itSigma{\unicode{x1D6F4}}\) \(\def\itTau{\unicode{x1D6F5}}\) \(\def\itUpsilon{\unicode{x1D6F6}}\) \(\def\itPhi{\unicode{x1D6F7}}\) \(\def\itChi{\unicode{x1D6F8}}\) \(\def\itPsi{\unicode{x1D6F9}}\) \(\def\itOmega{\unicode{x1D6FA}}\) \(\def\upalpha{\unicode{x03B1}}\) \(\def\upbeta{\unicode{x03B2}}\) \(\def\upvarbeta{\unicode{x03D0}}\) \(\def\upgamma{\unicode{x03B3}}\) \(\def\updigamma{\unicode{x03DD}}\) \(\def\updelta{\unicode{x03B4}}\) \(\def\upepsilon{\unicode{x03F5}}\) \(\def\upvarepsilon{\unicode{x03B5}}\) \(\def\upzeta{\unicode{x03B6}}\) \(\def\upeta{\unicode{x03B7}}\) \(\def\uptheta{\unicode{x03B8}}\) \(\def\upvartheta{\unicode{x03D1}}\) \(\def\upiota{\unicode{x03B9}}\) \(\def\upkappa{\unicode{x03BA}}\) \(\def\upvarkappa{\unicode{x03F0}}\) \(\def\uplambda{\unicode{x03BB}}\) \(\def\upmu{\unicode{x03BC}}\) \(\def\upnu{\unicode{x03BD}}\) \(\def\upxi{\unicode{x03BE}}\) \(\def\upomicron{\unicode{x03BF}}\) \(\def\uppi{\unicode{x03C0}}\) \(\def\upvarpi{\unicode{x03D6}}\) \(\def\uprho{\unicode{x03C1}}\) \(\def\upvarrho{\unicode{x03F1}}\) \(\def\upsigma{\unicode{x03C3}}\) \(\def\upvarsigma{\unicode{x03C2}}\) \(\def\uptau{\unicode{x03C4}}\) \(\def\upupsilon{\unicode{x03C5}}\) \(\def\upphi{\unicode{x03D5}}\) \(\def\upvarphi{\unicode{x03C6}}\) \(\def\upchi{\unicode{x03C7}}\) \(\def\uppsi{\unicode{x03C8}}\) \(\def\upomega{\unicode{x03C9}}\) \(\def\italpha{\unicode{x1D6FC}}\) \(\def\itbeta{\unicode{x1D6FD}}\) \(\def\itvarbeta{\unicode{x03D0}}\) \(\def\itgamma{\unicode{x1D6FE}}\) \(\def\itdigamma{\mathit{\unicode{x03DD}}}\) \(\def\itdelta{\unicode{x1D6FF}}\) \(\def\itepsilon{\unicode{x1D716}}\) \(\def\itvarepsilon{\unicode{x1D700}}\) \(\def\itzeta{\unicode{x1D701}}\) \(\def\iteta{\unicode{x1D702}}\) \(\def\ittheta{\unicode{x1D703}}\) \(\def\itvartheta{\unicode{x1D717}}\) \(\def\itiota{\unicode{x1D704}}\) \(\def\itkappa{\unicode{x1D705}}\) \(\def\itvarkappa{\unicode{x1D718}}\) \(\def\itlambda{\unicode{x1D706}}\) \(\def\itmu{\unicode{x1D707}}\) \(\def\itnu{\unicode{x1D708}}\) \(\def\itxi{\unicode{x1D709}}\) \(\def\itomicron{\unicode{x1D70A}}\) \(\def\itpi{\unicode{x1D70B}}\) \(\def\itvarpi{\unicode{x1D71B}}\) \(\def\itrho{\unicode{x1D70C}}\) \(\def\itvarrho{\unicode{x1D71A}}\) \(\def\itsigma{\unicode{x1D70E}}\) \(\def\itvarsigma{\unicode{x1D70D}}\) \(\def\ittau{\unicode{x1D70F}}\) \(\def\itupsilon{\unicode{x1D710}}\) \(\def\itphi{\unicode{x1D719}}\) \(\def\itvarphi{\unicode{x1D711}}\) \(\def\itchi{\unicode{x1D712}}\) \(\def\itpsi{\unicode{x1D713}}\) \(\def\itomega{\unicode{x1D714}}\) \(\let \lparen (\) \(\let \rparen )\) \(\newcommand {\cuberoot }[1]{\,{}^3\!\!\sqrt {#1}}\,\) \(\newcommand {\fourthroot }[1]{\,{}^4\!\!\sqrt {#1}}\,\) \(\newcommand {\longdivision }[1]{\mathord {\unicode {x027CC}#1}}\) \(\newcommand {\mathcomma }{,}\) \(\newcommand {\mathcolon }{:}\) \(\newcommand {\mathsemicolon }{;}\) \(\newcommand {\overbracket }[1]{\mathinner {\overline {\ulcorner {#1}\urcorner }}}\) \(\newcommand {\underbracket }[1]{\mathinner {\underline {\llcorner {#1}\lrcorner }}}\) \(\newcommand {\overbar }[1]{\mathord {#1\unicode {x00305}}}\) \(\newcommand {\ovhook }[1]{\mathord {#1\unicode {x00309}}}\) \(\newcommand {\ocirc }[1]{\mathord {#1\unicode {x0030A}}}\) \(\newcommand {\candra }[1]{\mathord {#1\unicode {x00310}}}\) \(\newcommand {\oturnedcomma }[1]{\mathord {#1\unicode {x00312}}}\) \(\newcommand {\ocommatopright }[1]{\mathord {#1\unicode {x00315}}}\) \(\newcommand {\droang }[1]{\mathord {#1\unicode {x0031A}}}\) \(\newcommand {\leftharpoonaccent }[1]{\mathord {#1\unicode {x020D0}}}\) \(\newcommand {\rightharpoonaccent }[1]{\mathord {#1\unicode {x020D1}}}\) \(\newcommand {\vertoverlay }[1]{\mathord {#1\unicode {x020D2}}}\) \(\newcommand {\leftarrowaccent }[1]{\mathord {#1\unicode {x020D0}}}\) \(\newcommand {\annuity }[1]{\mathord {#1\unicode {x020E7}}}\) \(\newcommand {\widebridgeabove }[1]{\mathord {#1\unicode {x020E9}}}\) \(\newcommand {\asteraccent }[1]{\mathord {#1\unicode {x020F0}}}\) \(\newcommand {\threeunderdot }[1]{\mathord {#1\unicode {x020E8}}}\) \(\newcommand {\Bbbsum }{\mathop {\unicode {x2140}}\limits }\) \(\newcommand {\oiint }{\mathop {\unicode {x222F}}\limits }\) \(\newcommand {\oiiint }{\mathop {\unicode {x2230}}\limits }\) \(\newcommand {\intclockwise }{\mathop {\unicode {x2231}}\limits }\) \(\newcommand {\ointclockwise }{\mathop {\unicode {x2232}}\limits }\) \(\newcommand {\ointctrclockwise }{\mathop {\unicode {x2233}}\limits }\) \(\newcommand {\varointclockwise }{\mathop {\unicode {x2232}}\limits }\) \(\newcommand {\leftouterjoin }{\mathop {\unicode {x27D5}}\limits }\) \(\newcommand {\rightouterjoin }{\mathop {\unicode {x27D6}}\limits }\) \(\newcommand {\fullouterjoin }{\mathop {\unicode {x27D7}}\limits }\) \(\newcommand {\bigbot }{\mathop {\unicode {x27D8}}\limits }\) \(\newcommand {\bigtop }{\mathop {\unicode {x27D9}}\limits }\) \(\newcommand {\xsol }{\mathop {\unicode {x29F8}}\limits }\) \(\newcommand {\xbsol }{\mathop {\unicode {x29F9}}\limits }\) \(\newcommand {\bigcupdot }{\mathop {\unicode {x2A03}}\limits }\) \(\newcommand {\bigsqcap }{\mathop {\unicode {x2A05}}\limits }\) \(\newcommand {\conjquant }{\mathop {\unicode {x2A07}}\limits }\) \(\newcommand {\disjquant }{\mathop {\unicode {x2A08}}\limits }\) \(\newcommand {\bigtimes }{\mathop {\unicode {x2A09}}\limits }\) \(\newcommand {\modtwosum }{\mathop {\unicode {x2A0A}}\limits }\) \(\newcommand {\sumint }{\mathop {\unicode {x2A0B}}\limits }\) \(\newcommand {\intbar }{\mathop {\unicode {x2A0D}}\limits }\) \(\newcommand {\intBar }{\mathop {\unicode {x2A0E}}\limits }\) \(\newcommand {\fint }{\mathop {\unicode {x2A0F}}\limits }\) \(\newcommand {\cirfnint }{\mathop {\unicode {x2A10}}\limits }\) \(\newcommand {\awint }{\mathop {\unicode {x2A11}}\limits }\) \(\newcommand {\rppolint }{\mathop {\unicode {x2A12}}\limits }\) \(\newcommand {\scpolint }{\mathop {\unicode {x2A13}}\limits }\) \(\newcommand {\npolint }{\mathop {\unicode {x2A14}}\limits }\) \(\newcommand {\pointint }{\mathop {\unicode {x2A15}}\limits }\) \(\newcommand {\sqint }{\mathop {\unicode {x2A16}}\limits }\) \(\newcommand {\intlarhk }{\mathop {\unicode {x2A17}}\limits }\) \(\newcommand {\intx }{\mathop {\unicode {x2A18}}\limits }\) \(\newcommand {\intcap }{\mathop {\unicode {x2A19}}\limits }\) \(\newcommand {\intcup }{\mathop {\unicode {x2A1A}}\limits }\) \(\newcommand {\upint }{\mathop {\unicode {x2A1B}}\limits }\) \(\newcommand {\lowint }{\mathop {\unicode {x2A1C}}\limits }\) \(\newcommand {\bigtriangleleft }{\mathop {\unicode {x2A1E}}\limits }\) \(\newcommand {\zcmp }{\mathop {\unicode {x2A1F}}\limits }\) \(\newcommand {\zpipe }{\mathop {\unicode {x2A20}}\limits }\) \(\newcommand {\zproject }{\mathop {\unicode {x2A21}}\limits }\) \(\newcommand {\biginterleave }{\mathop {\unicode {x2AFC}}\limits }\) \(\newcommand {\bigtalloblong }{\mathop {\unicode {x2AFF}}\limits }\) \(\newcommand {\arabicmaj }{\mathop {\unicode {x1EEF0}}\limits }\) \(\newcommand {\arabichad }{\mathop {\unicode {x1EEF1}}\limits }\)

B.3 Data Classes¶

Data classes are classes that are primarily used for storing data. In contrast to most other classes, data classes don’t contain nontrivial logic or internal state. Here is a simple example of a data class that stores a person’s name in a field of type String and their age in a field of type int:

class Person {
    final String name;
    int age;

    Person(String name, int age) {
        this.name = name;
        this.age = age;
    }
}

By default, classes and their fields and methods can only be accessed from code in the same package. To change this default visibility, the two access modifiers public and private can be used. In the following example, we use public to make Person and its constructor visible to other packages and private to hide its two fields:

public class Person {
    private final String name;
    private int age;
  
    public Person(String name, int age) {
        this.name = name;
        this.age = age;
    }
  
    ...
}

It is common practice to declare all fields of public classes as private and to define specialized accessor methods to allow selective access to the fields.

public String name() { return name; }
public String age() { return age; }
public String setAge(int age) { 
    if (age < 0)
        throw new IllegalArgumentException();
    this.age = age; 
}

The two methods name() and age() simply return the current values of the corresponding field; such methods are known as getter methods or simply getters. The setAge() methods can be used to change the value of the age field; such methods are known as setter methods or setters.

All classes in Java are derived from a common base class called Object, which defines several methods that you may want or need to override. The first is toString(), which produces a string representation of an object:

public String toString() {
    return String.format("Person(\"%s\", %d)", name, age));
}

toString() is often used for debugging, so its output should usually include the values of all fields.

For objects that are meant to be stored in containers such as hash tables and hash sets, you probably also need to override equals() and hashCode(). The purpose of equals() is to compare this to another object and return whether they are equal. In the case of data classes, two objects are generally considered equal if they have the same type and all fields have the same values:

public boolean equals(Object other) {
    return (other instanceof Person o)
        && name.equals(o.name) && age == o.age;
}

Here, we use String.equals() to compare the contents of the two name fields and the equality operator “==” to compare the values of the age fields.

In addition, objects that are used as keys in hash tables must override the hashCode() method, which computes an object’s hash code of type int:

public int hashCode() {
    return Objects.hash(person, age);
}

JDK: Objects

Classes that override equals() must usually also override hashCode() and vice versa. Containers such as HashMap that are based on hash tables require that hashCode() is consistent with equals() in the sense that two objects that are equal must also have the same hash code.²

² There are a few additional subtleties regarding the implementation of toString(), equals(), and hashCode(); for details, see [17, Item 10–12].

Records

Starting with Java 14, records provide much simpler way of defining data classes. With records, a Person class similar to the one discussed above can be defined as follows:

public record Person(String name, int age) {
}

The fields of a record are declared by listing them in parentheses directly after the record’s name. From this definition, Java automatically generates a class with the following properties:

• The specified fields are defined as final and have package-internal visibility.
• There is one constructor that takes the initial values of all fields as its arguments.
• The class provides implementations of toString(), equals(), and hashCode() that are equivalent to the hand-written versions shown above.
• Public getters are defined for all declared fields, here name() and age(). There are no corresponding setter methods — all fields are implicitly final, so records are immutable.

If needed, you can provide additional constructors, override methods such as toString() or equals(), and even define your own methods. Records can also implement one or more interfaces.

Apart from the way they are defined, records behave like ordinary classes:

var person = new Person("Einstein", 1879);
System.out.printf("%s was born in %d\n", person.name(),
    person.yearOfBirth());

There are a few important restrictions, however. Unlike normal classes, records are always immutable since all its fields are implicitly declared as final and no setter methods are provided. In addition, records cannot extend other classes or used as base classes themselves.