Java讲义5.ppt

入门 第五讲类 继承 作业4 Card javaConstructorprintOutPlayer javaConstructorplayFirst play winTurn Belote javamain Card类 publicclassCard intvalue Stringname intsuit Card构造函数 publicclassCard publicintvalue publicStringname publicintsuit publicCard intv Strings intn value v name s suit n Cardprintout publicclassCard publicintvalue publicStringname publicintsuit publicCard intv Strings intn value v name s suit n publicprintOut System out println value name suit Player类 publicclassPlayer publicStringname publicintscore publicintcounter publicCard cards Player构造函数 publicclassPlayer publicStringname publicintscore publicintcounter publicCard cards publicPlayer Strings name s cards newCard 13 score 0 counter 0 PlayeraddCard publicclassPlayer publicStringname publicintscore publicintcounter publicCard cards publicCardaddCard Cardc cards counter c counter Belote类 publicclassBelote staticCardcards staticPlayerplayers Belote构造函数 publicclassBelote staticCardcards staticPlayerplayers publicBelote createplayersplayers newPlayer 4 players 0 newPlayer Adam players 1 newPlayer Bob players 2 newPlayer Charlie players 3 newPlayer Daniel 卡片排列 publicclassBelote staticCardcards staticPlayerplayers publicBelote intcount 0 distributecardsfor inti 0 i 4 i for intj 1 j 13 j players i addCard cards count count Belotemain publicclassBelote staticCardcards staticPlayerplayers staticpublicvoidmain String args createanewinstanceofBeloteBelotegame newBelote publicclassBelote staticCardcards staticPlayerplayers staticpublicvoidmain String args createanewinstanceofBeloteBelotegame newBelote loopthroughplayersfor inti 0 i 4 i for intj 0 j 13 j players i cards j printOut 空对象 空对象意味着什么都没有 删掉数组中的一个元素很容易 但是也意味着没有办法挽回 publicclassPlayer publicStringname publicintscore publicintcounter publicCard cards publicCardplayFirst if counter 0 returnnull else counter returncards 0 错误 cards 0 maybenull publicclassPlayer publicCardplayFirst for inti 0 i cards length i if cards i null counter cards i null returncards i 错误 returnnull publicclassPlayer publicStringname publicintscore publicintcounter publicCard cards publicCardplayFirst for inti 0 i cards length i if cards i null counter CardreturnedCard cards i cards i null returnreturnedCard 正确 returnnull 空指针异常 publicclassPlayer publicCardplayFirst for inti 0 i cards length i System out println cards i suit WRONG 当访问对象cards i 的变量suit的时候 cards i 可能是空对象 空指针异常 publicclassPlayer publicCardplayFirst for inti 0 i cards length i if cards i null System out println cards i suit ok publicclassPlayer publicStringname publicintscore publicintcounter publicCard cards publicCardplay intsuit for inti 0 i cards length i if cards i null publicclassBelote staticCardcards staticPlayerplayers staticpublicvoidmain String args createanewinstanceofBeloteBelotegame newBelote playaslongasplayershavecardsintdone 0 while done 0 CardpCards newCard 4 pCards 0 players 0 playFirst intsuit pCard 0 suit 错误pCards 1 players 1 play suit pCards 2 players 2 play suit pCards 3 players 3 play suit if playedCards 0 null done 1 staticCardcards staticPlayerplayers staticpublicvoidmain String args createanewinstanceofBeloteBelotegame newBelote playaslongasplayershavecardsintdone 0 while done 0 CardpCards newCard 4 pCards 0 players 0 playFirst if pCards 0 null intsuit pCard 0 suit OKpCards 1 players 1 play suit else done 1 完成一轮 publicclassPlayer publicStringname publicintscore publicintcounter publicCard cards publicvoidwinTurn Cardc1 Cardc2 Cardc3 Cardc4 score c1 value c2 value c3 value c4 value 作业4 总结 Makelittlesteps检查空指针注意实例和对象的区别 实例和对象 Bicyclebike1 newBicycle Bicyclebike2 newBicycle 实例和对象 Bicyclebike1 newBicycle Bicyclebike2 bike1 抽象 对象是抽象的工具抽象是计算机科学中的一个基本概念 抽象 在Java中 对象是类的一个实例类中包含变量和方法 这些是对象的本质对象使用new进行实例化 抽象为什么很重要 模块化 将一个大问题细分为一个个的小问题 很好的展现了现实世界的问题 继承 在生活中 我们继承了父母的特征在计算机科学中 对象从父类继承了变量和方法 继承 当你在Java中定义一个类时 你也可以将它定义为另一类的子类例如 类Bike Car和Motorcycle可以作为类Vehicle的子类 为什么要继承 避免代码的重复增强程序的模块化 重复的代码 publicclassBike publicintspeed publicvoidspeedUp ints speed s publicclassCar publicintspeed publicvoidspeedUp ints speed s 模块化 继承规则 子类继承父类所有的变量和方法使用关键字extends来表明一个类是从另一个类继承而来在子类的构造函数中使用关键字super调用父类的构造函数 案例 publicclassVehicle publicintnWheels numberofwheelspublicVehicle intn nWheels n intgetNWheels returnnWheels 案例 publicclassCarextendsVehicle publicCar intn super n publicvoidopenWindow 案例 publicclassBicycleextendsVehicle publicBicycle intn super