檢視 SCJP 1.6版考題 200 的原始碼

Given: 
<br>11. &nbsp;public class Person{ 
<br>12. &nbsp; &nbsp; &nbsp; private String name; 
<br>13. &nbsp; &nbsp; &nbsp; public Person(String name){ 
<br>14. &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; this.name = name; 
<br>15. &nbsp; &nbsp; &nbsp; &nbsp;} 
<br>16. &nbsp; &nbsp; &nbsp; public boolean equals(Object o){ 
<br>17. &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp;if(!(o instanceof Person)) return false; <br>18. &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp;Person p = (Person)o; 
<br>19. &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp;return p.name.equals(this.name);
<br>20. &nbsp; &nbsp; &nbsp; &nbsp;} <br>21. &nbsp; } 

<br>Which statement is true? <br> 
<br>&nbsp; A. Compilation fails because the hashCode method is not overridden. 
<br>&nbsp; B. A HashSet could contain multiple Person objects with the same name. 
<br>&nbsp; C. All Person objects will have the same hash code because the hashCode method is not overridden. <br>&nbsp; D. If a HashSet contains more than one Person object with name="Fred", then removing another Person, also with name="Fred", will remove them all. 

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<span style="font-size:medium;">解答</span>

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<span style="font-size: medium;">Ans: B   </span>

<span style="font-size:medium;"> 解說:
&nbsp;&nbsp;B. A HashSet could contain multiple Person objects with the same name. 
<br>&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; HashSet不會有重複的元素，但前題類別必須重寫hashCode與equals提供元素間的比較方法 <br>&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; Person類別只提供equals方法，沒有提供hashCode方法… <br>&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; 二個物件會先用hashCode()方法傳回的值是否相同，如果相同，則再使用equals()方法比較，如果兩者都相同，則視為相同的物件。 <br> <br> 
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{{SCJP 1.6版考題講解}}