檢視 SCJP 1.6版考題 211 的原始碼

Given: 
<br>11. &nbsp; public class Key{ 
<br>12. &nbsp; &nbsp; &nbsp; &nbsp; &nbsp;private long id1, 
<br>13. &nbsp; &nbsp; &nbsp; &nbsp; &nbsp;private long id2; 
<br>14. 
<br>15. &nbsp; &nbsp; &nbsp; &nbsp; //class Key methods 
<br>16. } 

<br>A programmer is developing a class Key, that will be used as a key in a standard java.util.HashMap. 
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Which two methods should be overridden to assure that Key works correctly as a key? (Choose two.) 
<br>&nbsp; &nbsp;A. public int hashCode() 
<br>&nbsp; &nbsp;B. public void hashCode() 
<br>&nbsp; &nbsp;C. public int compareTo(Object o) 
<br>&nbsp; &nbsp;D. public boolean equals(Object o) 
<br>&nbsp; &nbsp;E. public boolean compareTo(Key k) 
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<span style="font-size:medium;">解答</span>

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

<span style="font-size:medium;"> 解說:
此題問說，一個程式設計師正在開發一個類別Key，這個類別將當成一個鍵值使用於 java.util.HashMap中， 
&nbsp; &nbsp; &nbsp; &nbsp; &nbsp;那二個方法應該被覆寫以保證Key類別以鍵值的形式正確地運作？ 
<br> &nbsp; &nbsp; &nbsp; &nbsp; &nbsp;一個物件被放在HashMap中，是靠著一個鍵值來進行物件的索引。 
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&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; 
<br>&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; hashCode 用來計算一個物件的雜湊值 
<br>&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; equals 用來比較二個物件的內含是否一樣 
<br> <br>&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; 一般來說，會先用hashCode計算雜湊值以索引出物件在Map中的位置，由於不同的物件有可能會有相同的雜湊值，
<br>&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; 若發生此種現象(碰撞)，則必須再用更精確的比較方法equals來找出在Map中的物件。 <br> 
<br>&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; 雜湊在各位的資料結構課程中會有一些概念的解說，不妨可以回頭看一下資料結構中對雜湊的介紹。 
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{{SCJP 1.6版考題講解}}