檢視 SCJP 1.6版考題 204 的原始碼

Given: 
<br>11. &nbsp;public class Person{ 
<br>12. &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp;private name; 
<br>13. &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp;public Person(String name){ 
<br>14. &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; this.name = name; <br>15. &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp;} 
<br>16. &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp;public int hashCode(){ 
<br>17. &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; return 420; <br>18. &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp;} 
<br>19. } 

<br>Which statement is true? //那一個敘述是正確的？ <br> 
<br>&nbsp; A. The time to find the value from HashMap with a Person key depends on the size of the map. <br>&nbsp; &nbsp; &nbsp; 用一個Person的鍵值從HashMap 找一筆資料的時間取決於map的大小。 
<br>&nbsp; B. Deleting a Person key from a HashMap will delete all map entries for all keys of type Person. <br>&nbsp; &nbsp; &nbsp;從HashMap 中刪除掉一個Person的鍵值將會刪除對型態為Person的全部鍵值的所有對映(map)的入口。 
<br>&nbsp; C. Inserting a second Person object into a HashSet will cause the first Person object to be removed as a duplicate. 
<br>&nbsp; &nbsp; 插入一個第二個Person物件到HashSet中將會導致第一個Person物件被當成重複而被移除。 
<br>&nbsp; D. The time to determine whether a Person object is contained in a HashSet is constant and does NOT depend on the size of the map. <br>&nbsp; &nbsp;決定一個Person物件是否存在於一個HashSet的時間是一個常數時間，並且不是取決於map的大小。 

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

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

<span style="font-size:medium;"> 解說:
雜湊是用HashCode(雜湊值)來當成map的入口索引，使用湊雜的原則是不同的物件算出來的雜湊值要儘量避免一樣， 
<BR>&nbsp; &nbsp; &nbsp; &nbsp; &nbsp;如果一樣，也就代表不同的物件其map中的索引入口一樣，這些不同的物件但因雜湊值一樣， 
<BR>&nbsp; &nbsp; &nbsp; &nbsp; &nbsp;會被map以線性的方式串起來，也就是說，若要找一個物件，是計算其雜湊值找出map的入口(entry)， 
<BR>&nbsp; &nbsp; &nbsp; &nbsp; &nbsp;如果這個入口的物件不只一個，那就要用線性搜尋的方式一個一個去找。 
<BR>&nbsp; &nbsp; &nbsp; &nbsp; &nbsp;在這個題目，Person的鍵值(HashCode)產生都是常數值，420，這樣會導致所有的Person物件都會被串到420這個map入口， 
<BR>&nbsp; &nbsp; &nbsp; &nbsp; &nbsp;尋找物件是否存於這個map的時間要看有多少的物件在裏面，也就是說，找尋的時間跟map的大小有關。 <br> 
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{{SCJP 1.6版考題講解}}