檢視 SCJP 1.6版考題 206 的原始碼

Given: 
<br>1. &nbsp; public class Drink implements Comparable{ 
<br>2. &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; public String name; 
<br>3. &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; public int compareTo(Object o){ 
<br>4. &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; return 0; <br>5. &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; } 
<br>6. &nbsp;} 
<br>and: 
<br>20. &nbsp;Drink one = new Drink(); 
<br>21. &nbsp;Drink two = new Drink(); 
<br>22. &nbsp;one.name = "Coffee"; 
<br>23. &nbsp;two.name = "Tea"; 
<br>24. &nbsp;TreeSet set = new TreeSet(); 
<br>25. &nbsp;set.add(one); <br>26. &nbsp;set.add(two); 
<br> 

A programmer iterates over the TreeSet and prints the name of each Drink object. 

一個程式設計走訪上面的TreeSet(一個物件集合)並且印出每一個Drink物件的name。 <br> <br>What is the result? 結果是什麼？ <br>

<br>&nbsp; A. Tea 
<br>&nbsp; B. Coffee 
<br>&nbsp; C. Coffee Tea 
<br>&nbsp; D. Compilation fails. 
<br>&nbsp; E. The code runs with no output. 
<br>&nbsp; F. An exception is thrown at runtime. 
<br> 


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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:
TreeSet是一個沒有重複物件的集合，類別Drink實作了compareTo方法(這個方法是用來比較2個物件是否一樣，或大或小)， 

&nbsp; &nbsp; &nbsp; &nbsp; &nbsp;這個方法固定傳回0(0代表2個物件比較是相同，實際上沒比較)， 

&nbsp; &nbsp; &nbsp; &nbsp; &nbsp;因此當把二個物件one和two加到TreeSet這個集合時，會使用compareTo來比較集合中是否已經有"相同"的物件在裏面， 

&nbsp; &nbsp; &nbsp; &nbsp; &nbsp;因此，只有one加入到集合中(第一個)，當two要加入到集合中，因為集合加入的方法用compareTo判斷已經有相同的物件one在集合中， 

&nbsp; &nbsp; &nbsp; &nbsp; &nbsp;two物件就沒有加入集合中。 
<br>&nbsp; &nbsp; &nbsp; &nbsp; &nbsp;因此，若印出集合中所有的物件，就只有one一個，而one物件的名稱就是Coffee。 
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