每天几条java题（2）

 Question: 6

Given:
1. import java.util.*;
2. public class Old {
3. public static Object get0(List list) {
4. return list.get(0);
5. }
6. }
Which three will compile successfully? (Choose three.)
A. Object o = Old.get0(new LinkedList());
B. Object o = Old.get0(new LinkedList<?>());
C. String s = Old.get0(new LinkedList<String>());
D. Object o = Old.get0(new LinkedList<Object>());
E. String s = (String)Old.get0(new LinkedList<String>());

Answer: A, D, E

题目分析：

C没有强制转换。

？是不确定类的时候用，但是不确定是只在子类型父类型里面应用，例如list<? Extends X>,那么这里就可以是X或者其子类，list的泛型实际类型就是初始化时候的那个类型。

范例代码（来源于网上）

import  java.util.ArrayList;

import  java.util.List;

 

class  Parent  {

      //  your code

     }

class  Child  extends  Parent {

//  your code

}

public   class  test  {

        // 这里更改一下，利用问号解决问题  

     public   static   void  doTest(List <?   extends  Parent >  list) {

       

   }

     public   static   void  main(String[] args) {

       List < Parent >  parentList  =   new  ArrayList < Parent > ();

       List < Child >  childList  =   new  ArrayList < Child > ();

       doTest(parentList);

        // 注意这里编译正确        

         doTest(childList);    }

}

 

Question: 7
Given:
1. import java.util.*;
2. public class Example {
3. public static void main(String[] args) {
4. // insert code here
5. set.add(new Integer(2));
6. set.add(new Integer(1));
7. System.out.println(set);
8. }
9. }
Which code, inserted at line 4, guarantees that this program will output [1, 2]?
A. Set set = new TreeSet();
B. Set set = new HashSet();
C. Set set = new SortedSet();
D. List set = new SortedList();
E. Set set = new LinkedHashSet();

Answer: A

题解：

首先不用想A肯定是对的，sortedset，之前也说过，接口。还有java是没有sortedlist的，你要用collection.sort来排序，为啥这样设计？？Stackoverflow上面有人问过。

Linkedhashset跟插入顺序有关，错。

至于Hashset，我也不懂，但是把代码写过去，hashset是输出[1,2]的。

个人认为答案为A，B，特意在stackoverflow上面找到这么一段东西：

A hashcode is an integer value that represents the state of the object upon which it was called. That is why an Integer that is set to 1 will return a hashcode of "1" because an Integer's hashcode and its value are the same thing. A character's hashcode is equal to it's ASCII character code. If you write a custom type you are responsible for creating a good hashCode implementation that will best represent the state of the current instance.

然后引申出一个新的问题：hashset是真的无序的么？至少，对于Integer来说它不是无序的。