100 Recipes for Programming Java Learn Java Today

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关键词：Programming Program Recipes Recipe Today

1. Solution
   
2. There are several differences between HashMap
   
3. and Hashtable in Java:
   
4. 1. Hashtable is synchronized, whereas HashMap is
   
5. not. This makes HashMap better for nonthreaded
   
6. applications, as unsynchronized
   
7. Objects typically perform better than
   
8. synchronized ones.
   
9. 2. Hashtable does not allow null keys or values.
   
10. HashMap allows one null key and any number of
    
11. null values.
    
12. 3. One of HashMap's subclasses is LinkedHashMap,
    
13. so in the event that you'd want predictable
    
14. iteration order (which is insertion order
    
15. by default), you could easily swap out the
    
16. HashMap for a LinkedHashMap. This wouldn't be
    
17. as easy if you were using Hashtable.
    
18. Since synchronization is not an issue for you,
    
19. I'd recommend HashMap. If synchronization
    
20. becomes an issue, you may also look at
    
21. ConcurrentHashMap.

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1. Solution
   
2. Here is code for secure, easy, but a little bit
   
3. more expensive session identifiers.
   
4. import java.security.SecureRandom;
   
5. import java.math.BigInteger;
   
6. public final class SessionIdentifierGenerator {
   
7. private SecureRandom random = new SecureRandom();
   
8. public String nextSessionId() {
   
9. return new BigInteger(130, random).toString(32);
   
10. }
    
11. }

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