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Question
why HashTable not allow null key and value
 Question Submitted By :: Core-Java
I also faced this Question!!     Answer Posted By  
 
Answer
# 1
To successfully store and retrieve objects from a hashtable,
the objects used
as keys must implement the hashCode method and the equals
method.

In a nutshell, since null isn't an object, you can't call
.equals() or .hashCode() on it, so the Hashtable can't
compute a hash to use it as a key.

HashMap is newer, and has more advanced capabilities, which
are basically just an improvement on the Hashtable
functionality. As such, when HashMap was created, it was
specifically designed to handle null values as keys and
handles them as a special case.

Specifically, the use of null as a key is handled like this
when issuing a .get(key):

(key==null ? k==null : key.equals(k))
 
Is This Answer Correct ?    4 Yes 0 No
Eknath
 
Answer
# 2
There is null check in the put method implementation of
hashtable, so it does not support null values and null keys.


public Object put(Object key, Object value) {
// Make sure the value is not null
if (value == null) throw new NullPointerException();
}

above is HashTable put method logic implemented by Sun.
 
Is This Answer Correct ?    4 Yes 0 No
Naveen
 
 
 

 
 
 
Other Core Java Interview Questions
 
  Question Asked @ Answers
 
how we can write the string concatenation program in java.   3
What modifiers are used with a top level class?   3
Is it possible to write JAVA program without including any of the packages,such as "import java.io.*"; bcoz I instantly wrote a code without "import..." statement and runned the program on command Line & it worked. the code is: class Person { String name; int age; Person(String s,int a) { name = s; age = a; } accept() { System.out.println(name+"Hi!!!!!"); System.out.println(age); } } class Demo { public static void main(Strings args[])throws IOException { String name = args[0]; int age = Integer.parseInt(args[1]); Person p = new Person(name,age); p.accept(); } }   3
explain the classification of exception and hoew to handle the exceptions   2
Can you explain the difference b/n abtract and interface with a good example,?In what cases we have use abtract and what case interface? Satyam 4
Suppose there is an array list [10,5,20,19,15,24].Print them in ascending & descending order in collection framework concept ??? Cognizent 2
How 'java' got its name and what it stands for? Wipro 10
Different types of modifiers? RoboCom 8
If I have 1000 objects and my requirement is to sort them quickly, then which collection would you recommend and why? KPIT 5
what is Thread?   6
When does Exception occurs?   3
what is the Arraylist limit (maximum) by default ? Fidelity 11
 
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