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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
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
 
Answer
# 2
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 ?    3 Yes 0 No
Eknath
 
 
 

 
 
 
Other Core Java Interview Questions
 
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What is the use of static keyword in "public static void main()" Infosys 10
If I will write String s=new String("XYZ"); String s1=new String("XYZ"); if(s.equals(s1)){ sop("True"); } else{ sop("False"); } This program will give me "True". But When I am creating my own class suppose class Employee{ public Employee(String name); } Employee e= new Employee("XYZ"); Employee e1 = neew Employee("XYZ"); if(e.equals(e1)){ sop("True"); } else{ sop("False"); } Then it will give the output as "False". Can I know what is happening internally?   4
Who developed JScript language?   3
Suppose i have two threads t1 and t2 are running.How the main thread will know that the two threads t1,t2 execution has completed? Subex 2
They had given one progam final HashMap hm=new HashMap(); public void show() { hm.put("name",value); } in this prg here the final hashtable value can be changed in put method,its the prg run? DNS 3
What modifiers can be used with a local inner class?   2
What we have to do, when we don't want to implement all methods of an interface?   5
Write a program that takes a 5 digit number and calculates 2 power that number and prints it.   1
what's diff between struts 1.1 & 1.2 Wipro 3
Different types of modifiers? RoboCom 8
What is the purpose of the finally clause?   3
String Reverse in Java...!   5
 
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