INTERVIEW QUESTIONS J2EE DETAILS

Question: Define Hash table?

Answer: Hash tables are good for doing a quick search on things.

For instance if we have an array full of data (say 100 items).If we knew the position that a specific item is stored in an array, then we could quickly access it. For instance, we just happen to know that the item we want it is at position 3; I can apply:
myitem=myarray[3];



There are many different hash functions. Some hash functions will take an integer key and turn it into an index. A common one is the division method.

Hash tables are good for doing a quick search on things.

For instance if we have an array full of data (say 100 items).If we knew the position that a specific item is stored in an array, then we could quickly access it. For instance, we just happen to know that the item we want it is at position 3; I can apply:
myitem=myarray[3];



There are many different hash functions. Some hash functions will take an integer key and turn it into an index. A common one is the division method. Source: CoolInterview.com

HashTable is one of the implementations of Collections frame work and it implements Map interface.There are two main factors of HashTable one is initial capacity & load factor. We can pass these two parameter while creation of Hash table other wise it will create the same by asigning default values (i.e 11 & 0.75 respectively).
Hash Table is dynamic (i.e it can grow), when we specify initial capacity & load factor(i.e Hash Table ht=new HashTable(100,0.75))it doesn't mean that the Hash Table size is restricted to 100 only. but it creates a Hash Table with capacity of 100 & increases itself when the capacity is reached to 75 not to 100.
We can put an elements by using put(). It accepts 2 arguments one is key & a value. Source: CoolInterview.com

Hashtable is a collection object ,is Synchronozed and it does not allow duplicate values but it allows null values. Source: CoolInterview.com

Defn: It is a data structure that associates keys with values. The primary operation it supports efficiently is a lookup: given a key (e.g. a person's name), find the corresponding value (e.g. that person's telephone number). It works by transforming the key using a hash function into a hash, a number that is used as an index in an array to locate the desired location ("bucket") where the values should be.

Feature:Hashtable is synchronized (thread-safe) and does not allow duplicate values. Source: CoolInterview.com

Like Vector, Hashtable has been in from prehistoric Java times. For
fun, don’t forget to note the naming inconsistency: HashMap vs. Hashtable. Where’s
the capitalization of “t”? Oh well, you won’t be expected to spell it. Anyway, just as
Vector is a synchronized counterpart to the sleeker, more modern ArrayList, Hashtable
is the synchronized counterpart to HashMap. Remember that you don’t synchronize a
class, so when we say that Vector and Hashtable are synchronized, we just mean that
the key methods of the class are synchronized. Another difference, though, is that
while HashMap lets you have null values as well as one null key, a Hashtable doesn’t
let you have anything that’s null. Source: CoolInterview.com

Hashtable implements Map interface which is a collection of objects from where you can retrieve those objects with the help of key and value pair. Source: CoolInterview.com

HashTable is just like Hash Map,Collection having key(Unique),value pairs.
But there are two differeces between HashMap and HashTable -
1) HashTable is synchronized while HashMap is not.
2)HashMap allows one null key and multiple null values but HashTable doent allow any of them. Source: CoolInterview.com

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