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Java - Interview Questions and Answers on HashCode

Q1.  There are two objects a and b with same hashcode. I am inserting these two objects inside a hashmap.

hMap.put(a,a);
hMap.put(b,b);

where a.hashCode()==b.hashCode()

Now tell me how many objects will be there inside the hashmap?

Ans. There can be two different elements with the same hashcode. When two elements have the same hashcode then Java uses the equals to further differentation. So there can be one or two objects depending on the content of the objects.

Q2.  What is the use of hashcode in Java ?

Ans. Hashcode is used for bucketing in Hash implementations like HashMap, HashTable, HashSet etc. The value received from hashcode() is used as bucket number for storing elements. This bucket number is the address of the element inside the set/map. when you do contains() then it will take the hashcode of the element, then look for the bucket where hashcode points to and if more than 1 element is found in the same bucket (multiple objects can have the same hashcode) then it uses the equals() method to evaluate if object are equal, and then decide if contain() is true or false, or decide if element could be added in the set or not.

Q3.  Why String is popular HashMap key in Java?


Ans. Since String is immutable, its hashcode is cached at the time of creation and it doesn’t need to be calculated again. This makes it a great candidate for key in a Map and it’s processing is fast than other HashMap key objects. This is why String is mostly used Object as HashMap keys.


Q4.  

public class a {
     public static void main(String args[]){
          final String s1="job";
          final String s2="seeker";
          String s3=s1.concat(s2);
          String s4="jobseeker";
          System.out.println(s3==s4); // Output 1
          System.out.println(s3.hashCode()==s4.hashCode()); Output 2
     }
}

What will be the Output 1 and Output 2 ?

Ans. S3 and S4 are pointing to different memory location and hence Output 1 will be false.

Hash code is generated to be used as hash key in some of the collections in Java and is calculated using string characters and its length. As they both are same string literals, and hence their hashcode is same.Output 2 will be true.

Q5.  What is the use of HashCode in objects ?

Ans. Hashcode is used for bucketing in Hash implementations like HashMap, HashTable, HashSet etc. 

Q6.  Why is String immutable in Java ?

Ans. 1. String Pool

When a string is created and if the string already exists in the pool, the reference of the existing string will be returned, instead of creating a new object. If string is not immutable, changing the string with one reference will lead to the wrong value for the other references.

2. To Cache its Hashcode

If string is not immutable, One can change its hashcode and hence not fit to be cached.

3. Security

String is widely used as parameter for many java classes, e.g. network connection, opening files, etc. Making it mutable might possess threats due to interception by the other code segment.

Q7.  What are the methods of Object Class ?

Ans. clone() - Creates and returns a copy of this object.
equals() - Indicates whether some other object is "equal to" this one.
finalize()  - Called by the garbage collector on an object when garbage collection determines that there are no more references to the object
getClass() - Returns the runtime class of an object.
hashCode() - Returns a hash code value for the object.
toString() - Returns a string representation of the object.

notify(), notifyAll(), and wait() - Play a part in synchronizing the activities of independently running threads in a program.

Q8.  What is rule regarding overriding equals and hasCode method ?


Ans. A Class must override the hashCode method if its overriding the equals method.




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