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Interview Questions and Answers for 'Hashcode' - 9 question(s) found - Order By Newest

 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?
Core Java
Anonymous
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.

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Frequently asked in all types of companies especially Indian Services companies. Frequently asked in CTS (Based on 2 feedback)
 Q2. What is the use of hashcode in Java ?Core Java
Anonymous
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.

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 Q3. Why String is popular HashMap key in Java?Core Java
Admin
info@buggybread.com
Ans. Since String is immutable, its hashcode is cached at the time of creation and it doesnt need to be calculated again. This makes it a great candidate for key in a Map and its processing is fast than other HashMap key objects. This is why String is mostly used Object as HashMap keys.

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 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 ?
Core Java
Admin
info@buggybread.com
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.

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 Q5. What is the use of HashCode in objects ?Core Java
Admin
info@buggybread.com
Ans. Hashcode is used for bucketing in Hash implementations like HashMap, HashTable, HashSet etc.

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   Like      Discuss      Correct / Improve     java   string   hashcode   hash code   string comparison     Verilogue


Very Frequently asked. Favorite question in walkins and telephonic interviews. Usually among first few questions. Asked in different variants. Must know for intermediate and expert professionals.Among Top 10 frequently asked questions.
  Q6. What is rule regarding overriding equals and hashCode method ?Core Java
Admin
info@buggybread.com
Ans. A Class must override the hashCode method if its overriding the equals method.

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 Q7. What is collision in hash collections ?Core Java
Admin
info@buggybread.com
Ans. Collision is the situation when two different elements have the same Hash Code.

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 Q8. Do you feel that its useless to define hashCode method for a class ?Core Java2017-01-19 11:24:10

Ans. Yes its useless if we are not going to use its objects within Hash collection, For example - HashSet , HashMap. HashCode is used internally by these collections for Search.

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 Q9. Which of the following is false ?Core Java
a. A Class cannot override both hashcode and equals method.
b. A class can override both hashcode and equals method.
c. A Class must override hashCode method if its overridding equal method.
d. A Class can override hashCode even if its not overridding equals method.

Ans.a. A Class cannot override both hashcode and equals method.



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