What is the purpose of the hashCode() method
💡 Concept: hashCode() Method
The hashCode() method returns an integer hash code value for an object used by hashing data structures like HashMap and HashSet.
📘 Quick Intro
hashCode() provides a numeric representation of an object for efficient lookup, insertion, and retrieval in hash-based collections.
🧠 Analogy
Think of hashCode() as a shortcut or index number that helps you quickly find a book in a library without scanning all shelves.
🔧 Technical Explanation
- hashCode() returns an int representing the object's memory address or computed hash.
- Equal objects must return the same hashCode.
- Used by hash-based collections for efficient data storage and lookup.
- Override hashCode() when equals() is overridden to maintain contract.
- Helps in distributing objects uniformly in hash buckets.
🎯 Use Cases
- ✅ HashMap, HashSet, and Hashtable internal operations rely on hashCode().
- ✅ Efficient retrieval of objects in hash-based collections.
- ✅ Prevents collisions and improves performance when properly implemented.
💻 Example: Overriding hashCode()
public class Person {
private String name;
private int age;
@Override
public int hashCode() {
int result = 17;
result = 31 * result + (name == null ? 0 : name.hashCode());
result = 31 * result + age;
return result;
}
}
❓ Interview Q&A
Q1: What does hashCode() return?
A: An integer hash code of the object.
Q2: Why override hashCode()?
A: To maintain consistency with equals().
Q3: How is hashCode() used in collections?
A: For efficient object lookup in hash tables.
Q4: Can two different objects have same hashCode()?
A: Yes, but rare and called collision.
Q5: Is hashCode() unique?
A: Not guaranteed.
Q6: What is the contract between equals() and hashCode()?
A: Equal objects must have same hashCode.
Q7: Can hashCode() change during object lifetime?
A: Should not change if object is used in collections.
Q8: What happens if hashCode() is not overridden?
A: Default method returns memory address based hash.
Q9: How to implement good hashCode()?
A: Use prime numbers and immutable fields.
Q10: Does hashCode() affect performance?
A: Yes, good hashCode() improves collection speed.
📝 MCQs
Q1. What does hashCode() return?
- String value
- Integer hash code
- Boolean value
- Object reference
Q2. Why override hashCode()?
- For performance
- Consistency with equals()
- For debugging
- For serialization
Q3. How is hashCode() used in collections?
- For logging
- Efficient lookup
- For encryption
- For sorting
Q4. Can different objects have same hashCode()?
- No
- Yes
- Sometimes
- Never
Q5. Is hashCode() unique?
- Yes
- No
- Sometimes
- Always
Q6. What is contract between equals() and hashCode()?
- No relation
- Equal objects same hashCode
- Opposite values
- Depends on JVM
Q7. Can hashCode() change after creation?
- Yes
- No
- Should not
- Sometimes
Q8. What if hashCode() not overridden?
- Error
- Default memory-based
- Random
- Null
Q9. How to implement good hashCode()?
- Prime numbers
- Random values
- Strings
- Objects
Q10. Does hashCode() affect performance?
- No
- Yes
- Maybe
- Unknown
💡 Bonus Insight
A well-implemented hashCode() is essential for the performance and correctness of hash-based collections in Java.
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