Part 4
What will be the impact if hashcode()
method return constant value every-time? What will be the size of
hashmap, In Employee class I have overridden equals and hashcode method
and hashcode method always return 1. I have 5 employee objects in
hashmap. What will be the size?
- You would lose any performance given by a hashmap, that can
retrieve items from a collection in O(1) time for objects with different
hashes, which is what we want to achieve when using HashMaps.
- If the equals method is implemented as per the contract and
the hashcode method returns a constant value, then we will still be able to
retrieve the value for the key from a hashMap, but the performance will be
slow compared to the method returning a unique hashcode.
- The size would be 5.
What will be the impact if I don't
override equals() method?
- Only Override HashCode, Use the default Equals: Only
the references to the same object will return true. In other words, those
objects you expected to be equal will not be equal by calling the equals
method.
- Only Override Equals, Use the default HashCode: There
might be duplicates in the HashMap or HashSet. We write the equals method
and expect {"xyz", "XYZ"} to be equals. However, when using a HashMap, they
might appear in different buckets, thus the contains() method will not
detect each other.
Why String and wrapper classes are considered good keys?
- Strings and wrapper classes are immutable so immutable
objects are really a good option for keys because once you insert the key’s
hashcode never change throughout life.
- String and wrapper classes also overrides the equals() and
hashcode() methods.
- Immutable objects are thread-safe this is an advantage as well.
What is LinkedHashMap? Can you explain
internal working of LinkedHashMap?
- LinkedHashMap is a data structure that preserves the
insertion order and stores the objects in the form of key/value pair like a
Hashmap.
- LinkedHashMap is a child class of Hashmap and implements a
Map interface.
- LinkedHashMap is not a thread-safe.
- It can contain only one null key and multiple null
values.
- It contains only unique elements.
- Please refer to the below example.
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package simplifiedjava.crackedInterview; import java.util.LinkedHashMap; public class LinkedHashMapDemo { public static void main(String[] args) {
LinkedHashMap<Integer, String> linkHm = new
LinkedHashMap<Integer, String>();
linkHm.put(100, "One Hundred");
linkHm.put(200, "Two Hundred");
linkHm.put(300, "Three Hundred");
linkHm.put(500, "Five Hundred");
linkHm.put(200, "Two Hundred");
linkHm.put(100, "One Hundred");
linkHm.put(null, "NULL Key");
linkHm.put(600, null);
linkHm.put(700, null);
System.out.println(linkHm);
}
}
Output: {100=One Hundred, 200=Two Hundred, 300=Three Hundred, 500=Five
Hundred, null=NULL Key, 600=null, 700=null} |
Why map doesn't extends Collection
interface?
- Map doesn’t extend the Collection interface because they are
not compatible.
- Internal data structure of Collection implementations are
like a list of objects and the internal data structure of the map is
key-value pair.
- Collection implementations are index-based collections.
- Map is not an indexed based collection but it is key-value
based collection.
- For adding objects to the collection we use add(Object o)
method.
- For adding objects to the map we use the put(key, value)
method.
- In the case of iterator, we directly put the iterator on the
collection implementation class.
- But, In the case map, we have to put the iterator on the entry set.
What is hash-collision in Hashtable and
how will you handled in Collection API?
- Two different keys with the same hash value are known as
hash-collision.
- Two separate entries will be kept in a single hash bucket to
avoid the collision.
- There are two ways to avoid hash-collision. Separate Chaining
and Open Addressing.
1.
Separate Chaining:
§ Keys are stored inside and outside the hashtable.
§ Extra space is required for the pointers to store the keys
outside the hashtable.
§ Some buckets of the hash table are never used which leads to
wastage of space.
§ Cache performance is poor. This is because of linked lists
which store the keys outside the hash table.
2.
Open Addressing:
§ All the keys are stored inside only in the hash table. No key
is present outside the Hashtable.
§ No extra space is required.
§ Buckets may be used even if non-mapped keys to those
particular buckets.
§ Cache performance is better. This is because there is no
linked lists are used.
Which data structure has implemented
for HashMap?
- Hashtable data structure has been implemented for
HashMap.
Which data structure has implemented
for TreeMap?
- RED-BLACK Tree data structure has been implemented for
TreeMap.
Can I convert Map into List?
- We can convert the map into a List.
- But we have to create a different list for the key set and
value set.
- Please refer to the below examples.
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package simplifiedjava.crackedInterview;
import
java.util.ArrayList;
import
java.util.HashMap; import java.util.List; public class ConvertMapToList {
public static void
main(String[] args) {
HashMap<Integer, String> hMap = new
HashMap<Integer, String>();
hMap.put(1, "One");
hMap.put(10, "Ten");
hMap.put(100, "Hundred"); hMap.put(1000, "Thousand");
List<Integer> keyList = new
ArrayList<Integer>(hMap.keySet()); List<String> valueList = new ArrayList<String>(hMap.values());
System.out.println("Key List "+ keyList);
System.out.println("Value List "+ valueList);
}
}
Output:
Key List [1, 100, 1000, 10]
Value List [One, Hundred, Thousand, Ten]
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Can you write a code to iterate a
hashMap object?
-
We can iterate Hashmap couple of ways.
1.
The first way is we can collect keyset and iterate the map
and print the vales.
2.
The second way is we can collect the entry set and iterate
map and print the values.
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package simplifiedjava.crackedInterview;
import
java.util.HashMap;
import
java.util.Iterator; import java.util.Map; public class HashMapIteratorDemo {
public
static
void
main(String[] args) {
HashMap<Integer, String> hMap
= new
HashMap<Integer, String>();
hMap.put(1, "One");
hMap.put(10, "Ten");
hMap.put(100, "Hundred"); hMap.put(1000, "Thousand");
System.out.println("========== Using eySet===========");
Iterator itr1
= hMap.keySet().iterator();
while(itr1.hasNext()) {
Integer key
= (Integer) itr1.next();
System.out.println("Key : "+ key
+ "\t Value : "+ hMap.get(key)); }
System.out.println("========== Using EntrySet===========");
Iterator itr2
= hMap.entrySet().iterator();
while(itr2.hasNext()) {
Map.Entry<Integer, String> pair
= (Map.Entry<Integer, String>)itr2.next();
System.out.println("Key : "
+ pair.getKey()+ "\tValue : "+ pair.getValue());
}
}
}
Output:
=============== Using KeySet===============
Key : 1
Value : One
Key : 100
Value : Hundred
Key : 1000
Value : Thousand
Key : 10
Value : Ten
=============== Using EntrySet===============
Key : 1 Value : One
Key : 100 Value : Hundred
Key : 1000 Value : Thousand
Key : 10 Value : Ten |
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