Part 9
Interview questions on Collections in java for experienced covered in this post:
How null
key is handled in HashMap?
-
Hashmap handles the different ways to handle null keys while
putting and retrieving.
-
Null key always map to the 0th index.
-
There are a couple of separate methods for that,
putForNullKey (V value)
and
getForNullKey ().
-
Equals() and hashcode() methods are not used in the case of
null keys.
-
For getting value by null key it also has a method
getForNullKey
which it calls from inside the get method.
-
Please refer to the below example.
|
private V getForNullKey() {
if (size == 0) {
return null;
}
for (Entry<K,V> e = table[0]; e != null; e = e.next) {
if (e.key == null)
return e.value;
}
return null;
} |
What is the time Complexity for map.get()?
-
The time complexity of hashmap for get operation and put
operation is generally O(1).
-
O(1) certainly isn't guaranteed - but it's usually what you
should assume when considering which algorithms and data structures to
use.
 
Note: Please click on Image to Purchase the books from Amazon.in |
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.
|
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} |
What is WeakHashMap?
-
In the case of hashmap even though the object doesn’t have any
reference still it is not eligible for garbage collection. If it is associated
with HashMap i.e. Hashmap dominates garbage collector.
-
In the case of WeakHashMap if the object doesn’t contain any
reference is eligible for garbage collection. Even though the object is
associated with WeakHashMap. Garbage collector dominates WeakHashMap.
- Please refer to the below example.
|
package simplifiedjava.crackedInterview; public class Temp {
public
String toString() {
return
"temp"; }
public
void
finalize() {
System.out.println("Finalize() method called.");
} } // Demo for HashMap package simplifiedjava.crackedInterview; import java.util.HashMap; public class HashMapDemoForWeaknessCompare {
public
static
void
main(String[] args) throws
InterruptedException {
HashMap m
= new
HashMap();
Temp t
= new
Temp();
m.put(t, "Testing");
System.out.println(m);
t
= null;
System.gc();
Thread.sleep(5000);
System.out.println(m);
}
}
Output:
{temp=Testing} {temp=Testing} // Demo for WeakHashMap package simplifiedjava.crackedInterview; import java.util.WeakHashMap; public class WeakHashMapDemo {
public
static
void
main(String[] args) throws
InterruptedException {
WeakHashMap m
= new
WeakHashMap();
Temp t
= new
Temp();
m.put(t, "Testing");
System.out.println(m);
t
= null;
System.gc();
Thread.sleep(5000);
System.out.println(m);
}
}
Output:
{temp=Testing}
Finalize() method called. {}
Please cross verify the output for both HashMap and WeakHashMap. In
case of WeakHashMap finalize() method has been called means garbage
collections performed his task and second time we are not able to get
the object which has evacuated by GC. |
What is IdentityHashMap?
-
In the case of IdentityHashMap JVM will use “==” to identify
duplicate keys which are meant for reference comparison.
-
For normal HashMap, JVM will use the .equals() method to
identify duplicate keys which is meant for content comparison.
-
Please refer couple of examples below.
|
package simplifiedjava.crackedInterview;
import
java.util.HashMap; import java.util.IdentityHashMap; public class HashMapDemoForIdentityHashMap { public static void main(String[] args) {
HashMap<Integer, String> hm
= new
HashMap<Integer, String>();
hm.put(new
Integer(10), "Yogesh");
hm.put(new
Integer(10), "Arpita");
System.out.println(hm);
IdentityHashMap<Integer, String> ihm
= new
IdentityHashMap<Integer, String>();
ihm.put(new
Integer(10), "Yogesh");
ihm.put(new
Integer(10), "Arpita");
System.out.println(ihm);
}
}
{10=Arpita}
{10=Yogesh, 10=Arpita} |
What is SortedMap? How objects will be stored in SortedMap bydefault?
-
SortedMap is a child interface of Map interface.
-
The map is ordered according to the natural sorting order of
its keys but not the values.
-
All keys inserted into a sorted map must implement the
Comparable interface (or be accepted by the specified comparator).
-
TreeMap is the implementation class of the SortedMap
interface.
-
Please refer to the below example. The reference is SortedMap
and its implementation is TreeMap.
|
package simplifiedjava.crackedInterview;
import
java.util.SortedMap; import java.util.TreeMap; public class SortedMapDemo {
public
static
void
main(String[] args) {
SortedMap<Integer, String>
sMap
= new
TreeMap<Integer, String>();
sMap.put(1000, "Thousand");
sMap.put(1, "One");
sMap.put(10000, "Ten Thousand");
sMap.put(100, "Hundred");
sMap.put(10, "Ten");
System.out.println(sMap);
}
}
Output:
{1=One, 10=Ten, 100=Hundred, 1000=Thousand, 10000=Ten
Thousand} |
-
Java interview questions and answers all MNC - Click here
-
Basic core java interview questions and answers for freshers
- Click here
-
Core java interview questions for 3 years experience - Click here
-
Core java interview questions and answers for 3-5 years exp
- Click here
-
Core java interview questions and Answers for 5 - 7 Years exp
- Click here
-
Basic Java Interview Questions and Answers - Click here
-
Java interview questions and answers on oops - Click here
-
Java interview questions and answers on Strings - Click here
-
Java interview questions on exception handling - Click here
-
Interview questions on multithreading in java for experienced
- Click here
-
Interview questions on serialization in java for experienced
- Click here
-
Interview questions on inner class in java for experienced
- Click here
-
Interview questions on Collections in java for experienced
- Click here
|
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