Java / Map and its implementations
1. List the differences between HashTable and HashMap in Java collections.
HashMap. HashTable. HashMap is not synchronized and therefore it is not thread safe. HashTable is internally synchronized and therefore it is thread safe. HashMap allows maximum one null key and any number of null values. HashTable does not allow null keys and null values. Iterators returned by t...
2. Similarities between HashMap and HashTable in Java.
Both store the data in the form of key-value pairs. Both use Hashing technique to store the key-value pairs. Both implement Map interface. Both doesn't maintain insertion order for elements. Both give constant time performance for insertion and retrieval operations.
3. What is a resource bundle in Java?
A resource bundle is represented by a text file containing keys and a text value for each key.
4. What is the Big-O notation for operations in a Hashmap?
Hashmap best and average case for Search, Insert and Delete is O(1) and worst case is O(n). Hash tables are the implementation of associative arrays. Associative arrays, the abstract data structure mapping keys to values. Implementations of associative arrays using self-balancing binary search tr...
5. Is HashMap faster compared to ArrayList in terms of search?
The ArrayList has O(n) performance for every search, so for n searches its performance is O(n^2). The HashMap has O(1) performance for every search (on average), so for n searches its performance will be O(n). While the HashMap will be slower at first and take more memory, it will be faster for l...
6. Difference between HashMap and ConcurrentHashMap.
In multiple threaded environment HashMap is usually faster than ConcurrentHashMap . ConcurrentHashMap does not allow NULL values as well as key can not be null in ConcurrentHashMap .While In HashMap there can only be one null key . ConcurrentHashMap is thread-safe while HashMap is not thread-safe .
7. Is ConcurrentHashmap good for frequently updating concurrent data structure?
8. What are the commonly used implementations of Map in Java?
HashMap, TreeMap, Hashtable, ConcurrentHashMap, and LinkedHashMap.
9. Difference between Collections.synchronizedMap(Map) and ConcurrentHashMap.
Both maps are thread-safe implementations of the Map interface. ConcurrentHashMap allows concurrent modification of the Map from several threads without the need to block them while synchronizedMap creates a blocking Map which will degrade performance. synchronizedMap ensures data consistency and...
10. Difference between Collections.synchronizedMap and Hashtable.
Both are thread-safe implementations of the Map interface. Both provide the same degree of synchronization. HashTable is part of Java's legacy code. The Hashtable class has all its methods synchronized i.e. the locking is done at the method level and hence one can say that the mutex is always at ...
11. Why is ConcurrentHashMap faster than Hashtable in Java?
ConcurrentHashMap divides the whole map into different segments and locks only a particular segment during the update operation, instead of Hashtable, which locks whole Map. The ConcurrentHashMap also provides lock-free read, which is not possible in Hashtable. ConcurrentHashMap is faster than Ha...
12. What are the different Reference types available in Java?
The different available reference types are WeakReference and softReference (PhantomReference). These reference types are defined in java.lang.ref package and provided to assist Java Garbage Collector in a case of low memory issues. If you wrap an object with WeakReference then it will be eligibl...
13. Which methods you need to override to use an object as key in HashMap ?
To use an object as key in HashMap, it needs to implement equals() and hashCode() method.
14. Why Map does not extend the Collection interface in Java Collections Framework?
Map interface is not compatible with the Collection interface. Since Map requires key as well as value , for example, if we want to add key-value pair then we will use put(Object key, Object value). So there are two parameters required to add element to the HashMap object . In Collection interfac...
15. How Remove method works internally in Java HashMap?
HashMap remove method calls removeEntryForKey method internally, which calculate the final hashValue of the key object, and then use that hashValue in the indexFor(int,int) method to find the first entry object in the appropriate bucket. Since bucket is a LinkedList we start traversing from the f...
16. Explain NavigableMap in Java collection.
NavigableMap Map was added in Java 1.6 that provides navigation capability to Map data structure. It provides methods such as lowerKey() to get keys which is less than specified key, floorKey() to retrieve keys which is less than or equal to specified key, ceilingKey() to get keys which is greate...
17. Explain rehashing process in Java HashMap.
If the number of elements in the map exceeds a given threshold defined by load-factor (default .75) it will re-size the map once it is 75% occupied. Java HashMap re-size itself by creating a new bucket array of it's size twice of the previous size of HashMap and puts every old element into that n...
18. How null key is handled in HashMap?
The null key is handled specially in HashMap using 2 separate methods putForNullKey(V value) and getForNullKey(). Null key is mapped always to 0th Index. This null key scenario is treated in isolated methods to eliminate null key code check and improve performance of get and put methods. The equa...
19. What is the data-structure behind the Java HashMap implementation?
HashMap is backed by the Array of LinkedList.
20. Why are immutable objects in HashMap so effective?
Immutability allows caching the hashcode of different keys which makes the overall retrieval process very fast and suggest that String and various wrapper classes such as Integer provided by Java Collection API are very good HashMap keys.
21. Why null is not allowed in ConcurrentHashmap?
The reason is that if map.get(key) returns null, you can't detect whether the key explicitly maps to null vs the key isn't mapped. In a non-concurrent map, you can check this via map.contains(key), but in a concurrent one, the map might have changed between calls.
22. Does ConcurrentHashMap allow null keys and null values ?
No.
23. Difference between remove() and clear() methods in HashMap.
We can remove entries from HashMap using remove(key) or clear() methods. remove method removes the mapping for the key specified in the parameter while clear() method removes all the entries from the HashMap and returns void.
24. Is hashcode method invoked internally when calling get and put method in Java?
Yes, key object hashcode method is called internally when get or put method is calculated to identify the memory location in hashmap. It is ideal to cache hashcode to avoid calculating every time especially when key is immutable.
25. How hash collision is handled in HashMap?
Prior to Java8, map implementations such as Hashmap handles collision by chaining using a linked list when multiple elements (key) end up in same bucket location. This may degrade the performance from O(1) (one bucket location one key) to O(n) for that particular bucket. In Java8, to address the ...
26. What are the interfaces that TreeMap implements?
TreeMap implements, NavigableMap is a subtype of java.util.SortedMap has navigation methods such as ceilingKey(), floorKey(), higherKey() and lowerKey(). AbstractMap provides a skeletal implementation of the Map interface, to minimize the effort required to implement this interface. SortedMap pro...
27. What interfaces that HashMap implements?
HashMap implements Serializable, Cloneable and Map interfaces. It also extends java.util.AbstractMap .
28. Big O notation for TreeMap in Java.
TreeMap guarantees log(n) time cost for operatons like containsKey, get, put and remove.
29. Is TreeMap synchronized in Java?
TreeMap is not synchronized, so to achieve thread safety, we may make it synchronized by Collections.synchronizedSortedMap method.
30. Does TreeMap allow null key?
No. TreeMap doesn't allow null keys.
31. Key object constraints on a TreeMap.
The key object need to implement Comparable interface otherwise provide explicit comparator while constructing TreeMap object.
32. Does IdentityHashMap use hashcode method?
No. IdentityHashMap does not use hashCode method instead it uses System.identityHashCode() method. It neither use equals method instead uses == reference equality operator. IdentityHashMap class is not a general-purpose Map implementation; it intentionally violates Map's general contract to use e...
33. Does EnumMap allow null key?
No, it does not allow null key however allow null values.
34. What design pattern that Collections.synchronizedMap implement?
Decorator design pattern.
35. Difference between HashMap and IdentityHashMap.
HashMap. IdentityHashMap. HashMap uses equals method to compare keys. IdentityHashMap uses == operator to compare its keys. HashMap uses hashcode method to find bucket location. IdentityHashMap uses System.identityHashCode(object) to find bucket location. HashMap may not be faster when compared t...
36. What is a ConcurrentSkipListMap?
ConcurrentSkipListMap is a TreeMap equivalent concurrent collection . ConcurrentSkipListMap Implements ConcurrentNavigableMap that keep its key elements sorted in a natural order. The order can also be defined by a custom comparator set while initializing the Map object (using constructor). This ...
37. How do you sort a TreeMap by comparing its value?
You can't have the TreeMap itself sort on the values, since that defies the SortedMap specification, "A Map that further provides a total ordering on its keys". However, using an external collection, you can always sort Map.entrySet() either by key, values, or other sorting as needed. package net...
38. Default initial table size of ConcurrentHashMap.
Default size is 16.
39. Does Map implement Iterable?
Map is not an Iterable object, since it doesn't implement the Iterable Interface. We have to use keys() method to get access to an Iterable object, which will be used to iterate over the keys.
