Java / Map and its implementations
1. List the differences between HashTable and HashMap in Java collections.
HashMap. HashTable....
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.
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....
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)....
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 ....
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.
10. Difference between Collections.synchronizedMap and Hashtable.
Both are thread-safe implementations of the Map interface. Both provide the same degree of synchronization....
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....
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....
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)....
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...
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...
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...
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....
19. What is the data-structure behind the Java HashMap implementation?
HashMap allows one null key; Hashtable allows none. IdentityHashMap compares keys with ==. Choose the map by order, null policy, and concurrency.
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...
22. Does ConcurrentHashMap allow null keys and null values ?
No.
23. Difference between remove() and clear() methods in HashMap.
Map stores key/value pairs. HashMap is unordered; LinkedHashMap keeps insertion order; TreeMap sorts keys; ConcurrentHashMap is safe for concurrent writes.
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)...
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.
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.
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.
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.
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,...
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.
