1. What are IdentityHashMap and WeakHashMap?
2. Explain ConcurrentHashMap? How it works?
3. How hashmap works?
4. How to design a good key for hashmap?
5. When to use HashMap or TreeMap?
6. Difference between HashMap and HashTable?
7. Difference between HashMap and ConcurrentHashMap
8. Difference between ConcurrentHashMap and Collections.synchronizedMap( HashMap )
9. Difference between HashTable and Collections.synchronized(HashMap)
So far you must have got the core idea of the similarities between them. Both are synchronized version of collection. Both have synchronized methods inside class. Both are blocking in nature i.e. multiple threads will need to wait for getting the lock on instance before putting/getting anything out of it.
So what is the difference. Well, NO major difference for above said reasons. Performance is also same for both collections.
Only thing which separates them is the fact HashTable is legacy class promoted into collection framework. It got its own extra features like enumerators.
10. How HashMap works in Java
11. [Update] HashMap improvements in Java 8
As part of the work for JEP 180, there is a performance improvement for HashMap objects where there are lots of collisions in the keys by using balanced trees rather than linked lists to store map entries. The principal idea is that once the number of items in a hash bucket grows beyond a certain threshold, that bucket will switch from using a linked list of entries to a balanced tree. In the case of high hash collisions, this will improve worst-case performance from O(n) to O(log n).
Basically when a bucket becomes too big (currently: TREEIFY_THRESHOLD = 8), HashMap dynamically replaces it with an ad-hoc implementation of the treemap. This way rather than having pessimistic O(n) we get much better O(log n).
Bins (elements or nodes) of TreeNodes may be traversed and used like any others, but additionally support faster lookup when overpopulated. However, since the vast majority of bins in normal use are not overpopulated, checking for the existence of tree bins may be delayed in the course of table methods.
Tree bins (i.e., bins whose elements are all TreeNodes) are ordered primarily by hashCode, but in the case of ties, if two elements are of the same “class C implements Comparable<C>“, type then their compareTo() method is used for ordering.
Because TreeNodes are about twice the size of regular nodes, we use them only when bins contain enough nodes. And when they become too small (due to removal or resizing) they are converted back to plain bins (currently: UNTREEIFY_THRESHOLD = 6). In usages with well-distributed user hashCodes, tree bins are rarely used.
2. Explain ConcurrentHashMap? How it works?
3. How hashmap works?
4. How to design a good key for hashmap?
5. When to use HashMap or TreeMap?
6. Difference between HashMap and HashTable?
7. Difference between HashMap and ConcurrentHashMap
8. Difference between ConcurrentHashMap and Collections.synchronizedMap( HashMap )
9. Difference between HashTable and Collections.synchronized(HashMap)
So far you must have got the core idea of the similarities between them. Both are synchronized version of collection. Both have synchronized methods inside class. Both are blocking in nature i.e. multiple threads will need to wait for getting the lock on instance before putting/getting anything out of it.
So what is the difference. Well, NO major difference for above said reasons. Performance is also same for both collections.
Only thing which separates them is the fact HashTable is legacy class promoted into collection framework. It got its own extra features like enumerators.
10. How HashMap works in Java
11. [Update] HashMap improvements in Java 8
As part of the work for JEP 180, there is a performance improvement for HashMap objects where there are lots of collisions in the keys by using balanced trees rather than linked lists to store map entries. The principal idea is that once the number of items in a hash bucket grows beyond a certain threshold, that bucket will switch from using a linked list of entries to a balanced tree. In the case of high hash collisions, this will improve worst-case performance from O(n) to O(log n).
Basically when a bucket becomes too big (currently: TREEIFY_THRESHOLD = 8), HashMap dynamically replaces it with an ad-hoc implementation of the treemap. This way rather than having pessimistic O(n) we get much better O(log n).
Bins (elements or nodes) of TreeNodes may be traversed and used like any others, but additionally support faster lookup when overpopulated. However, since the vast majority of bins in normal use are not overpopulated, checking for the existence of tree bins may be delayed in the course of table methods.
Tree bins (i.e., bins whose elements are all TreeNodes) are ordered primarily by hashCode, but in the case of ties, if two elements are of the same “class C implements Comparable<C>“, type then their compareTo() method is used for ordering.
Because TreeNodes are about twice the size of regular nodes, we use them only when bins contain enough nodes. And when they become too small (due to removal or resizing) they are converted back to plain bins (currently: UNTREEIFY_THRESHOLD = 6). In usages with well-distributed user hashCodes, tree bins are rarely used.
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