Updating a hashmap
Before , the number of “segments” required was relative to the number of threads accessing the table so that the update in progress per segment would be no more than one most of time.
That’s why constructors, compared to @Test(expected = Null Pointer Exception.class) public void given Concurrent Hash Map_when Put With Null Key_then Throws NPE() @Test(expected = Null Pointer Exception.class) public void given Concurrent Hash Map_when Put Null Value_then Throws NPE() Keep in mind micro-benchmarks are only looking at a single scenario and aren’t always a good reflection of real world performance.
Read operations do not block, and update contentions are minimized.
The number of segments required is relative to the number of threads accessing the table so that the update in progress per segment would be no more than one most of time.
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So for better performance, they only reflect the results of the most recently completed update operations, as stated in the official Javadoc.
We can get a value out of the hash map by providing its key to the Although the number of keys and values is growable, each key can only have one value associated with it at a time.
When we want to change the data in a hash map, we have to decide how to handle the case when a key already has a value assigned.
Many different programming languages support this kind of data structure, but often use a different name, such as hash, map, object, hash table, or associative array, just to name a few.
Hash maps are useful for when you want to look up data not by an index, as you can with vectors, but by using a key that can be of any type.