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Why does ConcurrentHashMap prevent null keys and values?
...oncurrentHashMap himself (Doug Lea):
The main reason that nulls aren't allowed in ConcurrentMaps
(ConcurrentHashMaps, ConcurrentSkipListMaps) is that ambiguities that
may be just barely tolerable in non-concurrent maps can't be
accommodated. The main one is that if map.get(key) returns nul...
Programmatically selecting text in an input field on iOS devices (mobile Safari)
How do you programmatically select the text of an input field on iOS devices, e.g. iPhone, iPad running mobile Safari?
10 A...
Start a git commit message with a hashmark (#)
...mit --cleanup=whitespace
If you do this you have to be careful to remove all # lines that you don't want to appear in the commit.
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What is the difference between save and insert in Mongo DB?
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Save Vs Insert :
In your given examples, the behavior is essentially the same.
save behaves differently if it is passed with an "_id" parameter.
For save, If the document contains _id, it will upsert querying the collection on the _id field, If not, it will insert.
If a document does not...
Why do we need a fieldset tag?
...abel for="colour_blue">Blue</label>
</fieldset>
This allows each radio button to be labeled while also providing a label for the group as a whole. This is especially important where assistive technology (such as a screen reader) is being used where the association of the contro...
Add floating point value to android resources/values
...ue = outValue.getFloat();
I know that this is confusing (you'd expect call like getResources().getDimension(R.dimen.text_line_spacing)), but Android dimensions have special treatment and pure "float" number is not valid dimension.
Additionally, there is small "hack" to put float number into d...
Is there a builtin identity function in python?
...hink about the signature and time costs.
So a better way to do it is actually (a lambda avoids naming the function):
_ = lambda *args: args
advantage: takes any number of parameters
disadvantage: the result is a boxed version of the parameters
OR
_ = lambda x: x
advantage: doesn't change...
How can I transform string to UTF-8 in C#?
...hould also write Console.OutputEncoding = System.Text.Encoding.UTF8;!!! Or all utf8 strings will be outputed as gbk...
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What's an easy way to read random line from a file in Unix command line?
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You can use shuf:
shuf -n 1 $FILE
There is also a utility called rl. In Debian it's in the randomize-lines package that does exactly what you want, though not available in all distros. On its home page it actually recommends the use of shuf instead (which didn't exist when it was cr...
What is mutex and semaphore in Java ? What is the main difference?
... has 10 flags, then your thread won't accept new connections
Mutex are usually used for guarding stuff. Suppose your 10 clients can access multiple parts of the system. Then you can protect a part of the system with a mutex so when 1 client is connected to that sub-system, no one else should have a...