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I am a complete newbie to Java programming and I am trying to learn caching and hash tables. I have seen tutorials online but they are complex, does anyone here know of any relatively short programs that utilise caching and hash tables?
Thanks for any help given
UPDATE:
I am basically starting from scratch. I know hash tables and sort of know caching (more simple caching programs would be much appreciated), but I don't get how the two work together. For example saving to a hash table and caching the data.
As the comments mention, a cache is just a store where you keep the output so you won't have to do the calculation again.
Here's a really simple example
Map<String,Double> answers = new HashMap<String,Double>();
// checking cache if we have the answer
If (answers.get("volatility") != null) {
System.out.println("volatility found in cache:" +
answers.get("volatility"));
}
// store a value in cache
answers.put("rate",1.887);
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In a recent interview with Amazon I have been rejected because I could not tell the advantage and disadvantage of making model and Entity classes different and same.
I have always created Model and Entity class same.
Can anybody help me on that with an example? Interviewer said you are making strong binding of UI+DB if you are keeping it same.
It's true by making same class for Entity and Model you are tightly binding UI and DB, simple example of why should avoid is -> most of the time, we modify response like adding DTO, modifying format of date and so on. which could impact your database calls (DTO layer).you can read more here
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I was reading about Garbage Collector performance and found the term max-jOPS and critical jOPS.
Link: http://openjdk.java.net/jeps/333
Can someone tell the full form and explain what is it?
These (jOPS, max-jOPS and critical-jOPS) are not GC terms.
I believe that you are referring to the terminology used in the SPECjbb2015 Benchmark; e.g. https://www.spec.org/jbb2015/docs/userguide.pdf. (This is confirmed by your update.)
The documents about the benchmark that I read don't specifically say what jOPS stands for. However the Glossary says that OPS stands for Operations Per Second, and I infer from the context that the j refers to jbb2015.
In other words, jOPS represents the rate at which a "unit of work" is performed by the jbb2015 benchmark. The unit is artificial, and is not designed to directly map to any real world measures ... though there will often be a correlation.
And the max-jOPS and critical-jOPS are specific points in the RT (Response-Throughput) curve that the benchmark captures.
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I have a hashtable that contains the information about some book titles and the number of times each book is purchased. I want to draw a bar chart that can show this information visually. Is there any library or method in java that can do this?
HashTable<String, Integer> bookPurchaseTable=new HashTable<String, Integer>();
JavaFX has bar chart capabilities. This page has a tutorial that should have enough information to get you started.
https://docs.oracle.com/javafx/2/charts/bar-chart.htm
Also, You should probably use a HashMap instead of a HashTable if possible.
Use myHashMap.keySet() to get the keys, and then for each key, use myHashMap.get(key) to get the integer.
it's a big topic about Java UI deployment.
you can try java swing tutorial first, and learn how to develop UI in java.
BTW, it's recommend to use HashMap not HashTable now.
hope you can enjoy the Java.
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I'm trying to solve the following problem:
I have some expensive work to do which I then cache the result of
The work is keyed by a string
Many requests may arrive simultaneously for the same key
I'd like to avoid doing the work more than once per key
I'd like to add callbacks against the key which will be invoked when the work is completed; not all of these are known when the work is first submitted.
This feels like a problem which ought to have been solved already; does anybody know of a Java framework or library which covers it?
I can imagine a wrapper around guava's LoadingCache but I'm not aware of a library which does everything out of the box.
While LoadingCache#get is synchronous, it does get you 1-4 and there may be some mileage in using refresh which can return a ListenableFuture (although to get all the features you list it might become a fairly chunky wrapper?)
For Reference:
http://docs.guava-libraries.googlecode.com/git/javadoc/com/google/common/cache/LoadingCache.html#refresh(K)
http://www.theotherian.com/2013/11/non-blocking-cache-with-guava-and-listenable-futures.html
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Where can I find more information on how exactly the following method works
and what it actually does? I found the single line of the documentation leaves a bit to be desired:
Class weka.associations.Apriori
public void buildAssociations(Instances instances) throws Exception
Method that generates all large itemsets with a minimum support, and from these all association rules with a minimum confidence.
Look at all the documentation, not only the method documentation tooltip in your IDE. You are missing out on a lot of the documentation.
Weka comes with a whole book, that will give you plenty of detail.
The Apriori class documentation also contains much more than that single line you quoted. You failed to access the JavaDoc class documentation; it's not the documentation that "leaves a bit to be desired", is it? It points to two publication giving the details on the algorithm.