Question

I am trying to learn a java-based program, but I am pretty new to java. I am quite confusing on the following two lines of java code. I think my confusion comes from the concepts including “class” and “cast”, but just do not know how to analyze.

For this one

XValidatingObjectCorpus<Classified<CharSequence>> corpus
        = new XValidatingObjectCorpus<Classified<CharSequence>>(numFolds);

What is <Classified<CharSequence>> used for in terms of Java programming? How to understand its relationships with XValidatingObjectCorpusand corpus

For the second one

LogisticRegressionClassifier<CharSequence> classifier
            = LogisticRegressionClassifier.<CharSequence>train(para1, para2, para3)

How to understand the right side of LogisticRegressionClassifier.<CharSequence>train? What is the difference between LogisticRegressionClassifier.<CharSequence>train and LogisticRegressionClassifier<CharSequence> classifier
?

Answer 1

These are called generics. They tell Java to make an instance of the outer class – either XValidatingObjectCorpus or LogisticRegressionClassifier – using the type of the inner object.

Normally, these are used for lists and arrays, such as ArrayList or HashMap.

What is the relationship between XValidatingObjectCorpus and corpus?

corpus is just a name given to the new XValidatingObjectCorpus object that you make with that statement (hence the = new... part).

What does LogisticRegressionClassifier.<CharSequence>train mean?

I have no idea, really. I suggest looking at the API for that (I think this is the right class).

What is the difference between LogisticRegressionClassifier.<CharSequence>train and LogisticRegressionClassifier<CharSequence> classifier?

You can’t really compare these two. The one on the left of the = is the object identifier, and the one on the right is the allocator (probably the wrong word, but it is what it does, kind of).

Together, the two define an instance of LogisticRegressionClassifier, saying to create that type of object, call it classifier, and then give it the value returned by the train() method. Again, look at the API to understand it more.

By the way, these look like wretched examples to be learning Java with. Start with something simple, or at least an easier part of the code. It looks like someone had way too much fun with long names (the API has even longer names). Seriously though, I only just got to fully understanding this, and Java was my main language for quite a while (It gets really confusing when you try and do simple things). Anyways, good luck!