How do you cope with Java coming from javascript?

Question

I learned some javascript as my introduction into programming, and now I'm trying to move on to Java and Python and from there to c++, c#, etc.

I've been caught up with schoolwork, so I've had very little chance to learn anything yet, but I have been lurking around SO looking for easy Java questions that I might have asked myself and trying to learn from the answers.

I saw one that was kind of interesting. It was a homework assignment about finding the most efficient order of jobs to do, assuming each job took two days and had to be in sequential order. I could have easily done this in javascript, but I'm having trouble figuring out how in Java. With javascript I would just return a two dimensional array. Each element being an array containing the profit and the days to work. Ignoring whether or not that's a stupid way to return data in javascript, what is the proper way to return data like this in Java?

In other words, the javascript array I would return would look something like:

[ ['1-3-5', 50], ['2-5-8', 90], ['2-4-7', 75] ]

In Java, I would have to convert the integers to strings to start off, and then find a way to concatenate two two dimensional arrays... Needless to say, this is the wrong way to go about it.

Answer 1

Using arrays for everything is a bad idea. Even in JavaScript. But in Java even more so, because arrays are far more narrow in Java.

The elements of your return array are in fact representing results. You should communicate it accordingly:

class Result {
     public int someInt;//no idea what this actually is, but a class will allow you giving it a proper name
     public String someString;//same as with the other field. Also, I suppose String is actually wrong here, since it rather looks like a '-'-separated list of integers, which should rather be represented as an array of integers
     Result(String someString, int someInt) {
           this.someString = someString;
           this.someInt = someInt;
     }
}

And then your return value should look something like:

ArrayList.ofList(new Result('1-3-5', 50), new Result('2-5-8', 90), new Result('2-4-7', 75))

Answer 2

Java is not JavaScript. You have to rid yourself of the JavaScript loose typing habits to appreciate Java's type system.

In loose languages you optimize for easy implementation of the function being called. Because the function can accept and return data in any format, it makes implementing the function without a lot of overhead easy. The downside is that whoever is calling your function has to look at the source code of your function to know what data it accepts and what data it returns. This is efficient for a single programmer (assuming you can remember your functions), and inefficient for a team (everyone has to read all the source code).

In strict languages, like Java, you optimize for easy implementation by the caller of the function. The caller knows what data format the function accepts, and what data format the function returns. The more strictly you define these data formats, the easier things become for the caller. They can rely on the function signature, without looking at the source code, to know how to use the function. This is inefficient for a single programmer, but efficient for a team (people can treat functions like black boxes).

In this specific case you would have to think carefully about how this function could define its input and output so that it becomes a black box. You wouldn't return arrays of strings. You would return lists of objects of a specific (possibly self-declared) type.

Answer 3

Firstly, I recommend you rid yourself of the notion that Java and Javascript are in any way related. It's a common misconception with newcomers, but apart from the name, and some of the basic syntax, they're about as closely related as C++ and Python.

Now, I'm not quite understanding the requirements of the actual problem (what does the string of numbers represent?) but if you're trying to decide what data structures to use in Java, the place to look is the java.utils package.

Answer 4

Needless to say, this is the wrong way to go about it.

Not necessarily. Whatever does the job at hand in an understandable, testable and reliable fashion is a good way to go about it. At the data structure abstraction level, one can think more-or-less language-agnosticly.

Now, languages with a strong type system like Java just nudge you a little bit to good and thought-out data structures, because dirty hacks like your 2xX string vector are not much easier to pull of.

So, what you'll have to think about in making your transition is how you think about your data and put this structure into a data structure. That is, you'd probably declare a WorkLoad class that consists of a list of WorkItem objects, each of which consists of a number of week days (or whatever is in your first array element, can't tell that from looking at your data) and a profit.

Answer 5

I would go for something like

public class Main {
  // ...
  public static class Item {
     private List<Integer> list = new ArrayList<Integer>(); // good when you need add elements
     //or: private int[] array; // instantiate when you know number of olements
     private int something;
     // getters, maybe setters (or fill data in constructor and don't allow changing them
     // int that case, don't expose array/list, make accesor methods instead
  }

  public List<Item> yourFunction() {
       // something
  }
}