Make a Method Which Generates the x and y values of Another Given Function

Question

I have just started to learn about Java Runnables and I have heard of Callables. However, I am very much struggling with this problem. I would like to make a method which takes a function as an argument (whether that be as a Callable, a Runnable, or something else, as long as I can simply call the function as coolNewFunction(() -> otherFunction(), 100) or some similar simple way) and the method would return an array of the returned values of the otherFunction. For example, say I defined the function

public static int square(int x){

    return x * x;

} 

I could then do something along the lines of:

coolNewFunction(() -> square(), 100)

And this would return an array of the first 100 numbers and their squares (i.e. {{1, 1}, {2, 4}, {3, 9}...}). Now right off the bat I know that the lambda () -> square() wouldn't work because square has to be passed a value. I though of creating an array of 100 Runnables each of which has the next argument for square, but still the method run() doesn't return anything. So, long story short, what would a method look like which evaluates another function which is given as an argument like square at different x values and returns an array of that evaluation? Also, preferably I don't want to start any new threads although if this is the only way that this can be achieved than that is okay. Finally, I don't want to have to implement the square (or other) function in a special way (preferably).

Answer 1

I hope you don't mind if I don't use an Array, but I will use your square method

public Map<Integer, Integer> lotsOfSquares(int limit) {

    return IntStream.rangeClosed(1,limit)                         // Creates a stream of 1, 2, 3, ... limit
                    .boxed()                                      //  Boxes int to Integer. 
                    .collect(Collectors.toMap(i -> i,             // Collects the numbers, i->i generates the map key
                                              i -> square(i));    // Generates the map value
}

This will give you a map containing {1=1, 2=4, 3=9, ... , 99=9801, 100=10000}.

You should probably add some validation on limit.

Update:

public <T> Map<Integer, T> lotsOfResults(int limit, Function<Integer, T> f) {

    return IntStream.rangeClosed(1,limit)                        
                    .boxed()                                      
                    .collect(Collectors.toMap(i -> i,             
                                              i -> f.apply(i));    
}

Now, you can call lotsOfResults(100, i -> square(i))

Note that T is the return type of f -- in case you get tired of squaring.

Answer 2

Hope this helps:

public int[][] fn2Array(Function<Integer, Integer> fn, int x) {
    int[][] result = new int[x][2];
    for (int i; i < x; i++) {
        result[i][0]=i+1;
        result[i][1]=fn.apply(i+1);
    }
    return result;
}