Java: Passing arrays from another method

Yes, you are correct you have to pass the 2 array to the forth function and you can return the result array to

public static double[] calculateSavings(double withoutGardens[], double withGardens[])
{
    double difference_array[withoutGardens.length()];
    //.....some code goes here for calculations
    return difference_array;
}

double[] diff = new double[12]; 
for(int i = 0; i < 12; i++){
    diff[i] = withoutGardens[i] - withGardens[i];
}
return diff;

I know an answer has already been accepted, still, there are couple of things I'd like to mention, that you/others might benefit from. First, let billWithoutGardens() return an array of doubles that the user has entered, so let's change it's signature to:

public static double[] billWithoutGardens() {

}

Next, you can greatly simplify data entry by using a loop in your method, eg.:

//an array of months
String[] months = {"January", "February", "March", "April", "May", "June", "July", "September", "October", "November", "December"};

System.out.println("Please enter energy bills for year without rooftop gardens");

//loop through the months and store the values in your array
for(int i=0; i<months.length; i++) {
    System.out.print("Please enter energy bill for " + months[i] + ": ");
    withoutGardens[i] = input.nextDouble();

    //!!!!!!!!!!!!!
    //also remember to skip newline character added when user pressed ENTER
    //you can consume it with this call, which will read newline character
    input.nextLine();
}

Then, return the array with the values by adding this as the last line in your method:

return withoutGardens;

Next, to calculate savings, let's also return an array in calculateSavings(), so change it's signature to:

public static double[] calculateSavings(double[] withoutGardens, double[] withGardens)

And to calculate the difference, create an array that will hold the difference for each month, and loop through the months:

double[] difference = new double[withoutGardens.length];

for (int i=0; i<difference.length; i++) {
    difference[i] = withoutGardens[i] - withGardens[i];
}

At the end, return the array:

return difference;

In similar fashion, you can rewrite your other methods. Then, to use them in main(), you can have the following:

public static void main(String[] args) {
    double[] wGardens = billWithGardens();
    double[] woGardens = billWithoutGardens();
    double[] savings = calculateSavings(woGardens, wGardens);

    System.out.println(Arrays.toString(savings));

}

I believe I can help answer your question.

Before I answer the question, let me point out that instead of the following parameters:

double withoutGardens[],  double withGardens[]

You should instead use:

double[] withoutGardens, double[] withGardens

To ensure that the type of the parameter is an array, and the brackets aren't just in the name.

Now, to determine the difference between each of the elements, you must loop through them. This can be done with a simple for loop or with a foreach loop. In my opinion, a for loop would be easier than a foreach loop since you're using 2 arrays, and a for loop can give you an integer for an index to use for both. If you used a foreach loop, you would have to use a method to hunt down the current index.

With a for loop, it would look something like this:

public static double[] calculateSavings(double[] withGardens, double[] withoutGardens) {
    double[] difference = new double[];
    for (int i = 0; i < withGardens.size(); i++) { 
        difference[i] = withGardens[i] - withoutGardens[i];
    }
    return difference;
}

I hope this helps!