Adding elements in an array in a loop

Change

for (int i = 0; i < totalSale.length; i++) {
      if (i == 0) {
       System.out.println("Total sale for category " + i + ": $" + totalSale[0]);
      }
      if (i == 1) {
       System.out.println("Total sale for category " + i + ": $" + totalSale[1]);
      }
      if (i == 2) {
       System.out.println("Total sale for category " + i + ": $" + totalSale[2]);
      }
      if (i == 3) {
       System.out.println("Total sale for category " + i + ": $" + totalSale[3]);
      }
}

to

for (int i = 0; i < totalSale.length; i++) {
  System.out.println("Total sale for category " + i + ": $ + totalSale[i]);
}

There's no need to create an if-statement for every single element in the array totalSale. Think about general cases for the array when dealing with loop statements involving arrays. Since you're printing out every single element in the array with the same line "Total sale for category", you can toy with it and think about what's changing and what isn't.

Clearly, the only things changing are i and the element in the array that's being printed out. What's changing can be represented by variables (i, totalSale[i]) and what's not changing can be represented by constants ("Total sale for category", "$"). Therefore, you only need one line in your for-loop to express this when printing out elements of an array.

EDIT: A way to deal with the problem of accepting a variable number of sales categories in your method is to instead make your method take in a 2D array, in which the number of rows is the number of categories and the number of columns ins the number of sales within each category. For example, int sales[][] = new int[30][20] represents an array of 30 categories with 20 sales in each category. So modify the header,

public static int[] totalSale( int[] mac, int[] iphone, int[] ipad, int[] ipod )

to

public static int[] totalSale(int[][] sales)

and change this,

int[] totalSale = {0,0,0,0};

for (int i = 0; i < mac.length ; i++) {
    totalSale[0] = totalSale[0] + mac[i]; 
    totalSale[1] = totalSale[1] + iphone[i]; 
    totalSale[2] = totalSale[2] + ipad[i];
    totalSale[3] = totalSale[3] + ipod[i];
}

to

int[] totalSale = new int[sales.length];//sales.length is the number of         
                                        //rows/categories in sales

for (int i = 0; i < sales.length ; i++) {
    for (int j = 0; j < sales[i].length; j++) { //sales[i].length is the number
                                                //of sales/columns per category
        totalSale[i] = totalSale[i] + sales[i][j];//sales[i][j] is the jth
                                                  //sale in the ith category
    }
}

And lastly in the main, those 4 arrays can now be replaced with a 2D array.

Replace,

int[] mac = {200,9000,13000,900};
int[] iphone = {500,5000,200,0};
int[] ipad = {900,4300,0,800};
int[] ipod = {0,300,120,500};

with

int[][] sales = { {200,9000,13000,900},
                  {500,5000,200,0},
                  {900,4300,0,800},
                  {0,300,120,500}}; //how you define a 2D array

Above is a 4x4 array but it can be flexible. You can make it 10x10, 20x20, 30x30, etc. Lastly, don't forget to change the method call to take in only one parameter now, the 2D array.

Change

popularDay(days,mac,iphone,ipad,ipod);

to

popularDay(sales);

Here, i can see that you are assuming mac, iphone, ipad and ipod arrays will always have the same number of elements (length). Your code can break in the case one of them have a different length.

I would rather suggest this approach:

public static int[] totalSale( int[] mac, int[] iphone, int[] ipad, int[] ipod ){
    int[] totalSale = {0,0,0,0};

    for (int i = 0; i < mac.length ; i++) {
        totalSale[0] = totalSale[0] + mac[i];
    }
    for (int i = 0; i < iphone.length ; i++) {
        totalSale[1] = totalSale[1] + iphone[i];
    }
    for (int i = 0; i < ipad.length ; i++) {
        totalSale[2] = totalSale[2] + ipad[i];
    }
    for (int i = 0; i < ipod.length ; i++) {
        totalSale[3] = totalSale[3] + ipod[i];
    }

    System.out.println("Total sale for category mac: $" + totalSale[0]);
    System.out.println("Total sale for category iphone: $" + totalSale[1]);
    System.out.println("Total sale for category ipad: $" + totalSale[2]);
    System.out.println("Total sale for category ipod: $" + totalSale[3]);

    return totalSale;
}

First, calculate the total sale for each category by its own, using a for loop for each of them. Then writing the result back to the console without the for loop, since it has no reason to be used.

Varargs will help you: https://docs.oracle.com/javase/1.5.0/docs/guide/language/varargs.html

public static int[] totalSale(int[]... categories) {
    int[] totalSale = new int[categories.length];

    for (int i = 0; i < totalSale.length; i++) {

        for (int j = 0; j < categories[i].length; j++) {
            totalSale[i] = totalSale[i] + categories[i][j];
        }
        System.out.println("Total sale for category " + i + ": $" + totalSale[i]);
    }
    return totalSale;
}

So, in this case you can either to pass any amount of parameters:

int[] totalSales = totalSale(mac, iphone, ipad, ipod, ipod2, iphone3, iphone4);

int[] totalSales2 = totalSale(ipod, ipod2, iphone3, iphone4);

Or make an array to pass only one parameter using the same method:

int[][] categories = {mac, iphone, ipad, ipod, ipod2, ipod3, iphone100500};
int[] totalSales2 = totalSale(categories);

And if you want use only the second way, then you can just define the method as:

public static int[] totalSale(int[][] categories)

I suppose you requirement is SUM all array data as below and store the result into the array too?

int[] mac = {200,9000,13000,900};
int[] iphone = {500,5000,200,0};
int[] ipad = {900,4300,0,800};
int[] ipod = {0,300,120,500};
int[] result equal to {macSUM, iphoneSUM, ipadSUM, ipodSUM}

So we can use two loops one if for each device, and other one is for each element in the array the code show below as example

 public static int totalSale(int[] devices){
        int sum;
        for (int i = 0; i < devices.length ; i++) {
            sum += devices[i];
        }

        return totalSale;
    }

public static void main(String[] arguments) {
    int[] days = {1,2,3,4};
    int[] mac = {200,9000,13000,900};
    int[] iphone = {500,5000,200,0};
    int[] ipad = {900,4300,0,800};
    int[] ipod = {0,300,120,500};
    numberOfDays(days);
    int[]totalSales = new int[4];
    for(int i = 0; i < 4; i++) {
            totalSales[i] = totalSale(mac);
        }
}