Thread: Subtracting arrays in switch case adds them instead

So I am trying to make my first basic calculator
my problem is in "case 2" which was suppose to subtract two numbers is adding them even though i used "-" in between them
here is the program

import java.util.Scanner;
public class MyClass {
    public static void main(String args[])
    {
    	Scanner input = new Scanner(System.in);
    	int s1=0, calculation = 0;
    	double sum = 0, sum1 = 1; int o;
    	String s;
    	do {
    	do {
    	System.out.println("enter the quantity of numbers to calculate");
    	s1 = input.nextInt();
    	if (s1<=1) {
    		System.out.println("please enter two number minimum");
    	} 
    	}while (s1<=1); /* this means that if we have only 1 number to calculate 
    	                      we cant do that so we ask user again to enter the number */
    	int quantity[]= new int[s1];
    	System.out.println("enter the numbers");
    	for (int i=0;i<s1;i++)  /*this means quantity[0] is the first empty box to input 
    	                              the first number, and its gonna continue till it reaches s1 number of quantity.*/
    	{
    		quantity[i] = input.nextInt();
    	}
    	if (s1<=2) /*we are making seperate condition for division and subtraction cuz
    	               they only use two numbers*/
    	{
    		    System.out.println("choose the method of calculation");
    		    System.out.println("type 1 for division");
    		    System.out.println("type 2 for subtraction");
    		    calculation = input.nextInt();
    	}
    	else {
    	            System.out.println("choose the method of calculation");
        	    System.out.println("type 3 for multiplication");
        	    System.out.println("type 4 for addition");
        	    calculation = input.nextInt();
    		}
    	switch (calculation) {
    	case 1: 
    			for (int i=0;i<2;i++) {
    			sum = quantity[i] / quantity[i++]; /* this means that its gonna its gonna 
    			                                             take the number stored in quantity[0](in first run) and divide it with 
    			                                             the value stored in quantity[1] */
    			}
    	case 2:
    			for (int i=0;i<2;i++) {
    				sum = quantity[i] - quantity[i++];
    			}
    	case 3: 
    			for (int i=0;i<s1;i++) {
    				sum1 = quantity[i] * sum1; /* we are taking sum1=1 from main class because 
    			                                              if we use sum whose value is 0 the result is alwasy gonna be 0 since anythngX0=0*/
    			}
    	case 4:
    			for (int i=0;i<s1;i++ ) {
    				sum = quantity[i] + sum;
    			}
    			}
    		   if ( (calculation)==3) /* since multiplication use variable sum1 to store the sum we 
    		                                are gonna have to make a seperate condition to print it*/
    		{ System.out.println("the sum of ");
    			for (int i=0;i<s1;i++) {
    			System.out.print(quantity[i]+", "); /* this code will print "the sum of"
    			                                              first and then loop itself by printing the numbers stored in quantity[0]
    			                                             (in first run) and then print quantity[1] in the same line till we reach the
    			                                              last quanity i.e s1 */
 
 
    			} 	System.out.println("is"+sum1);
    			}
    		 else {
    			System.out.println("the result is"+ sum);
    		}
    		   System.out.println("do you want to calculate again?");
    		   s = input.next();
    }         while (s.equalsIgnoreCase("yes") || (s.equalsIgnoreCase("yep") || (s.equalsIgnoreCase("yea") || (s.equalsIgnoreCase("ok") || 
               (s.equalsIgnoreCase("okay"))))));
 
    }
    }