trying to generate a fibonacci sequence 0,1,1,2,3,5,8,13,21,34 with for loop and without the final comma

The way you are doing the tour, you should set the conditionals in this way .. with two consecutive ifs , so that in the first loop of the for loop , read the first if the condition is met, and continue and also enter the else , with i=0 . So you will not miss another round in the for, as I was missing. I do not know if I explained myself well.

for (int i = 0; i < 4; i++) {
            if (i==0) {
                System.out.print(a+","+b+",");
            }
            if(i==3) {
                a=a+b;
                b=a+b;
                System.out.print(a+","+b);
            }else {
                a=a+b;
                b=a+b;
                System.out.print(a+","+b+",");
            }
        }

You can do it easily in the following way without needing if-else and solving the problem of the last comma:

public static void main(String[] args) {

        int n1 = 0, n2 = 1, n3, cantidad = 10;

        // Imprimir 0 y 1
        System.out.print(n1 + ", " + n2);

        // Comienza desde 2 porque 0 y 1 ya fueron imprimidos anteriormente
        for (int i = 2; i < cantidad; ++i) {
            n3 = n1 + n2;
            System.out.print(", " + n3);
            n1 = n2;
            n2 = n3;
        }
    }

Result: 0, 1, 1, 2, 3, 5, 8, 13, 21, 34