Recursion - If Else Loop To Check If Operation Is Completed

I am currently learning recursion, and i have a query. I have a program that receives user input, and outputs the decimal input into binary form. In my recursive function, I have a if else loop to check if operation is completed.

public static int Converter(int input, int weight) {
System.out.printf("Input is now %d, weight is now %d
",input,weight);
if(input==0) {
return 0;

[Code] .....

The final return, when input finally hits zero is concatenating 0 to whatever i have this far. e.g. Decimal 10 should give 1010, but because of the "return 0", instead of getting 1010+0, i am getting 10100.

I can solve the problem by creating a variable sum,and passing along, but i am just curious why the final return 0 is giving me not a +0, but a concatenate 0.