Could someone explain this code?

Question

public class Main {
public static void main(String[] args) {
System.out.println(tri(3));
}
static int tri(int n){
return n==0?0:n==1?1:tri(n-1)+tri(n-2);
}
}

Answer 1

You are calling tri(3 ) method with passing value 3.

The ternary conditional statement:
condition ? (True) :( False)
works like if condition is true then True block is executed else False block executed.
so if n == 0 then it returns 0 else goes to false part, it has again ternary condition nested. So that works same way as if n==1 then it returns 1 else executes tri( n -1 ) + tri( n-2 ) . call the self method in recursive passion.
So
tri(3) => tri(2) + tri(1)

Here both calls are new independent calls pushed into stack so
tri(2) => tri(1) + tri(0) => 1+0 = 1
And tri(1) => 1
So tri(3) => tri(2) + tri(1) => 1+1 =2
Finally returns 2

edit:
for clearence, simplipy it by :

public class Main {
public static void main(String[] args) {
System.out.println(tri(3));
}
static int tri(int n){
return n<=1? n : tri(n-1)+tri(n-2);
}
}

Answer 2

System.out.println( tri(3, 0) );
}
static int tri(int n, int level){
System.out.println( n +", level=" +level);
return
n==0? 0:
n==1? 1:
tri(n-1, level+1) +tri(n-2, level+1);
}