Here yo can see what is triangular number:
https://en.wikipedia.org/wiki/Triangular_number
public class NumShape {
public static void main(String[] args) {
class Number{
int number;
public boolean isTriangular(){
int x=1;
int triangularNumber=1;
while(triangularNumber<number)
{
x++;
triangularNumber=triangularNumber+x;
}
if(triangularNumber==number){
return true;
}else
{
return false;
}
}
}
Number myNumber = new Number();
myNumber.number=7;
System.out.println(myNumber.isTriangular());
}
}