New to Java

Hi, I just started working with java and I have probably an easy problem to solve.
I've made an array and I want to fill it with random integers from 0-39. Each integer can be used only once.

Here's the code:

for (i=0; i<40; i++) 
{
  kup[i] = (int)(Math.random() * 40);
  for (j=0; j<40; j++) 
  {
    if (j != i) 
    {
      while (kup[i] == kup[j])
      {
        kup[i] = (int)(Math.random() * 40);
      } 
    }
  }
}