Generating random numbers without duplicates [duplicate]

This question already has an answer here:

• How can I prevent the overlapping random numbers 7 answers

As the title says I'm trying to get my code to not generate the same number twice in a set array. EX: (comp is an array defined as int[] comp = new int[20];)

Random ran = new Random();
for(int i = 0; i < comp.length; i++)
{
    comp[i] = ran.nextInt(80)+1;
    for(int j = 0; j < comp.length; j++)
    {
        if(comp[i] == comp[j])
            comp[i] = ran.nextInt(80)+1;
    }
    System.out.println(comp[i]);
}