Q:

Java: do something x percent of the time

I need a few lines of Java code that run a command x percent of the time at random.
psuedocode:
boolean x = true 10% of cases.

if(x){
  System.out.println("you got lucky");
}

A:

You just need something like this:
Random rand = new Random();

if (rand.nextInt(10) == 0) {
    System.out.println("you got lucky");
}

Here's a full example that measures it:
import java.util.Random;

public class Rand10 {
    public static void main(String[] args) {
        Random rand = new Random();
        int lucky = 0;
        for (int i = 0; i < 1000000; i++) {
            if (rand.nextInt(10) == 0) {
                lucky++;
            }
        }
        System.out.println(lucky); // you'll get a number close to 100000
    }
}

If you want something like 34% you could use rand.nextInt(100) < 34.

A:

If by time you mean times that the code is being executed, so that you want something, inside a code block, that is executed 10% of the times the whole block is executed you can just do something like:
Random r = new Random();

...
void yourFunction()
{
  float chance = r.nextFloat();

  if (chance <= 0.10f)
    doSomethingLucky();
}

Of course 0.10f stands for 10% but you can adjust it. Like every PRNG algorithm this works by average usage. You won't get near to 10% unless yourFunction() is called a reasonable amount of times.

A:

To take your code as a base, you could simply do it like that:
if(Math.random() < 0.1){
  System.out.println("you got lucky");
}

FYI Math.random() uses a static instance of Random

