How to generate given number of random objects with given number of attributes?

My question is, how to generate a random challenge with given number (4-8) of trials ? I dont have any idea how to do that.
A challenge is built from trials(see in code).
The Trials are the attributes of Challenge class, built from an enum and an EnumMap
So when I get a random number of Integer in main (like r.nextInt(4)+4;) i will generate that number of trials for a challenge like
Weapons=25;
Hacking=32;
Vehicle=34;
Speed=56;
So for 1 Challenge, I dont need all the trials, just some of them. But I still need the possibility to choos from them randomly(4-8).
How can I generate these random Challenge objects with given number of attributes(and the values of them)? So basically some of them, from ALL of them.

Is this all wrong ?How should I start it ?
The code so far:

import java.util.*;

public class Challenge {
    Random r = new Random();

    // DRIVE, VEHICLE, ACCURACY, WEAPONS, REFLEX, STRATEGY, CHARISMA, HACKING,
    // SPEED, STEALTH;
    public static enum Trial {
        DRIVE, VEHICLE, ACCURACY, WEAPONS, REFLEX, STRATEGY, CHARISMA, HACKING, SPEED, STEALTH;
    }

    Map<Trial, Integer> challenge = new EnumMap<Trial, Integer>(Trial.class);

    public Challenge() {
        challenge.put(Trial.DRIVE, r.nextInt(100) + 25);
        challenge.put(Trial.VEHICLE, r.nextInt(100) + 25);
        challenge.put(Trial.ACCURACY, r.nextInt(100) + 25);
        challenge.put(Trial.WEAPONS, r.nextInt(100) + 25);
        challenge.put(Trial.REFLEX, r.nextInt(100) + 25);
        challenge.put(Trial.STRATEGY, r.nextInt(100) + 25);
        challenge.put(Trial.CHARISMA, r.nextInt(100) + 25);
        challenge.put(Trial.HACKING, r.nextInt(100) + 25);
        challenge.put(Trial.SPEED, r.nextInt(100) + 25);
        challenge.put(Trial.STEALTH, r.nextInt(100) + 25);
    }

    List<Trial> keys = new ArrayList<Trial>(challenge.keySet());
    Trial randomKey = keys.get(r.nextInt(keys.size()));
    Integer value = challenge.get(randomKey);
}