What dose it means

at Dice.rollDice(Dice.java:30)
java.lang.IllegalArgumentException: n must be positive
at java.util.Random.nextInt(Random.java:250)
at Dice.rollDice(Dice.java:30)
my code is :

import java.util.Random;

public class Dice
    // instance variables.
    private Random mRandom;
    private int mSides;

     * Constructor for objects of class Dice
    public Dice(int sides)
        sides = mSides;
        mRandom = new Random();
    public int rollDice()
        return mRandom.nextInt(mSides);

first of all this:
sides = mSides;
should be like this:
mSides = sides;

And the error you get is because the method:
doesn't take as argument a negative or zero number; as the error message says it must be positive.

The reason you get the error is because when you do:
sides = mSides
the sides is the argument and you simply change its value. The mSides which is the class variable doesn't change and remains to 0 and you get error.

If you had:
mSides = sides
the argument sides will give value to the mSides of the class, and it will take the value you give it, so the call:
will have what you give as argument

Thanks alot, I was realy blind.