Question

I have a simple question. I try to get random values from a container RandomList. It works with String and int, however doesn’t work with double what is wrong and how to fix it?

Thank you

import java.util.*;



public class RandomList<T> {

  private ArrayList<T> storage = new ArrayList<T>();

  private Random rand = new Random(47);

  public void add(T item) { storage.add(item); }

  public T select() {

    return storage.get(rand.nextInt(storage.size()));

  }

  public static void main(String[] args) {

    RandomList<String> rs = new RandomList<String>();
    RandomList<Integer> rs1 = new RandomList<Integer>();
    RandomList<Double> rs2 = new RandomList<Double>();
    double w = 1.23;

    for(String s: ("The quick brown fox jumped over " +

        "the lazy brown dog").split(" "))

      rs.add(s);

    for(int i = 0; i < 11; i++)

      System.out.print(rs.select() + " ");

    for (int i = 0; i < 5; i++)
        rs1.add(i);
    for (int i = 0; i < 5; i++)
        System.out.print(rs1.select() + " ");

    for (int i = 0; i < 5; i++)
        w = (double)i/5;
        rs2.add(w);
    for (int i = 0; i < 5; i++)
        System.out.print(rs2.select() + " ");

  }

}

Output is brown over fox quick quick dog brown The brown lazy brown 3 0 3 1 1 0.8 0.8 0.8 0.8 0.8 , so it works for string and int, but doesn’t for double (same value each time).

Answer 1

You don’t have any curly braces around the for loop where you add a bunch of doubles, so it’s not executing both of the indented statements. As a result, you’re just adding one number in, rather than five. Try adding the missing braces and see if that fixes things.

For future reference, if you had declared w as a local variable inside the for loop, this wouldn’t have even compiled at all because the statement that adds w to the random list would have been referencing a variable that’s out of scope. Scoping variables to just where they’re needed can thus help prevent bugs like this.

Hope this helps!