Question

I wrote this method. I don’t understand why it is write out this exception:

Exception in thread “main” java.lang.ClassCastException: Kocsma.Sör cannot be cast to java.lang.Comparable

Anybody know what is my mistake?

The compiler reference this line

beers.put(beer, dl);

This is my code:

private Map<Beer, Integer> beers = new TreeMap<Beer, Integer>();


public void Upload(Beer beer, int dl) {
    int d = 0;
    Beer s = null;
    for (Map.Entry<Beer, Integer> item : beers.entrySet()) {
        if (item.getKey().equals(beer)) {
            d = item.getValue();
            s = item.getKey();
        }
    }
    if (s != null) {
        beers.put(s, d + dl);
    }else
    beers.put(beer, dl); // Here is the problem by the Compiler
}

Class Kocsma:

public Kocsma() {
    Upload(new Beer("Borsodi sör", 160, 4.6), 1000);
    Upload(new Beer("Pilsner Urquell", 250, 4.4), 800);
    Upload(new Beer("Soproni Ászok", 150, 4.5), 900);
    Upload(new Beer("Dreher Classic", 200, 5.2), 600);
}

Answer 1

Your Beer class needs to implement Comparable<Beer> or you need to provide a Comparator<Beer> to the TreeMap constructor.

private static class BeerComparator implements Comparator<Beer> {
     @Override
     public int compare(Beer b1, Beer b2) {
         //return a value > 1 if b1 is greater than b2, < 1 if b2 greater than b1,
         //and exactly 0 if the two are equal
     }
}

beers = new TreeMap<Beer, Integer>(new BeerComparator());

TreeMap stores keys by using a binary search tree. This works without effort for some common classes like Integer and String because they are naturally sortable and implement Comparable out of the box. However, for your Beer class, you would have to implement it manually.

If Beer is not a good candidate for comparisons (most things aren’t) then consider using HashMap instead, and overriding equals() and hashCode() on Beer (see Effective Java Chapter 3 for a great reference on this).