Java error: Comparison method violates its general contract

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The exception message is actually pretty descriptive. The contract it mentions is transitivity: if A > B and B > C then for any A, B and C: A > C. I checked it with paper and pencil and your code seems to have few holes:

if (card1.getRarity() < card2.getRarity()) {
  return 1;

you do not return -1 if card1.getRarity() > card2.getRarity().

if (card1.getId() == card2.getId()) {
  //...
}
return -1;

You return -1 if ids aren’t equal. You should return -1 or 1 depending on which id was bigger.

Take a look at this. Apart from being much more readable, I think it should actually work:

if (card1.getSet() > card2.getSet()) {
    return 1;
}
if (card1.getSet() < card2.getSet()) {
    return -1;
};
if (card1.getRarity() < card2.getRarity()) {
    return 1;
}
if (card1.getRarity() > card2.getRarity()) {
    return -1;
}
if (card1.getId() > card2.getId()) {
    return 1;
}
if (card1.getId() < card2.getId()) {
    return -1;
}
return cardType - item.getCardType();  //watch out for overflow!

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