Why Does My Java Comparator Throw a 'Comparison Method Violates Its General Contract' Error?

Java Error: "Comparison Method Violates Its General Contract"

When sorting collections, Java utilizes a comparison method to establish object ordering. However, this method must adhere to the transitivity principle, which states that if object A is greater than B and B is greater than C, then A must be greater than C. In certain circumstances, this contract can be violated, leading to the "Comparison method violates its general contract" error.

Example

Consider the following custom comparator, intended to sort objects based on specific criteria. However, due to logical inconsistencies, it violates the transitivity rule:

@Override
public int compareTo(Object o) {
    ...
    if (card1.getRarity() < card2.getRarity()) {
        return 1;
    }
    ...
    if (card1.getSet() > card2.getSet()) {
        return -1;
    }
    ...
}

Transitivity Issue

In the above example, if card1.getRarity() is the deciding factor and card1.getRarity() is less than card2.getRarity(), the comparator correctly returns 1. However, if card2.getSet() is the deciding factor and card2.getSet() is less than card1.getSet(), the comparator incorrectly returns -1. This results in a non-transitive situation.

Correct Solution

To ensure transitivity, the comparator should adhere to the following logic:

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

In summary, to avoid the "Comparison method violates its general contract" error, ensure your comparator method adheres to the transitivity principle. This rule dictates that if object A is greater than B and B is greater than C, then A must be greater than C. By ensuring transitivity, you guarantee consistent and correct sorting behavior.

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