Java error: Comparison method violates its general contract

Question

I saw many questions about this, and tried to solve the problem, but after one hour of googling and a lots of trial & error, I still can\'t fix it. I hope some of you ca

Answer 1

I got the same error with a class like the following StockPickBean. Called from this code:

List beansListcatMap.getValue();
beansList.sort(StockPickBean.Comparators.VALUE);

public class StockPickBean implements Comparable {
    private double value;
    public double getValue() { return value; }
    public void setValue(double value) { this.value = value; }

    @Override
    public int compareTo(StockPickBean view) {
        return Comparators.VALUE.compare(this,view); //return 
        Comparators.SYMBOL.compare(this,view);
    }

    public static class Comparators {
        public static Comparator VALUE = (val1, val2) -> 
(int) 
         (val1.value - val2.value);
    }
}

After getting the same error:

java.lang.IllegalArgumentException: Comparison method violates its general contract!

I changed this line:

public static Comparator VALUE = (val1, val2) -> (int) 
         (val1.value - val2.value);

to:

public static Comparator VALUE = (StockPickBean spb1, 
StockPickBean spb2) -> Double.compare(spb2.value,spb1.value);

That fixes the error.