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I have this code: package org.optimization.geneticAlgorithm; import org.optimization.geneticAlgorithm.selection.Pair; public abstract class Chromosome implements Comparable<Chromosome> { public abstract double fitness(); public abstract Pair<Chromosome> crossover(Chromosome parent); public abstract void mutation(); public int compareTo(Chromosome o) { int rv = 0; if (this.fitness() > o.fitness()) { rv = -1; } else if (this.fitness() < o.fitness()) { rv = 1; } return rv; } } And every time I run this code I get this error: Exception in thread "main" java.lang.IllegalArgumentException: Comparison method violates its general contract! at java.util.ComparableTimSort.mergeHi(ComparableTimSort.java:835) at java.util.ComparableTimSort.mergeAt(ComparableTimSort.java:453) at java.util.ComparableTimSort.mergeCollapse(ComparableTimSort.java:376) at java.util.ComparableTimSort.sort(ComparableTimSort.java:182) at java.util.ComparableTimSort.sort(ComparableTimSort.java:146) at java.util.Arrays.sort(Arrays.java:472) at java.util.Collections.sort(Collections.java:155) at org.optimization.geneticAlgorithm.GeneticAlgorithm.nextGeneration(GeneticAlgorithm.java:74) at org.optimization.geneticAlgorithm.GeneticAlgorithm.execute(GeneticAlgorithm.java:40) at test.newData.InferenceModel.main(InferenceModel.java:134) I use OpenJDK7u3 and I return 0 when the objects are equal. Can someone explain this error to me?

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