Java Comparator：违反总承包

import java.util.Comparator;

public class SolutionComparator implements Comparator<ExpressionTree> {
    private final int target;

    public SolutionComparator(int target) {
        this.target = target;
    }

    @Override
    public int compare(ExpressionTree o1,ExpressionTree o2) {
        int v1 = o1.getValue();
        int v2 = o2.getValue();
        if (v1 == -1 && v2 == -1)
            return 0;
        else if (v1 == -1)
            return 1;
        else if (v2 == -1)
            return -1;
        else if (v1 == v2)
            return (int)Math.signum(solutionCost(o1) - solutionCost(o2));
        else
            return (int)Math.signum(Math.abs(target-v1) - Math.abs(target-v2));
    }

    private int solutionCost(ExpressionTree v) {
        int cost = 0;
        Operation o = v.getoperator();
        if (o != Operation.NONE) {
            cost += o.getCost();
            for (ExpressionTree e : v.getChildren())
                cost += solutionCost(e);
        }
        return cost;
    }
}

几个月来我一直在看这个代码,我无法找出它违反比较器总合同的原因.

这个想法是每个ExpressionTree可以被评估为一个结果. (getValue()方法).如果它返回-1,它总是高于其他数字.如果值不同,则比较它与目标的接近程度.如果值相同,则按解决方案成本进行比较.

使用此比较器,Java抛出IllegalStatesException.但如果我删除基于成本的比较,它就有效.

编辑：异常跟踪

Exception in thread "Thread-3" java.lang.IllegalArgumentException: Comparison method violates its general contract!
    at java.util.Timsort.mergeHi(Timsort.java:868)
    at java.util.Timsort.mergeAt(Timsort.java:485)
    at java.util.Timsort.mergeCollapse(Timsort.java:408)
    at java.util.Timsort.sort(Timsort.java:214)
    at java.util.Timsort.sort(Timsort.java:173)
    at java.util.Arrays.sort(Arrays.java:659)
    at java.util.Collections.sort(Collections.java:217)
    at ***.run(***:123)
    at java.lang.Thread.run(Thread.java:722)