关于jdk7中 使用Collections的排序方法时报Comparison method violates its general contract!异常

参考:

　　Comparison method violates its general contract

　　Comparison method violates its general contract!

　　比较器报错：Comparison method violates its general contract

　　图解JDK7的Comparison method violates its general contract异常

主要是因为在compare的实现方法中,没有处理好两个比较对象相等的情况.

错误比较代码:

Collections.sort(entryList, new Comparator>()
{
@Override
public int compare(Map.Entry o1, Map.Entry o2)
{
return (o1.getValue() - o2.getValue() > 0 ? -1 : 1);
}
});

参考Float的compare方法(因为Float实现了Comparable<Float>接口):

    public static int compare(float f1, float f2) {
        if (f1 < f2)
            return -1;           // Neither val is NaN, thisVal is smaller
        if (f1 > f2)
            return 1;            // Neither val is NaN, thisVal is larger

        // Cannot use floatToRawIntBits because of possibility of NaNs.
        int thisBits    = Float.floatToIntBits(f1);
        int anotherBits = Float.floatToIntBits(f2);

        return (thisBits == anotherBits ?  0 : // Values are equal
                (thisBits < anotherBits ? -1 : // (-0.0, 0.0) or (!NaN, NaN)
                 1));                          // (0.0, -0.0) or (NaN, !NaN)
    }

所以错误代码应该改为:　

        Collections.sort(entryList, new Comparator<Map.Entry<String, Float>>()
        {
            @Override
            public int compare(Map.Entry<String, Float> o1, Map.Entry<String, Float> o2)
            {
                return Float.compare(o1.getValue(),o1.getValue());
            }
        });

很多封装的数字对象都是有compare实现的.