不明白sun的这段代码什么意思

 

public static int min(int a, int b) {
	return (a <= b) ? a : b;
    }

public static long min(long a, long b) {
	return (a <= b) ? a : b;
    }

public static float min(float a, float b) {
        if (a != a) return a;	// a is NaN
	if ((a == 0.0f) && (b == 0.0f)
	    && (Float.floatToIntBits(b) == negativeZeroFloatBits)) {
	    return b;
	}
	return (a <= b) ? a : b;
    }

public static double min(double a, double b) {
        if (a != a) return a;	// a is NaN
	if ((a == 0.0d) && (b == 0.0d)
	    && (Double.doubleToLongBits(b) == negativeZeroDoubleBits)) {
	    return b;
	}
	return (a <= b) ? a : b;
    }

以上是Math类中的代码，同样是比较，为什么float和double弄的这么麻烦？a!=a是什么判断？