为什么不建议使用实数作为 HashMap 的 key？

import java.util.HashMap;
import java.util.Map;
//class Point {
//    int x;
//    int y;
//    Point(int a, int b) { x = a; y = b; }
//}
public class Solution {
    public int maxPoints(Point[] points) {
        if (points.length <= 2) {
            return points.length;
        }
        int max = 2;
        for (int i = 0; i < points.length - 1; i++) {
            Map<Float, Integer> map = new HashMap<>(16);
            // 记录垂直点数; 当前和Points[i]在一条线上的最大点数; 和Points[i]垂直的点数
            int ver = 0, cur, dup = 0;
            for (int j = i + 1; j < points.length; j++) {
                if (points[j].x == points[i].x) {
                    if (points[j].y != points[i].y) {
                        ver++;
                    } else {
                        dup++;
                    }
                } else {
                    float d = (float)((points[j].y - points[i].y) / (double) (points[j].x - points[i].x));
                    map.put(d, map.get(d) == null ? 1 : map.get(d) + 1);
                }
            }
            cur = ver;
            for (int v : map.values()) {
                cur = Math.max(v, cur);
            }
            max = Math.max(max, cur + dup + 1);
        }
        return max;
    }
}

public static void main(String[] args) {
    int[][] vals = {{2,3},{3,3},{-5,3}};
    Point[] points = new Point[3];
    for (int i=0; i<vals.length; i++){
        points[i] = new Point(vals[i][0], vals[i][1]);
    }
    Solution solution = new Solution();
    System.out.println(solution.maxPoints(points));
}

System.out.println(0.0 == -0.0);

map.put(d, map.get(d) == null ? 1 : map.get(d) + 1);

public V get(Object key) {
    Node<K,V> e;
    return (e = getNode(hash(key), key)) == null ? null : e.value;
}

static final int hash(Object key) {
    int h;
    return (key == null) ? 0 : (h = key.hashCode()) ^ (h >>> 16);
}

public native int hashCode();

public static void main(String[] args) {
    System.out.println(0.0 == -0.0);
    System.out.println(new Float(0.0).hashCode() == 
        new Float(-0.0).hashCode());
}

System.out.println(new Float(0.0).equals(0.0f));
System.out.println(new Float(0.0).equals((float) -0.0));

public boolean equals(Object obj) {
    return (obj instanceof Float)
           && (floatToIntBits(((Float)obj).value) == 
                   floatToIntBits(value));
}

/**
 * Returns a representation of the specified floating-point value
 * according to the IEEE 754 floating-point "single format" bit
 * layout.
 *
 * <p>Bit 31 (the bit that is selected by the mask
 * {@code 0x80000000}) represents the sign of the floating-point
 * number.
 * Bits 30-23 (the bits that are selected by the mask
 * {@code 0x7f800000}) represent the exponent.
 * Bits 22-0 (the bits that are selected by the mask
 * {@code 0x007fffff}) represent the significand (sometimes called
 * the mantissa) of the floating-point number.
 *
 * <p>If the argument is positive infinity, the result is
 * {@code 0x7f800000}.
 *
 * <p>If the argument is negative infinity, the result is
 * {@code 0xff800000}.
 *
 * <p>If the argument is NaN, the result is {@code 0x7fc00000}.
 *
 * <p>In all cases, the result is an integer that, when given to the
 * {@link #intBitsToFloat(int)} method, will produce a floating-point
 * value the same as the argument to {@code floatToIntBits}
 * (except all NaN values are collapsed to a single
 * "canonical" NaN value).
 *
 * @param   value   a floating-point number.
 * @return the bits that represent the floating-point number.
 */
public static int floatToIntBits(float value) {
    int result = floatToRawIntBits(value);
    // Check for NaN based on values of bit fields, maximum
    // exponent and nonzero significand.
    if (((result & FloatConsts.EXP_BIT_MASK) ==
          FloatConsts.EXP_BIT_MASK) &&
         (result & FloatConsts.SIGNIF_BIT_MASK) != 0)
        result = 0x7fc00000;
    return result;
}

System.out.println(Float.floatToIntBits((float) 0.0));
System.out.println(Float.floatToIntBits((float) -0.0));
System.out.println(Float.floatToRawIntBits(0.0f));
System.out.println(Float.floatToRawIntBits((float)-0.0));

0
-2147483648
0
-2147483648