Why Integer numberOfLeadingZeros and numberOfTrailingZeros use different implementations?

问题

In JDK 8:

public static int numberOfLeadingZeros(int i) {
    if (i == 0)
        return 32;
    int n = 1;
    // if the first leftest one bit occurs in the low 16 bits
    if (i >>> 16 == 0) { n += 16; i <<= 16; }
    if (i >>> 24 == 0) { n +=  8; i <<=  8; }
    if (i >>> 28 == 0) { n +=  4; i <<=  4; }
    if (i >>> 30 == 0) { n +=  2; i <<=  2; }
    n -= i >>> 31;
    return n;
}

By judging if the leftest first one bit is in the low x bits.

public static int numberOfTrailingZeros(int i) {
    int y;
    if (i == 0) return 32;
    int n = 31;
    // if the rightest first one bit occurs in the low 16 bits
    y = i <<16; if (y != 0) { n = n -16; i = y; }
    y = i << 8; if (y != 0) { n = n - 8; i = y; }
    y = i << 4; if (y != 0) { n = n - 4; i = y; }
    y = i << 2; if (y != 0) { n = n - 2; i = y; }
    return n - ((i << 1) >>> 31);
}

By judging if the rightest first one bit is in the low x bits.

Why numberOfTrailingZeros doesn't use the implementation like numberOfLeadingZeros?

    public static int numberOfTrailingZeros2(int i) {
        if (i == 0) return 32;
        int n = 0;
        if (i << 16 == 0) { n += 16; i >>>= 16; }
        if (i << 24 == 0) { n +=  8; i >>>=  8; }
        if (i << 28 == 0) { n +=  4; i >>>=  4; }
        if (i << 30 == 0) { n +=  2; i >>>=  2; }
        if (i << 31 == 0) { n +=  1; i >>>=  1; }
        return n;
    }

And why numberOfLeadingZeros doesn't use the implementation like numberOfTrailingZeros?

    public static int numberOfLeadingZeros2(int i) {
        int y;
        if (i == 0) return 32;
        int n = 31;
        y = i >>>16; if (y != 0) { n = n -16; i = y; }
        y = i >>> 8; if (y != 0) { n = n - 8; i = y; }
        y = i >>> 4; if (y != 0) { n = n - 4; i = y; }
        y = i >>> 2; if (y != 0) { n = n - 2; i = y; }
        return n - (i >>> 1);
    }

来源：https://stackoverflow.com/questions/59463728/why-integer-numberofleadingzeros-and-numberoftrailingzeros-use-different-impleme