private final static int ADDRESS_BITS_PER_WORD = 6;
private final static int BITS_PER_WORD = 1 << ADDRESS_BITS_PER_WORD;    //1左移6位是64，也就是说BITS_PER_WORD=64

一个默认大小的BitSet里面是64个bit，也就是说有64个开关量，可以用于标记64个数据是否存在


    /**
     * Creates a new bit set. All bits are initially {@code false}.
     */
    public BitSet() {
        initWords(BITS_PER_WORD);    //默认64，这个64不是指字节，而是指64个Bit，就是8个字节，所以一个long正好是8个字节
        sizeIsSticky = false;
    }

BitSet里面是用long数组来维持标记的，一个long是8个字节，一个长度为1的long[]就可以表示64个bit的开关量

    /**
     * Creates a bit set whose initial size is large enough to explicitly
     * represent bits with indices in the range {@code 0} through
     * {@code nbits-1}. All bits are initially {@code false}.
     *
     * @param  nbits the initial size of the bit set
     * @throws NegativeArraySizeException if the specified initial size
     *         is negative
     */
    public BitSet(int nbits) {
        // nbits can't be negative; size 0 is OK
        if (nbits < 0)
            throw new NegativeArraySizeException("nbits < 0: " + nbits);

        initWords(nbits);
        sizeIsSticky = true;
    }

BitSet bitSet = new BitSet(100);
        System.out.println("bitSet.size = " + bitSet.size() + ", bitSet.length = " + bitSet.length());

        bitSet.set(46000, true);

        System.out.println("bitSet = " + bitSet.get(45999));
        System.out.println("bitSet = " + bitSet.get(46000));
        System.out.println("bitSet = " + bitSet.get(46001));
        System.out.println("bitSet.size = " + bitSet.size() + ", bitSet.length = " + bitSet.length());