BitSet

    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; //标识words大小由用户指定
    }
　　 // 
    private void initWords(int nbits) {
        words = new long[wordIndex(nbits-1) + 1];
    }
　　 // 因为long类型是64位，所以bitIndex/64就可以得到bitIndex位于Long数组的第几位private static int wordIndex(int bitIndex) {
　　　　  // ADDRESS_BIT_PER_WORD = 6
        return bitIndex >> ADDRESS_BITS_PER_WORD;
    }

    public void set(int bitIndex) {
        if (bitIndex < 0)
            throw new IndexOutOfBoundsException("bitIndex < 0: " + bitIndex);

        int wordIndex = wordIndex(bitIndex);
        expandTo(wordIndex);
　　　　 // 给对应的bitIndex位设置为1（1 << 65 = 2：溢出后归0了，当bitIndex>64时，实际上bitIndex=bitIndex%64）
　　　　 // 1 << bitIndex = 2^bitIndex = 1000...(bitIndex个0)
        words[wordIndex] |= (1L << bitIndex); // Restores invariants

        checkInvariants();
    }

　　 // 判断是否需要扩容
    private void expandTo(int wordIndex) {
        int wordsRequired = wordIndex+1;
        if (wordsInUse < wordsRequired) {
            ensureCapacity(wordsRequired);
            wordsInUse = wordsRequired;
        }
    }
　　 // 扩容操作，2倍扩容
    private void ensureCapacity(int wordsRequired) {
        if (words.length < wordsRequired) {
            // Allocate larger of doubled size or required size
            int request = Math.max(2 * words.length, wordsRequired);
            words = Arrays.copyOf(words, request);
            sizeIsSticky = false;
        }
    }

    /**
     * Every public method must preserve these invariants.
     */
    private void checkInvariants() {
        assert(wordsInUse == 0 || words[wordsInUse - 1] != 0);
        assert(wordsInUse >= 0 && wordsInUse <= words.length);
        assert(wordsInUse == words.length || words[wordsInUse] == 0);
    }

    public boolean get(int bitIndex) {
        if (bitIndex < 0)
            throw new IndexOutOfBoundsException("bitIndex < 0: " + bitIndex);

        checkInvariants();

        int wordIndex = wordIndex(bitIndex);
        return (wordIndex < wordsInUse)
            && ((words[wordIndex] & (1L << bitIndex)) != 0);
    }