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private int mark = -1;
private int position = 0;
private int limit;
private int capacity;

Buffer(int mark, int pos, int lim, int cap) {       // package-private
    if (cap < 0)
        throw new IllegalArgumentException("Negative capacity: " + cap);
    this.capacity = cap;
    limit(lim);
    position(pos);
    if (mark >= 0) {
        if (mark > pos)
            throw new IllegalArgumentException("mark > position: ("
                                               + mark + " > " + pos + ")");
        this.mark = mark;
    }
}

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CharBuffer charBuffer = CharBuffer.allocate(16);
allocate 方法调用了HeapCharBuffer，默认mark=-1,position=0,limit=capacity;

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charBuffer.put("Hello,World".toCharArray());

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charBuffer.flip();

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charBuffer.get();//循环调用5次，读取"Hello"

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charBuffer.mark()