Javaソース-AbstractStringBuilder

    /**
     * The value is used for character storage.
     */
    char[] value;

    /**
     * The count is the number of characters used.
     */
    int count;

これはこのような2つのメンバー変数で、valueは文字を格納するために使用されます.value.lengthは文字容量を表します.
countは使用済みの長さを表します.
対応方法:

    /**
     * Returns the length (character count).
     *
     * @return  the length of the sequence of characters currently
     *          represented by this object
     */
    public int length() {
        return count;
    }

    /**
     * Returns the current capacity. The capacity is the amount of storage
     * available for newly inserted characters, beyond which an allocation
     * will occur.
     *
     * @return  the current capacity
     */
    public int capacity() {
        return value.length;
    }