一般會答: s.length()

看原始碼是如何實現的:

    /**

     * Returns the length of this string.

     * The length is equal to the number of <a href="Character.html#unicode">Unicode

     * code units</a> in the string.

     *

     * @return  the length of the sequence of characters represented by this

     *          object.

     */

    public int length() {

        return value.length;

    }

value就是一個字元陣列, 即s.length()其實是呼叫char[]的length屬性

    /** The value is used for character storage. */

    private final char value[];