String.valueOf 默认的方法

argument 可以为null 的

1. boolean b = null;
2. char c = null;
3. char[] data = null;
4. double d = null;
5. float f = null;
6. int i = null;
7. long l = null;
8. Object obj = null;

String.valueOf(null) 会调用更为具体valueOf(char[] data)

/**     * Returns the string representation of the char array     * argument. The contents of the character array are copied; subsequent     * modification of the character array does not affect the newly     * created string.     *     * @param   data   a char array.     * @return  a newly allocated string representing the same sequence of     *          characters contained in the character array argument.     */    public static String valueOf(char data[]) {        return new String(data);    }    /**     * Allocates a new {
    @code String} so that it represents the sequence of     * characters currently contained in the character array argument. The     * contents of the character array are copied; subsequent modification of     * the character array does not affect the newly created string.     *     * @param  value     *         The initial value of the string     */    public String(char value[]) {        this.value = Arrays.copyOf(value, value.length);    }

会在value.length 处抛空指针异常！

case1:

String.valueOf(null);

case2:

char[] data;

String.valueOf(data);