String.valueOf(null) 报空指针

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 <code>char</code> 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 <code>char</code> 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);