optString方法会在对应的key中的值不存在的时候返回一个空字符串，但是getString会抛一个JSONException 。

 /**
     * Returns the value mapped by {@code name} if it exists, coercing it if
     * necessary, or throws if no such mapping exists.
     *
     * @throws JSONException if no such mapping exists.
     */
    public String getString(String name) throws JSONException {
        Object object = get(name);
        String result = JSON.toString(object);
        if (result == null) {
            throw JSON.typeMismatch(name, object, "String");
        }
        return result;
    }

    /**
     * Returns the value mapped by {@code name} if it exists, coercing it if
     * necessary, or the empty string if no such mapping exists.
     */
    public String optString(String name) {
        return optString(name, "");
    }