There is a A Class

class A {
    private String str;

    public void setStr(String str) {
        this.str = str;
    }
}

• this method works fine;

    public static void main(String[] args) {
        A a = null;
        Optional.ofNullable((String) null)
                .ifPresent(str -> a.setStr(str));
    }

• This method Causing NullPointerException

    public static void main(String[] args) {
        A a = null;
        Optional.ofNullable((String) null)
                .ifPresent(a::setStr);
    }

Why?

Those two methods should be no difference in my perspective;
How this happend?