Malicious code vulnerability - May expose internal representation by incorporating reference to mutable object

Question

I have the following code in my dto class.

public void setBillDate(Date billDate) {
    this.billDate = billDate;
}

And I get an error in s

Answer 1

In addition to the existing answers, I propose a new version based on Optional class from Java 8.

public void setBillDate(Date billDate) {
    this.billDate = Optional
            .ofNullable(billDate)
            .map(Date::getTime)
            .map(Date::new)
            .orElse(null);
}