Re: How to "force" my object never been changed by other other objects using it?
"www" <www@nospam.com> wrote in message
news:eqffv3$smt$1@news.nems.noaa.gov...
Hi,
I have a class, Data.java. Once an object of this class is created,
several other kind objects will use it. For example:
Data myDataObj = new Data(...//params);
UserA userA = new UserA(myDataObj); //pass the object to another object.
double ave = userA.getAverage();
UserB userB = new UserB(myDataObj);
...
I want myDataObj state never been modified by users. Right now, I just
open my eyes widely to make sure that my code does not do it. I am
wondering if I can use some key words to enforce it, like:
final Data myDataObj = new Data(...); //my feeling this is wrong: it just
make sure the reference myDataObj (the memory address in heap being
referenced) cannot be changed. Or, the street address cannot be changed.
But the house can be modified. Am I correct?
You're right that "final" won't do it. I'm assuming that you want to
make Data immutable mainly for reducing bugs, and not for security purposes.
Simply provide two interfaces for your Data object, one which provides
getters, and one which provides setters.
public interface ReadableData {
int getX();
}
public class Data implements ReadableData {
private int x;
public int getX() {
return this.x;
}
public void setX(int newValue) {
this.x = newValue;
}
}
And when you want to make your object immutable, upcast it from Data to
ReadableData:
Data myDataObj = new Data();
ReadableData immutableViewOfMyDataObj = myDataObj;
immutableViewOfMyDataObj.setX(5); /*This causes a compile error, setX() is
not defined on the interface.*/
If you have "malicious programmers" on your team, they can get around
this by downcasting from ReadableData to Data, which is why I say this only
works for bug detecting and not security. If you need something stronger
than this, then you'll probably need to make proxy classes which can read
from the original data object, but never provides a mean to write to it.
- Oliver