Simple demo of avoiding side-effects by using Object.clone
/*
* Copyright (c) Ian F. Darwin, http://www.darwinsys.com/, 1996-2002.
* All rights reserved. Software written by Ian F. Darwin and others.
* $Id: LICENSE,v 1.8 2004/02/09 03:33:38 ian Exp $
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* The pioneering role of Dennis Ritchie and Bjarne Stroustrup, of AT&T, for
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*/
import java.util.*;
/**
* Simple demo of avoiding side-effects by using Object.clone()
* to duplicate an object before passing it to your enemy's methods.
* Cloneable is a "marker" interface: it has no methods, but is tested
* for by Object.clone. If you implement it, you tell Object.clone that
* your data is stable enough that field-by-field copy is OK.
*/
class Enemy {
public void munge(SideEffects md) {
System.out.println("Object is " + md);
md.year = 0;
md.td.setYear(71); // Ignore deprecation warnings
}
}
public class SideEffects implements Cloneable {
/** When we clone a "SideEffects", this REFERENCE gets cloned */
public Date td;
/** When we clone a "SideEffects", this integer does NOT get cloned */
volatile int year;
public static void main(String[] argv) throws CloneNotSupportedException {
new SideEffects().process();
}
SideEffects() {
td = new Date(); // today
year = td.getYear();
}
public void process() throws CloneNotSupportedException {
Enemy r = new Enemy();
System.out.println("We have seen the enemy, and he is " + r);
System.out.println("Today is " + td + "; nice weather, isn't it?");
System.out.println("And the year is " + year);
r.munge((SideEffects)this.clone());
System.out.println("Why, I believe it is now " + td);
if (year == 0) // should not happen!!
System.out.println("** PANIC IN YEAR ZERO **");
System.out.println("But wait, the year is still " + year);
r.munge(this);
System.out.println("Now I'm certain that it's " + td);
System.out.println("Now the year is " + year);
}
}
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