return "constant" value from method java

Question

I'm not quite sure if this is possible so that's why I ask you guys. I want to write a method that knows if it has been visited before and if it has return the same value it has lasttime it were visited. I can't use fields/instance varibles for this.

This is want I want to do, without the instance variable foo:

private FooObject foo = null;
public int setFoo(FooObject in)
{
    if(foo == null)
        foo = in;

    return foo.getX();
}

Could this be done?

possible duplicate of [What are the different techniques for memoization in Java?](http://stackoverflow.com/questions/3623754/what-are-the-different-techniques-for-memoization-in-java) — skaffman, Apr 30 '12 at 21:16
Is there a particular reason why you want to do this without the instance variable? — trutheality, Apr 30 '12 at 21:20
It's a school exercise/competition. We are creating a game, and there are some rules for what changes we can make to this game. One of them are that we can't add more instance variables. — KLIM8D, Apr 30 '12 at 21:22

score 5 · Accepted Answer · answered Apr 30 '12 at 21:15

5

that knows if it has been visited before

Knowledge of what happened before requires memory, aka state. Therefore you will need something to store that knowledge; you can't avoid it.

answered Apr 30 '12 at 21:15

Oliver Charlesworth

267,707
33
569
680

score 1 · Answer 2 · answered Apr 30 '12 at 21:35

I agree with @Oli Charlesworth, you cannot do this without state. But using a field to save state is just one of many options. You could, for instance, save the state as a system property using System.setProperty() and System.getProperty(), or save the state to a file. You could even save the state to a database.

score 0 · Answer 3 · answered Apr 30 '12 at 21:15

0

Without using an instance variable? no, it can't be done. You need a way for the method to "remember" something between calls - a local variable won't work, it has to be something that "lives" outside a method call. And that's what an instance variable is for: saving state, visible from all instance methods.

answered Apr 30 '12 at 21:15

Óscar López

232,561
37
312
386

1

Okay, thanks for the reply. I need too figure out something else then :) – KLIM8D Apr 30 '12 at 21:20

score 0 · Answer 4 · answered Apr 30 '12 at 21:46

0

You can use a cache. Read from cache for looking for the input. If the input is already in cache, then return the value from cache. If don't, put the entry in cache and return the new calculated value.

CacheManager singletonManager = CacheManager.create();
Cache memoryOnlyCache = new Cache("testCache", 5000, false, false, 5, 2);
manager.addCache(memoryOnlyCache);
Cache cache = singletonManager.getCache("testCache");

Put new entry

Element element = new Element("key1", "value1");
cache.put(element);

Looking for in chache:

Element element = cache.get("key1");
Object value = element.getObjectValue();

For remove:

cache.remove("key1");

You can persist this to disk.

For more information, see in http://ehcache.org/

answered Apr 30 '12 at 21:46

Paul Vargas

41,222
15
102
148

I this case, I need to declare the CacheManager as a instance variable right? – KLIM8D Apr 30 '12 at 22:05
Like a static variable and initialized in static block – Paul Vargas Apr 30 '12 at 22:09
I can't write any variables outside this method. Actaully I only have other methods too work with. – KLIM8D Apr 30 '12 at 22:51
Can write to file for store the values? – Paul Vargas Apr 30 '12 at 22:54

score 0 · Answer 5 · answered Apr 30 '12 at 22:39

0

You could use the Preferences API. That allows you to store memory across application loads though, so if you want it to be restarted each time you load, you'll need to set a shutdown hook to clear the value from the Preferences API.

answered Apr 30 '12 at 22:39

wchargin

15,589
12
71
110

return "constant" value from method java

5 Answers5