I have a class in charge of responding to an input event and maybe triggering another event. To decide, it has several rules to check.

I’m trying to get away from a class looking like this:

public class Translator {
    public void maybeTranslate() {
        if (!condition1 || !condition2 || !condition3) {
            return;
        }

        // All rules have passed
        translate();
    }
}

I’d like this class to know that it has rules to pass, but not know about the implementations of rules. My attempt was to transform it like so:

public class Translator {
    private Rule[] rules;

    public Translator (Rule[] rules) { this.rules = rules; }

    public void maybeTranslate() {
        for (Rule rule : rules) {
            if (!rule.passes()) {
                return;
            }
        }

        translate();
    }
}

where a Rule has this interface:

public interface Rule {
    boolean passes();
}

Here comes my problem: I now have a new rule that takes in a parameter. Its passes method would require a String parameter. Is my foundation wrong with this new requirement? I don’t see how I can adapt this pattern to accommodate rules having various parameters.

This parameter would indicate the source of the input event. I can modify maybeTranslate to receive this value from whoever calls it, and change the Rule interface to be boolean passes(String str). Only now, the majority of my rules are getting a parameter they don’t care about.

This is in the context of an Android app, where Translator would be instantiated on different fragments.

UPDATE: Some changes from my original question.

This Translator will be used on multiple screens. There is only one screen active at a time, and each screen can have a different set of rules.

Instead of making a rule take in a parameter and act differently, I created multiple screen-dependent rules (ScreenOneRule, ScreenTwoRule, etc.).

Now I would use the class like this:

public class ScreenOne {
    Translator translator;

    public Screen(Translator translator) { ... }

    public void onSomeEvent() { translator.maybeTranslate(); }
}

and I would create the set of rules for ScreenOne in my IoC configuration (this is an attempt at pseudo code):

Rule[] screenOneRules = new Rule[] { SharedRule1, SharedRule2, ScreenOneRule1 }
ScreenOne screenOne = new Screen(new Translator(screenOneRules))

This solves my problem, though I feel like the design is not perfect. What if I get a rule later on that depends on a value only known after the screen has been instantiated? For example, say a rule needs to know the (x,y) coordinates of where the user tapped on screen to trigger onSomeEvent. I have a feeling ScreenOne would need to construct the rules itself in onSomeEvent. I think this is coming back to my original question.

In that case, I would want to have the instantiation of rules somewhere else. Is the solution to have a factory like this?

public class RuleFactory {
    public Rule[] getRulesForScreenOne(int x, int y, String anotherParameter) {
        return new Rule[] { new SharedRule1(x, y), new SharedRule2(anotherParameter) };
    }
}

public void onSomeEvent() { 
    Rule[] rules = RuleFactory.getRulesForScreenOne(x, y, str);
    translator.maybeTranslate(rules);
}

At that point, Translator would not receive rules in its constructor. I can’t put my finger on it, but I feel like the factory implementation isn’t right. Something about that method name.