Super class of String and Integer in Java 8

Question

I'm writing a simple Predicate function as follows:

public static Predicate<String> stringPredicate(String op, String compVal) {
    if (op.equals("=")){
        return p -> p.compareTo(compVal) == 0;
    }
    else if (op.equals("<")){
        return p -> p.compareTo(compVal) == -1;
    }
    else if (op.equals("<=")){
        return p -> p.compareTo(compVal) == 0 || p.compareTo(compVal) == -1;
    }
    else if (op.equals(">")){
        return p -> p.compareTo(compVal) == 1;
    }
    else if (op.equals(">=")){
        return p -> p.compareTo(compVal) == 1 || p.compareTo(compVal) == 0;
    }
    return p -> -1 == 0; //return false 
}

I'd like this function to work for both Integers, Floats and Strings. So, rather than writing 2 identical predicate functions, one for Strings and one for Integer+Float, I'd like to write a single one as compareTo() method works for all 3 of them. I guess I can do it by replacing Predicate<String> with Predicate<Super class of Integer/Float/String>. I tried Object but that didn't work. So, what should I put instead of <String> to use the function in the way I want?

I don't understand what you mean? I've if-else,if there right? Am I missing someting ? — SpiderRico, Apr 18 '17 at 15:20
Ah I see now, yes. But that won't change much in terms of the correctness, right ? — SpiderRico, Apr 18 '17 at 15:21
@SpiderRico yes it will: http://stackoverflow.com/questions/513832/how-do-i-compare-strings-in-java — assylias, Apr 18 '17 at 15:21
@SpiderRico Depends on which "correctness" you're referring to. — Dave Newton, Apr 18 '17 at 15:22
@Pshemo Yeah you're right. Was a bit sleepy when writing this last night I guess. — SpiderRico, Apr 18 '17 at 15:22

score 7 · Accepted Answer · edited May 23 '17 at 12:32

7

There is no common "superclass" of Integer and String other than Object, which is useless for you since it doesn't have compareTo.

But there is a common interface between the two — Comparable<T>, which provides the compareTo method.

public static <T extends Comparable<T>> Predicate<T> predicate(String op, T compVal) {
    switch (op) {
        case "=": return p -> p.compareTo(compVal) == 0;
        case "<": return p -> p.compareTo(compVal) < 0;
        case "<=": return p -> p.compareTo(compVal) <= 0;
        case ">": return p -> p.compareTo(compVal) > 0;
        case ">=": return p -> p.compareTo(compVal) >= 0;
        default: return p -> false;
    }
}

(Starting from Java 7 you could switch on strings, so there is no point using an if/else chain here. But in your case an enum for op may even be more suitable.)

edited May 23 '17 at 12:32

Community

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answered Apr 18 '17 at 15:26

kennytm

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Thank you. This looks exactly what I need. – SpiderRico Apr 18 '17 at 15:31
Consider **currying** based on a `Map`. `functionMap.put(">", (a,b) -> x.compareTo(y) > 0)` etc. Then `return p -> functionMap.get(op).apply(p,compVal)`. Or an enum instead of the map. – slim Apr 18 '17 at 15:40
@slim That's just moving the `switch` into a `Map`, you still need to duplicate the same amount of code. I don't how this will improve anything :) – kennytm Apr 18 '17 at 15:44

Super class of String and Integer in Java 8

1 Answers1