Java type parameter in static method compile time

Question

I'm writing universal compare utility, it should be type safe as possible. Idea is to prevent comparing of different class Objects. For example this works OK:

UtilityClazz.<String>compare("dd", 1); //OK - compile time error

Is there more elegant way to call Utility class, that has type parameter. This below is not OK because it will compare Date and int

compare(SomeObject.getDate(), 1); //NOT OK - compiles OK no type is passed

Method Code:

public static<T> int compare(T value1, T value2) {
    if (value1 instanceof Time && value2 instanceof Time) {
        return DateComparator.compareTimes((Time) value1, (Time) value2);
    } else if (value1 instanceof Date && value2 instanceof Date) {
        return DateComparator.compareDates((Date) value1, (Date) value2);
    } else if (value1 instanceof Number && value2 instanceof Number) {
        return NumberUtility.compare((Number) value1, (Number) value2);
    } else {
        //....
        return 0;
    }

}

}

How to make method call elegant as possible ?

This answer could be useful : http://stackoverflow.com/questions/16069106/how-to-compare-two-java-objects — cihan adil seven, May 17 '16 at 11:52
first remove that **** from _UtilityClazz.compare("dd", 1);_ — bananas, May 17 '16 at 11:53
Refer to https://github.com/jeevatkm/generic-repo/blob/master/genericComparator/src/main/java/com/myjeeva/comparator/GenericComparator.java — LONGHORN007, May 17 '16 at 11:58
Question is more about how would you do it then about generics. — Levijatanu, May 17 '16 at 12:15

score 1 · Answer 1 · answered May 17 '16 at 11:53

You can make it easier with method overload rather than with generics:

public static int compare(Time value1, Time value2) {
   return DateComparator.compareTimes(value1, value2);
}

public static int compare(Number value1, Number value2) {
   return NumberUtility.compare(value1, value2);
}

This eliminates unnecessary instanceof checks and gives some compile time type guarantees.

nikli · Answer 2 · 2016-05-23T07:21:05.147

0

This one you checked generic type as String, and doesnt compile because 1 is not String:

UtilityClazz.<String>compare("dd", 1);

but below you dont check generic type, and automatically compiler checks it as Object, and Object type is passed.

compare(SomeObject.getDate(), 1);

You must add Object case to your compare method

edited May 23 '16 at 07:21

answered May 17 '16 at 11:54

nikli

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score 0 · Answer 3 · answered May 17 '16 at 12:00

Instead of having a serie of if, you can make a map which associates a class to its comparator: Map<Class<?>,Comparator<?>>. The class is available through the getClass() method.

If you want to handle polymorphism, then you would have to call getSuperclass() on each object until you find a common ancestor.

Java type parameter in static method compile time

3 Answers3