Question

I’m trying to write a generic class in Java. A few methods in that class require that T extends Comparable<T>. How can I make it such that T is required to be comparable only if one of those methods is used? Or maybe there’s some other way I should organize my class?

Here’s the class I’m trying to implement. Its and array that I plan to use on both comparable and non-comparable types.

// I know Java has its own containers, but this
// is homework and I'm not allowed to use them
class Array<T>
{
    // Some methods that pose no
    // special restrictions on T

    // These require that T be comparable
    public Array<T> union(...) {...}
    public Array<T> intersect(...) {...}
}

Answer 1

You can hide type T for method. T of Test is not the same as T of CompareMethodhere.

public static class Test<T> {
    <T extends Comparable<T>> void compareMethod(T t, Class<T> classt) {
    }
    void normalMethod(T t) {
    }
}

Now example

    Test<String> test = new Test<String>();//Comparable class
    test.compareMethod("",String.class);//works fine
    Test<Random> tes1t = new Test<Random>();//Non Comparable class
    tes1t.compareMethod(new Random(),Random.class);//Compilation error here 
    tes1t.normalMethod(new Random());//Works fine
    new Test<Random>().compareMethod("",String.class);// Not a good but can be valid
new Test<String>().compareMethod(new Random(),Random.class);//Compilation error here

Update:

After being cursed about this solution I did some search in java API and this practice gets followed for toArray() method

ArrayList<String> string = new ArrayList<String>();
string.toArray(new Integer[5]);<--Illegal however <T> is hide by toArray method