CODEWITHSUNDEEP
javagenericsinterface
user7845429
user7845429

Reputation: 103

How is this valid generic method in java?

class GenMethDemo {
// Determine if an object is in an array.

static <T extends Comparable<T>, V extends T> boolean isIn(T x, V[] y) {
     for(int i=0; i < y.length; i++)
         if(x.equals(y[i])) return true;
   }
}

how can Comparable interface be extended rather than being implemented being an Interface itself?

Upvotes: 2

Views: 378

Answers (2)

Thomas Fritsch
Thomas Fritsch

Reputation: 10127

The meaning of extends within < ... > is a little different from its normal meaning (like in class FileInputStream extends InputStream).

Quoted from the tutorial Upper Bounded Wildcards:

To declare an upper-bounded wildcard, use the wildcard character (?), followed by the extends keyword, followed by its upper bound. Note that, in this context, extends is used in a general sense to mean either "extends" (as in classes) or "implements" (as in interfaces).

Upvotes: 0

Eran
Eran

Reputation: 393811

The Comparable interface is not extended here.

<T extends Comparable<T>, V extends T> are generic type parameters with type bounds. It means that the isIn method has two generic type parameters - one of them is called T and must implement Comparable<T>, and the other is called V, and must be a sub-type of T.

BTW, your specific method doesn't require V and T to implement Comparable, since it doesn't call compareTo.

However, it would be required if you changed the code to:

static <T extends Comparable<T>, V extends T> boolean isIn(T x, V[] y) {
     for(int i=0; i < y.length; i++)
         if(x.compareTo(y[i]) == 0) return true;
     return false;
}

Then you can call this method only with parameters that implement Comparable.

For example:

if (isIn ("str",new String[]{"a","b","c"})) {

}

Upvotes: 3

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