具有多个参数的Java泛型

Question

I have seen examples on the site that deal with generics with multiple parameters but none that work for my situation.

So here is the deal: I am trying to learn Java generics and have decided to create a simple binary array search utility function. I am testing it out using custom objects and integers. To get feedback on errors and warning I am using Eclipse. Here is what I have:

public static int binarySearch(Comparable[] array, Comparable item, int start, int end) {
    if(end < start) {
        return -1;
    }
    int mid = (start + end) / 2;
    if(item.compareTo(array[mid]) > 0) {
        return binarySearch(array, item, mid + 1, end);
    } else if(item.compareTo(array[mid]) < 0) {
        return binarySearch(array, item, start, mid - 1);
    } else {
        return mid;
    }
}

So obviously I get the warnings for Raw types saying the generics should be parameterized. How can I do this correctly given that I have multiple parameters which both need to be the same type?

SOLUTION

Here is the working solution using generics with the correct parameter checks:

public static <T extends Comparable<? super T>> int binarySearch(T[] array, T item, int start, int end) {
    if(array.length == 0) {
        return -1;
    }
    if(item == null) {
        return -1;
    }
    if(start < 0) {
        return -1;
    }
    if(end < start) {
        return -1;
    }
    int mid = (start + end) / 2;
    if(item.compareTo(array[mid]) > 0) {
        return binarySearch(array, item, mid + 1, end);
    } else if(item.compareTo(array[mid]) < 0) {
        return binarySearch(array, item, start, mid - 1);
    } else {
        return mid;
    }
}

Answer 1

you can specify function-specific generic parameter like so

public static <T extends Comparable<? super T>> int binarySearch(T[] arr,T elem,int start,int end){
    //...
}

Answer 2

This is the typical way to create generic functions:

public static <T extends Comparable<? super T>> int binarySearch(T[] array, T item, int start, int end) { ... }

For extra generality, item doesn't have to be of the same type the things in the array are, and things in the array don't have to be Comparable since you're not comparing them to anything, so

public static <T> int binarySearch(T[] array, Comparable<T> item, int start, int end) { ... }

gives some additional flexibility.