Question

Given the following code:

interface IParam {}

class Parameter implements IParam {}

void foo(Collection<? extends IParam> params) {
    SortedSet<? extends IParam> sortedParams;
    if (params instanceof SortedSet)
        sortedParams = (SortedSet<? extends IParam>) params;
    else
        sortedParams = new TreeSet<IParam>(params);
}

What I get is some collection of parameters.
What I need is a SortedSet of the parameters.
If the given collection already is a SortedSet, I want to use that.
Otherwise, I’ll create a new TreeSet with just the content of the given collection.

However, this code doesn’t compile with warnings about unchecked casts.

Is there any way to achieve what I want, without suppressing warnings?

Answer 1

Compiles just fine for me. I would restructure the code slightly though to make it easier to conceptualize and maintain:

protected <T extends IParam> void foo(Collection<T> params) {
    SortedSet<T> sortedParams;
    if (params instanceof SortedSet)
        sortedParams = (SortedSet<T>) params;
    else
        sortedParams = new TreeSet<T>(params);

    System.out.println(sortedParams);
}

As an aside, checking for instanceof within a method, though not illegal, is sometimes a sign of code smell. Your method can be refactored with some overloading:

protected <T extends IParam> void foo(SortedSet<T> params) {
    System.out.println("Doing something with sorted set);
}

protected <T extends IParam> void foo(Collection<T> params) {
    SortedSet<T> sortedParams = new TreeSet<T>(params);
    System.out.println("Doing something with other collection types");
    foo(sortedParams);
}