是否可以在堆栈上使用提供的Java Collections方法，例如max，min，sort，ect…？

Question

I was working on a problem involving Stacks on HackerRank ( See Here ). One of the parts of the question asked to provide the max value within the Stack. I thought an easy way to do this was to just write an extended Stack class with a max() method (see below). That worked but I thought an even easier way might be to just take advantage of Java's Collections methods. So I built the craftyMax() method (also seen below).

class MyStack<T> extends Stack<T> {
    public T craftyMax() {
        return Collections.max(this);
    }

    public T max() {
        Integer max = Integer.MIN_VALUE;

        for (T item: this) {
            max = Math.max((Integer)item, max);
        }

        return (T) max;
    }
}

Of course this did not work as the compiler replied with:

Solution.java:6: error: no suitable method found for max(MyStack<T#1>)
        return Collections.max(this);
                          ^
    method Collections.<T#2>max(Collection<? extends T#2>) is not applicable
      (inferred type does not conform to upper bound(s)
        inferred: T#1
        upper bound(s): Comparable<? super T#1>,Object)
    method Collections.<T#3>max(Collection<? extends T#3>,Comparator<? super T#3>) is not applicable
      (cannot infer type-variable(s) T#3
        (actual and formal argument lists differ in length))
  where T#1,T#2,T#3 are type-variables:
    T#1 extends Object declared in class MyStack
    T#2 extends Object,Comparable<? super T#2> declared in method <T#2>max(Collection<? extends T#2>)
    T#3 extends Object declared in method <T#3>max(Collection<? extends T#3>,Comparator<? super T#3>)
Note: Solution.java uses unchecked or unsafe operations.
Note: Recompile with -Xlint:unchecked for details.
1 error

Since, I have tried a few different things and been looking around but I can't seem to find if what I am trying to do here is possible or not. So my question is:

Is it possible to use the provided Java Collections methods, like max, min, sort, ect..., on / within a Stack? Or am I expecting a little too much?

Answer 1

public static <T extends Object & Comparable<? super T>> T max(Collection<? extends T> coll)

only works for Collection s whose element type implements the Comparable interface.

Therefore, your code will work with the proper type bound:

class MyStack<T extends Comparable<T>> extends Stack<T> {
    public T craftyMax() {
        return Collections.max(this);
    }
}

I'm not sure about your second method ( max() ), though. You are casting T to Integer . If you are certain T is an Integer , why not define MyStack as class MyStack extends Stack<Integer> ?

Answer 2

You can only call Collections.max with a single argument on types that implement the Comparable interface. Change your type declaration to

class MyStack<T extends Comparable<T>> extends Stack<T> {

and it should work. Or, as an alternative, you can take the two-argument max that takes a Comparator as the second argument.

Answer 3

you can override the comparator to match your functionality like below.

if (!MyStack.isEmpty()) {
Integer max = Collections.min(MyStack, new Comparator<Integer>() {
@Override
public int compare(Integer o1, Integer o2) {
  return o2.compareTo(o1);
}
});
}