Having an abstract method requires that the class be made abstract. Stack extends Vector. Used for sorting and searching. Big problems here. These are classes in the java.util package.
Collections consists exclusively of static methods that operate on or return collections.
Use SortedMap when you want things sorted. HashMap for speed. To indicate when an operation is unsupported by the function that throws it. It's used when you try to modify an unmodifiable collection, etc. Definitions were okay.