How to make sure that Collections in java hold only one Type?

Question

I want to make sure that a Collection can hold only one Type.

Lets say there is such a method.

public Collection<Students> getStudents();

One can write following code.

Collection students = getStudents();
students.add(new Book());

Book does not extend Student . Now the Collection students contains a wrong Object. How can I make sure that this line Collection students = getStudents(); is not possible?

Answer 1

This is called a raw type . Raw types were introduced for backward compatibility - so that old code still worked against newer JDKs, basically.

You can make javac warn about the use of raw types using -Xlint:rawtypes , and in IDEs you may be able to make it an actual error, but it's still fundamentally valid Java.

Answer 2

You can enforce it at runtime if you use Collections.checkedCollection() every time you instantiate a Collection. Admittedly it would be a bit painful. :)

Answer 3

Jon Skeet made the right answer. You can define your Java Compiler on Properties (Or Project-Properties) to get an error if you use a raw-type.

Answer 4

你应该使用这种格式：

    Collection<Students> s = students.getStudents();