How to set constraints on generic types in Java?

java generics constraints
citronas picture citronas · Jan 17, 2010 · Viewed 34.9k times · Source

I have a generic class:

public class ListObject<T>
{
    // fields
    protected T _Value = null;
      // ..
}

Now I want to do something like the following:

ListObject<MyClass> foo = new ListObject<MyClass>();
ListObject<MyClass> foo2 = new ListObject<MyClass>();
foo.compareTo(foo2);

Question:

How can I define the compareTo() method with resprect to the generic T?

I guess I have to somehow implement a constraint on the generic T, to tell that T implements a specific interface (maybe Comparable, if that one exists).

Can anyone provide me with a small code sample?

Answer

nanda picture nanda · Jan 17, 2010

Read also the discussion here: Generics and sorting in Java

Short answer, the best you can get is:

class ListObject<T extends Comparable<? super T>> {
    ...
}

But there is also reason to just use:

class ListObject<T extends Comparable> {
    ...
}

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