When to use generic methods and when to use wild-card?

Question

I am reading about generic methods from OracleDocGenericMethod. I am pretty confused about the comparison when it says when to use wild-card and when to use generic methods. Quoting from the document.

interface Collection<E> {
    public boolean containsAll(Collection<?> c);
    public boolean addAll(Collection<? extends E> c);
}

We could have used generic methods here instead:

interface Collection<E> {
    public <T> boolean containsAll(Collection<T> c);
    public <T extends E> boolean addAll(Collection<T> c);
    // Hey, type variables can have bounds too!
}

[…] This tells us that the type argument is being used for polymorphism; its only effect is to allow a variety of actual argument types to be used at different invocation sites. If that is the case, one should use wildcards. Wildcards are designed to support flexible subtyping, which is what we're trying to express here.

Don't we think wild card like (Collection<? extends E> c); is also supporting kind of polymorphism? Then why generic method usage is considered not good in this?

Continuing ahead, it states,

Generic methods allow type parameters to be used to express dependencies among the types of one or more arguments to a method and/or its return type. If there isn't such a dependency, a generic method should not be used.

What does this mean?

They have presented the example

class Collections {
    public static <T> void copy(List<T> dest, List<? extends T> src) {
    ...
}

[…]

We could have written the signature for this method another way, without using wildcards at all:

class Collections {
    public static <T, S extends T> void copy(List<T> dest, List<S> src) {
    ...
}

The document discourages the second declaration and promotes usage of first syntax? What's the difference between the first and second declaration? Both seems to be doing the same thing?

Can someone put light on this area.

Answer 1

There are certain places, where wildcards, and type parameters do the same thing. But there are also certain places, where you have to use type parameters.

1. If you want to enforce some relationship on the different types of method arguments, you can't do that with wildcards, you have to use type parameters.

Taking your method as example, suppose you want to ensure that the src and dest list passed to copy() method should be of same parameterized type, you can do it with type parameters like so:

public static <T extends Number> void copy(List<T> dest, List<T> src)

Here, you are ensured that both dest and src have same parameterized type for List. So, it's safe to copy elements from src to dest.

But, if you go on to change the method to use wildcard:

public static void copy(List<? extends Number> dest, List<? extends Number> src)

it won't work as expected. In 2nd case, you can pass List<Integer> and List<Float> as dest and src. So, moving elements from src to dest wouldn't be type safe anymore. If you don't need such kind of relation, then you are free not to use type parameters at all.

Some other difference between using wildcards and type parameters are:

• If you have only one parameterized type argument, then you can use wildcard, although type parameter will also work.
• Type parameters support multiple bounds, wildcards don't.

• Wildcards support both upper and lower bounds, type parameters just support upper bounds. So, if you want to define a method that takes a List of type Integer or it's super class, you can do:

  public void print(List<? super Integer> list)  // OK
  

  but you can't use type parameter:

   public <T super Integer> void print(List<T> list)  // Won't compile
  

---

References:

• Angelika Langer's Java Generics FAQs

Answer 2

Consider following example from The Java Programming by James Gosling 4th edition below where we want to merge 2 SinglyLinkQueue:

public static <T1, T2 extends T1> void merge(SinglyLinkQueue<T1> d, SinglyLinkQueue<T2> s){
    // merge s element into d
}

public static <T> void merge(SinglyLinkQueue<T> d, SinglyLinkQueue<? extends T> s){
        // merge s element into d
}

Both of the above methods have the same functionality. So which is preferable? Answer is 2nd one. In the author's own words :

"The general rule is to use wildcards when you can because code with wildcards is generally more readable than code with multiple type parameters. When deciding if you need a type variable, ask yourself if that type variable is used to relate two or more parameters, or to relate a parameter type with the return type. If the answer is no, then a wildcard should suffice."

Note: In book only second method is given and type parameter name is S instead of 'T'. First method is not there in the book.