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when to use generic methods and when to use wild card


when to use generic methods and when to use wild card  using -'java,generics,wildcard'

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.
    

asked Sep 10, 2015 by KristanRutzb
0 votes
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5 Answers

0 votes

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  void copy(List dest, List 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 dest, List src)

it won't work as expected. In 2nd case, you can pass List and List 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 list)  // OK
    

    but you can't use type parameter:

     public  void print(List list)  // Won't compile
    

References:

answered Sep 10, 2015 by BlaHecht
0 votes

In your first question: It means that if there is a relation between the parameter's type and the method's return type then use a generic.

For example:

public T giveMeMaximum(Collection items);
public Collection applyFilter(Collection items);

Here you are extracting some of the T following a certain criteria. If T is Long your methods will return Long and Collection; the actual return type is dependent on the parameter type, thus it is useful, and advised, to use generic types.

When this is not the case you can use wild card types:

public int count(Collection items);
public boolean containsDuplicate(Collection items);

In this two example whatever the type of the items in the collections the return types will be int and boolean.

In your examples:

interface Collection {
    public boolean containsAll(Collection c);
    public boolean addAll(Collection c);
}

those two functions will return a boolean whatever is the types of the items in the collections. In the second case it is limited to instances of a subclass of E.

Second question:

class Collections {
    public static  void copy(List dest, List src) {
    ...
}

This first code allow you to pass an heterogeneous List src as a parameter. This list can contain multiple elements of different classes as long as they all extends the base class T.

if you had:

interface Fruit{}

and

class Apple implements Fruit{}
class Pear implements Fruit{}
class Tomato implements Fruit{}

you could do

List basket = new ArrayList();
basket.add(new Apple());
basket.add(new Pear());
basket.add(new Tomato());
List fridge = new ArrayList(); 

Collections.copy(fridge, basket);// works 

On the other hand

class Collections {
    public static  void copy(List dest, List src) {
    ...
}

constrain List src to be of one particular class S that is a subclass of T. The list can only contain elements of one class (in this instance S) and no other class, even if they implement T too. You wouldn't be able to use my previous example but you could do:

List basket = new ArrayList();
basket.add(new Apple());
basket.add(new Apple());
basket.add(new Apple());
List fridge = new ArrayList();

Collections.copy(fridge, basket); /* works since the basket is defined as a List of apples and not a list of some fruits. */
answered Sep 10, 2015 by BrittAmato3
0 votes

I will try and answer your question, one by one.

Don't we think wild card like (Collection c); is also supporting kind of polymorphism?

No. The reason is that the bounded wildcard has no defined parameter type. It is an unknown. All it "knows" is that the "containment" is of a type E (whatever defined). So, it cannot verify and justify whether the value provided matches the bounded type.

So, it's no sensible to have polymorphic behaviours on wildcards.

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?

The first option is better in this case as T is always bounded, and source will definitely have values (of unknowns) that subclasses T.

So, suppose that you want to copy all list of numbers, the first option will be

Collections.copy(List dest, List src);

src, essentially, can accept List, List, etc. as there is an upper bound to the parameterized type found in dest.

The 2nd option will force you to bind S for every type you want to copy, like so

//For double 
Collections.copy(List dest, List src); //Double extends Number.

//For int
Collections.copy(List dest, List src); //Integer extends Number.

As S is a parameterized type that needs binding.

I hope this helps.

answered Sep 10, 2015 by PhoebeMcdani
0 votes

Wildcard method is also generic - you could call it with some range of types.

The syntax defines a type variable name. If a type variable has any use (e.g. in method implementation or as a constraint for other type), then it makes sense to name it, otherwise you could use ?, as anonymous variable. So, looks like just a short-cut.

Moreover, the ? syntax is not avoidable when you declare a field:

class NumberContainer
{
 Set numbers;
}
answered Sep 10, 2015 by DanEru
0 votes

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

public static  void merge(SinglyLinkQueue d, SinglyLinkQueue s){
    // merge s element into d
}

public static  void merge(SinglyLinkQueue d, SinglyLinkQueue 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.

answered Sep 10, 2015 by ChaEYP

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