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Wildcards

In generic code, the question mark (?), called the wildcard, represents an unknown type. The wildcard can be used in a variety of situations: as the type of a parameter, field, or local variable, or sometimes as a return type (though it is better programming practice to be more specific). The wildcard is never used as a type argument for a generic method invocation, a generic class instance creation, or a supertype.

The following sections discuss wildcards in more detail, including upper-bounded wildcards, lower-bounded wildcards, and wildcard capture.

Upper-Bounded Wildcards

You can use an upper-bounded wildcard to relax the restrictions on a variable. For example, say you want to write a method that works on List<Integer>, List<Double>, and List<Number>; you can achieve this by using an upper-bounded wildcard.

To declare an upper-bounded wildcard, use the wildcard character (?), followed by the extends keyword, followed by its upper bound. Note that, in this context, extends is used in a general sense to mean either extends (as in classes) or implements (as in interfaces).

To write the method that works on lists of Number and the subtypes of Number, such as Integer, Double, and Float, you would specify List<? extends Number>. The term List<Number> is more restrictive than List<? extends Number> because the former matches a list of type Number only, whereas the latter matches a list of type Number or any of its subclasses.

Consider the following process method:

public static void process(List<? extends Foo> list) { /* . . . */ }

The upper-bounded wildcard, <? extends Foo>, where Foo is any type, matches Foo and any subtype of Foo. The process method can access the list elements as type Foo:

public static void process(List<? extends Foo> list) {
    for (Foo elem : list) {
        // . . .
    }
}

In the foreach clause, the elem variable iterates over each element in the list. Any method defined in the Foo class can now be used on elem.

The sumOfList method returns the sum of the numbers in a list:

public static double sumOfList(List<? extends Number> list) {
    double s = 0.0;
    for (Number n : list)
        s += n.doubleValue();
    return s;
}

The following code, using a list of Integer objects, prints sum = 6.0:

List<Integer> li = Arrays.asList(1, 2, 3);
System.out.println("sum = " + sumOfList(li));

A list of Double values can use the same sumOfList method. The following code prints sum = 7.0:

List<Double> ld = Arrays.asList(1.2, 2.3, 3.5);
System.out.println("sum = " + sumOfList(ld));

Unbounded Wildcards

The unbounded wildcard type is specified using the wildcard character (?)—for example, List<?>. This is called a list of unknown type. There are two scenarios where an unbounded wildcard is a useful approach:

  • It is useful if you are writing a method that can be implemented using functionality provided in the Object class.
  • It is useful when the code is using methods in the generic class that don’t depend on the type parameter (e.g., List.size or List.clear). In fact, Class<?> is often used because most of the methods in Class<T> do not depend on T.

Consider the following method, printList:

public static void printList(List<Object> list) {
    for (Object elem : list)
        System.out.println(elem + " ");
    System.out.println();
}

The goal of printList is to print a list of any type, but it fails to achieve that goal: It prints only a list of Object instances. It cannot print List<Integer>, List<String>, List<Double>, and so on. This is because they are not subtypes of List<Object>. To write a generic printList method, use List<?>:

public static void printList(List<?> list) {
    for (Object elem: list)
        System.out.print(elem + " ");
    System.out.println();
}

For any concrete type A, List is a subtype of List<?>. Thus you can use printList to print a list of any type:

List<Integer> li = Arrays.asList(1, 2, 3);
List<String>  ls = Arrays.asList("one", "two", "three");
printList(li);
printList(ls);

It’s important to note that List<Object> and List<?> are not the same. You can insert an Object, or any subtype of Object, into a List<Object>. But you can only insert null into a List<?>. The “Guidelines for Wildcard Use” section has more information on how to determine what kind of wildcard, if any, should be used in a given situation.

Lower-Bounded Wildcards

The “Upper-Bounded Wildcards” section shows that an upper-bounded wildcard restricts the unknown type to be a specific type or a subtype of that type and is represented using the extends keyword. In a similar way, a lower-bounded wildcard restricts the unknown type to be a specific type or a supertype of that type. A lower-bounded wildcard is expressed using the wildcard character (?), followed by the super keyword, followed by its lower bound: <? super A>.

Say you want to write a method that puts Integer objects into a list. To maximize flexibility, you would like the method to work on List<Integer>, List<Number>, and List<Object>—anything that can hold Integer values.

To write the method that works on lists of Integer and the supertypes of Integer, such as Integer, Number, and Object, you would specify List<? super Integer>. The term List<Integer> is more restrictive than List<? super Integer> because the former matches a list of type Integer only, whereas the latter matches a list of any type that is a supertype of Integer.

The following code adds the numbers 1 through 10 to the end of a list:

public static void addNumbers(List<? super Integer> list) {
    for (int i = 1; i <= 10; i++) {
        list.add(i);
    }
}

The “Guidelines for Wildcard Use” section in this chapter provides guidance on when to use upper-bounded wildcards and when to use lower-bounded wildcards.

Wildcards and Subtyping

As described previously in “Generics, Inheritance, and Subtypes,” generic classes or interfaces are not related merely because there is a relationship between their types. However, you can use wildcards to create a relationship between generic classes or interfaces.

Consider the following two regular (nongeneric) classes:

class A { /* . . . */ }
class B extends A { /* . . . */ }

For these classes, it would be reasonable to write the following code:

B b = new B();
A a = b;

This example shows that inheritance of regular classes follows the rule of subtyping: class B is a subtype of class A if B extends A. This rule does not apply to generic types:

List<B> lb = new ArrayList<>();
List la = lb;   // compile-time error

Given that Integer is a subtype of Number, what is the relationship between List<Integer> and List<Number>? Although Integer is a subtype of Number, List<Integer> is not a subtype of List<Number> and, in fact, these two types are not related. The common parent of List<Number> and List<Integer> is List<?> (Figure 6.4).

Figure 6.4

Figure 6.4. The Common Parent Is List<?>

In order to create a relationship between these classes so that the code can access Number’s methods through List<Integer>’s elements, use an upper-bounded wildcard:

List<? extends Integer> intList = new ArrayList<>();
// OK. List<? extends Integer> is a subtype of List<? extends Number>
List<? extends Number>  numList = intList;

Because Integer is a subtype of Number, and numList is a list of Number objects, a relationship now exists between intList (a list of Integer objects) and numList. Figure 6.5 shows the relationships between several List classes declared with both upper- and lower-bounded wildcards.

Figure 6.5

Figure 6.5. A Hierarchy of Several Generic List Class Declarations

The “Guidelines for Wildcard Use” section later on in this chapter has more information about the ramifications of using upper- and lower-bounded wildcards.

Wildcard Capture and Helper Methods

In some cases, the compiler infers the type of a wildcard. For example, a list may be defined as List<?>, but when evaluating an expression, the compiler infers a particular type from the code. This scenario is known as wildcard capture. For the most part, you don’t need to worry about wildcard capture, except when you see an error message that contains the phrase capture of.

The WildcardError example produces a capture error when compiled:

import java.util.List;

public class WildcardError {

    void foo(List<?> i) {
        i.set(0, i.get(0));
    }
}

In this example, the compiler processes the i input parameter as being of type Object. When the foo method invokes List.set(int, E)2, the compiler is not able to confirm the type of object that is being inserted into the list and an error is produced. When this type of error occurs, it typically means that the compiler believes that you are assigning the wrong type to a variable. Generics were added to the Java language for this reason—to enforce type safety at compile time.

The WildcardError example generates the following error when compiled by Oracle’s JDK 7 javac implementation:

WildcardError.java:6: error: method set in interface List<E> cannot
   be applied to given types;
    i.set(0, i.get(0));
     ^
  required: int,CAP#1
  found: int,Object
  reason: actual argument Object cannot be converted to CAP#1 by method
     invocation conversion
  where E is a type-variable:
    E extends Object declared in interface List
  where CAP#1 is a fresh type-variable:
    CAP#1 extends Object from capture of ?
1 error

In this example, the code is attempting to perform a safe operation, so how can you work around the compiler error? You can fix it by writing a private helper method, which captures the wildcard. In this case, you can work around the problem by creating the private helper method, fooHelper, as shown in WildcardFixed:

public class WildcardFixed {

    void foo(List<?> i) {
        fooHelper(i);
    }

    // Helper method created so that the wildcard can be captured
    // through type inference.
    private <T> void fooHelper(List<T> l) {
        l.set(0, l.get(0));
    }
}

Thanks to the helper method, the compiler uses inference to determine that T is CAP#1, the capture variable, in the invocation. The example now compiles successfully.

By convention, helper methods are generally named originalMethodNameHelper. Now consider a more complex example, WildcardErrorBad:

import java.util.List;

public class WildcardErrorBad {

    void swapFirst(List<? extends Number> l1, List<? extends Number> l2) {
      Number temp = l1.get(0);
      l1.set(0, l2.get(0)); // expected a CAP#1 extends Number,
                            // got a CAP#2 extends Number;
                            // same bound, but different types
      l2.set(0, temp);      // expected a CAP#1 extends Number,
                            // got a Number
    }
}

In this example, the code is attempting an unsafe operation. For example, consider the following invocation of the swapFirst method:

List<Integer> li = Arrays.asList(1, 2, 3);
List<Double>  ld = Arrays.asList(10.10, 20.20, 30.30);
swapFirst(li, ld);

While List<Integer> and List<Double> both fulfill the criteria of List<? extends Number>, it is clearly incorrect to take an item from a list of Integer values and attempt to place it into a list of Double values.

Compiling the code with Oracle’s JDK javac compiler produces the following error:

WildcardErrorBad.java:7: error: method set in interface List<E> cannot be
   applied to given types;
      l1.set(0, l2.get(0)); // expected a CAP#1 extends Number,
        ^
  required: int,CAP#1
  found: int,Number
  reason: actual argument Number cannot be converted to CAP#1 by method
     invocation conversion
  where E is a type-variable:
    E extends Object declared in interface List
  where CAP#1 is a fresh type-variable:
    CAP#1 extends Number from capture of ? extends Number
WildcardErrorBad.java:10: error: method set in interface List<E> cannot
   be applied to given types;
      l2.set(0, temp);      // expected a CAP#1 extends Number,
        ^
  required: int,CAP#1
  found: int,Number
  reason: actual argument Number cannot be converted to CAP#1 by method
     invocation conversion
  where E is a type-variable:
    E extends Object declared in interface List
  where CAP#1 is a fresh type-variable:
    CAP#1 extends Number from capture of ? extends Number
WildcardErrorBad.java:15: error: method set in interface List<E> cannot
   be applied to given types;
        i.set(0, i.get(0));
         ^
  required: int,CAP#1
  found: int,Object
  reason: actual argument Object cannot be converted to CAP#1 by method
     invocation conversion
  where E is a type-variable:
    E extends Object declared in interface List
  where CAP#1 is a fresh type-variable:
    CAP#1 extends Object from capture of ?
3 errors

There is no helper method to work around the problem because the code is fundamentally wrong.

Guidelines for Wildcard Use

One of the more confusing aspects when learning to program with generics is determining when to use an upper-bounded wildcard and when to use a lower-bounded wildcard. This section provides some guidelines to follow when designing your code.

For purposes of this discussion, it is helpful to think of variables as serving one of two functions:

  • An in variable serves up data to the code. Imagine a copy method with two arguments: copy(src, dest). The src argument provides the data to be copied, so it is the in parameter.
  • An out variable holds data for use elsewhere. In the copy example, copy(src, dest), the dest argument accepts data, so it is the out parameter.

Of course, some variables are used for both in and out purposes, as discussed later. You can use the in and out principles when deciding whether to use a wildcard and what type of wildcard is appropriate. The following list provides the guidelines that you should follow:

  • An invariable is defined with an upper-bounded wildcard, using the extends keyword.
  • An out variable is defined with a lower-bounded wildcard, using the super keyword.
  • In the case where the in variable can be accessed using methods defined in the Object class, use an unbounded wildcard.
  • In the case where the code needs to access the variable as both an in and an out variable, do not use a wildcard.

These guidelines do not apply to a method’s return type. Using a wildcard as a return type should be avoided because it forces programmers using the code to deal with wildcards.

A list defined by List<? extends . . . > can be informally thought of as read only, but that is not a strict guarantee. Suppose you have the following two classes:

class NaturalNumber {

    private int i;

    public NaturalNumber(int i) { this.i = i; }
    // . . .
}

class EvenNumber extends NaturalNumber {

    public EvenNumber(int i) { super(i); }
    // . . .
}

Consider the following code:

List<EvenNumber> le = new ArrayList<>();
List<? extends NaturalNumber> ln = le;
ln.add(new NaturalNumber(35));  // compile-time error

Because List<EvenNumber> is a subtype of List<? extends NaturalNumber>, you can assign le to ln. But you cannot use ln to add a natural number to a list of even numbers. The following operations on the list are possible:

  • You can add null.
  • You can invoke clear.
  • You can get the iterator and invoke remove.
  • You can capture the wildcard and write elements that you’ve read from the list.

You can see that the list defined by List<? extends NaturalNumber> is not read only in the strictest sense of the word, but you might think of it that way because you cannot store a new element or change an existing element in the list.

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