Java 教程是为 JDK 8 编写的。本页中描述的示例和实践未利用在后续版本中引入的改进。
在某些情况下,编译器会推断出通配符的类型。例如,列表可以定义为 List<?> 但是,在计算表达式时,编译器会从代码中推断出特定类型。此方案称为 wildcard capture (通配符捕获)。
在大多数情况下,你不必担心通配符捕获,除非你看到包含短语 "capture of" 的错误消息。
WildcardError
示例在编译时产生捕获错误:
import java.util.List; public class WildcardError { void foo(List<?> i) { i.set(0, i.get(0)); } }
在此示例中,编译器将 i 输入参数处理为 Object 类型。当 foo 方法调用 List.set(int, E) 时,编译器无法确认插入到列表中的对象类型,并生成错误。发生此类错误时,通常意味着编译器认为你为变量分配了错误的类型。出于这个原因,泛型被添加到 Java 语言中 - 在编译时强制类型安全。
当由 Oracle 的 JDK 7 javac 实现编译时,WildcardError 示例生成以下错误:
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
在此示例中,代码尝试执行安全操作,那么如何解决编译器错误?你可以通过编写捕获通配符的 private helper method 来修复它。在这种情况下,你可以通过创建私有辅助方法 fooHelper 来解决此问题,如 WildcardFixed
中所示:
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)); } }
由于辅助方法,编译器使用推断来确定调用中 T 是 CAP#1(捕获变量)。该示例现在成功编译。
按照规范,辅助方法通常命名为 originalMethodNameHelper。
现在考虑一个更复杂的例子,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 } }
在此示例中,代码正在尝试不安全的操作。例如,考虑以下对 swapFirst 方法的调用:
List<Integer> li = Arrays.asList(1, 2, 3); List<Double> ld = Arrays.asList(10.10, 20.20, 30.30); swapFirst(li, ld);
List<Integer> 和 List<Double> 都满足 List<? extends Number>,从 Integer 值列表中取一个项目并尝试将其放入 Double 值列表中显然是不正确的。
使用 Oracle 的 JDK javac 编译器编译代码会产生以下错误:
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
没有辅助方法来解决这个问题,因为代码根本就是错误的。