Java™ 教程（泛型通配符捕获和Helper方法）

泛型通配符捕获和Helper方法在某些情况下，编译器会推断出通配符的类型，例如，列表可以定义为List<?>，但是在评估表达式时，编译器会从代码中推断出特定类型，此场景称为通配符捕获。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的类型，并且会产生错误，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 — 在编译时强制执行类型安全。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私有Helper方法来修复它，在这种情况下，你可以通过创建私有Helper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Helper方法，编译器使用推断来确定T是调用中的CAP#1（捕获变量），该示例现在成功编译。按照惯例，Helper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 }}在这个例子中，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>都符合List<? extends Number>的标准，从Integer值列表中获取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 NumberWildcardErrorBad.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 NumberWildcardErrorBad.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这里没有Helper方法来解决这个问题，因为代码根本就是错误的。上一篇：泛型通配符