通配符捕获和帮助方法
原文: https://docs.oracle.com/javase/tutorial/java/generics/capture.html
在某些情况下,编译器会推断出通配符的类型。例如,列表可以被定义为List<?>。
但是,在评估表达式时,编译器会从代码中推断出特定的类型。这种情况称为通配符捕获。
在大多数情况下,您不必担心通配符捕获,除非您看到包含短语“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,并产生错误。发生此类错误时,通常意味着编译器认为您为变量分配了错误的类型。由于这个原因,泛型被添加到 Java 语言中 - 在编译时强制类型安全。
WildcardError
示例在由 Oracle 的 JDK 7 javac
实现编译时生成以下错误:
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?您可以通过编写捕获通配符的私有帮助程序方法来修复它。在这种情况下,您可以通过创建私有帮助方法fooHelper
来解决问题,如 WildcardFixed
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T
是CAP#1
,即调用中的捕获变量。该示例现在已成功编译。
按照惯例,辅助方法通常被命名为_originalMethodName_ Helper
。
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,代码尝试进行不安全的操作。例如,考虑以下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);
而List< Integer>
和列表< Double>
均符合List<?的标准 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
没有帮助方法来解决这个问题,因为代码根本就是错误的。