我在尝试Java 8的Lambda表达式时有一个问题。
通常它工作得很好，但现在我有了抛出IOException的方法。
最好看看下面的代码:
class Bank{
....
public Set<String> getActiveAccountNumbers() throws IOException {
Stream<Account> s = accounts.values().stream();
s = s.filter(a -> a.isActive());
Stream<String> ss = s.map(a -> a.getNumber());
return ss.collect(Collectors.toSet());
}
....
}
interface Account{
....
boolean isActive() throws IOException;
String getNumber() throws IOException;
....
}
问题是，它不能编译，因为我必须捕获isActive-和getNumber-Methods的可能异常。但是，即使我显式地使用如下所示的try-catch-Block，它仍然不能编译，因为我没有捕获异常。所以，要么是JDK有bug，要么是我不知道如何捕捉这些异常。
class Bank{
....
//Doesn't compile either
public Set<String> getActiveAccountNumbers() throws IOException {
try{
Stream<Account> s = accounts.values().stream();
s = s.filter(a -> a.isActive());
Stream<String> ss = s.map(a -> a.getNumber());
return ss.collect(Collectors.toSet());
}catch(IOException ex){
}
}
....
}
我怎样才能让它工作呢?谁能给我点提示吗?
扩展@marcg解决方案，您通常可以在Streams中抛出和捕获检查异常;也就是说，编译器会要求你捕获/重新抛出，就像你在流外一样!
@FunctionalInterface
public interface Predicate_WithExceptions<T, E extends Exception> {
boolean test(T t) throws E;
}
/**
* .filter(rethrowPredicate(t -> t.isActive()))
*/
public static <T, E extends Exception> Predicate<T> rethrowPredicate(Predicate_WithExceptions<T, E> predicate) throws E {
return t -> {
try {
return predicate.test(t);
} catch (Exception exception) {
return throwActualException(exception);
}
};
}
@SuppressWarnings("unchecked")
private static <T, E extends Exception> T throwActualException(Exception exception) throws E {
throw (E) exception;
}
然后，您的示例将如下所示(添加测试以更清楚地显示它):
@Test
public void testPredicate() throws MyTestException {
List<String> nonEmptyStrings = Stream.of("ciao", "")
.filter(rethrowPredicate(s -> notEmpty(s)))
.collect(toList());
assertEquals(1, nonEmptyStrings.size());
assertEquals("ciao", nonEmptyStrings.get(0));
}
private class MyTestException extends Exception { }
private boolean notEmpty(String value) throws MyTestException {
if(value==null) {
throw new MyTestException();
}
return !value.isEmpty();
}
@Test
public void testPredicateRaisingException() throws MyTestException {
try {
Stream.of("ciao", null)
.filter(rethrowPredicate(s -> notEmpty(s)))
.collect(toList());
fail();
} catch (MyTestException e) {
//OK
}
}
也可以使用一些外部(流)错误指示器在更高级别抛出异常:
List<String> errorMessages = new ArrayList<>(); // error indicator
//..
errorMessages.clear();
List<String> names = new ArrayList<>(Arrays.asList("andrey", "angela", "pamela"));
names.stream()
.map(name -> {
if (name != "pamela") {
errorMessages.add(name + " is wrong here!");
return null; // triggering the indicator
}
return name;
} )
.filter(elem -> (elem != null)) // bypassing propagation of only current unwanted data
//.filter(elem -> (errorMessages.size() == 0)) // or blocking any propagation once unwanted data detected
.forEach(System.out::println);
if (errorMessages.size() > 0) { // handling the indicator
throw new RuntimeException(String,join(", ", errorMessages));
}
