引发异常的Java 8 Lambda函数？
我知道如何创建对具有引发异常的Java 8 Lambda函数？,java,lambda,java-8,Java,Lambda,Java 8,我知道如何创建对具有字符串参数并返回int的方法的引用，它是： Function<String, Integer> 我将如何定义此引用？免责声明：我还没有使用Java 8，只是阅读了有关它的内容 函数不会抛出IOException，因此不能在其中放入任何抛出IOException的代码。如果调用的方法需要函数，则传递给该方法的lambda不能抛出IOException，句点。您可以这样编写lambda（我认为这是lambda语法，不确定）： 或者，如果要将lambda传递给的方法是
字符串intFunction<String, Integer>
函数IOException抛出IOException的代码。如果调用的方法需要函数
，则传递给该方法的lambda不能抛出IOException
，句点。您可以这样编写lambda（我认为这是lambda语法，不确定）：
或者，如果要将lambda传递给的方法是您自己编写的方法，则可以定义一个新的函数接口，并将其用作参数类型，而不是函数
：
通过IOException的公共接口函数{
O应用（I输入）引发IOException；
}
您需要执行以下操作之一
- 如果是您的代码，则定义您自己的功能接口来声明选中的异常：
@FunctionalInterface
public interface CheckedFunction<T, R> {
R apply(T t) throws IOException;
}
public Integer myWrappedMethod(String s) {
try {
return myMethod(s);
}
catch(IOException e) {
throw new UncheckedIOException(e);
}
}
- 否则，将
整数myMethod（字符串s）
包装到一个不声明选中异常的方法中：
@FunctionalInterface
public interface CheckedFunction<T, R> {
R apply(T t) throws IOException;
}
public Integer myWrappedMethod(String s) {
try {
return myMethod(s);
}
catch(IOException e) {
throw new UncheckedIOException(e);
}
}
然后：
Function<String, Integer> f = (String t) -> myWrappedMethod(t);
函数f=（字符串t）->myWrappedMethod（t）；
或：
函数f=
（字符串t）->{
试一试{
返回myMethod（t）；
}
捕获（IOE异常）{
抛出新的未选中异常（e）；
}
};
这不是Java 8特有的。您正在尝试编译与以下内容等效的内容：
interface I {
void m();
}
class C implements I {
public void m() throws Exception {} //can't compile
}
使用函数包装器的另一个解决方案是，如果一切顺利，则返回结果包装器的一个实例（如成功），或者返回结果包装器的一个实例（如失败）
一些代码可以澄清一些事情：
public interface ThrowableFunction<A, B> {
B apply(A a) throws Exception;
}
public abstract class Try<A> {
public static boolean isSuccess(Try tryy) {
return tryy instanceof Success;
}
public static <A, B> Function<A, Try<B>> tryOf(ThrowableFunction<A, B> function) {
return a -> {
try {
B result = function.apply(a);
return new Success<B>(result);
} catch (Exception e) {
return new Failure<>(e);
}
};
}
public abstract boolean isSuccess();
public boolean isError() {
return !isSuccess();
}
public abstract A getResult();
public abstract Exception getError();
}
public class Success<A> extends Try<A> {
private final A result;
public Success(A result) {
this.result = result;
}
@Override
public boolean isSuccess() {
return true;
}
@Override
public A getResult() {
return result;
}
@Override
public Exception getError() {
return new UnsupportedOperationException();
}
@Override
public boolean equals(Object that) {
if(!(that instanceof Success)) {
return false;
}
return Objects.equal(result, ((Success) that).getResult());
}
}
public class Failure<A> extends Try<A> {
private final Exception exception;
public Failure(Exception exception) {
this.exception = exception;
}
@Override
public boolean isSuccess() {
return false;
}
@Override
public A getResult() {
throw new UnsupportedOperationException();
}
@Override
public Exception getError() {
return exception;
}
}
公共接口ThrowableFunction{
B应用（A）抛出异常；
}
公共抽象类尝试{
公共静态布尔isSuccess（Try-tryy）{
返回tryy成功实例；
}
公共静态功能tryOf（一次性功能）{
返回一个->{
试一试{
B结果=函数。应用（a）；
返回新的成功（结果）；
}捕获（例外e）{
返回新故障（e）；
}
};
}
公共抽象布尔isSuccess（）；
公共布尔isError（）{
return！issucess（）；
}
公共摘要一个getResult（）；
公共抽象异常getError（）；
}
公开课的成功扩展了尝试{
私人最终结果；
公众的成功（结果）{
this.result=结果；
}
@凌驾
公共布尔值isSuccess（）{
返回true；
}
@凌驾
公开一个getResult（）{
返回结果；
}
@凌驾
公共异常getError（）{
返回新的UnsupportedOperationException（）；
}
@凌驾
公共布尔等于（该对象）{
如果（！（成功的那一刻））{
返回false；
}
返回Objects.equal（result，（（Success）that）.getResult（））；
}
}
公共类失败扩展了Try{
私人最终例外；
公共故障（例外）{
this.exception=异常；
}
@凌驾
公共布尔值isSuccess（）{
返回false；
}
@凌驾
公开一个getResult（）{
抛出新的UnsupportedOperationException（）；
}
@凌驾
公共异常getError（）{
返回异常；
}
}
一个简单的用例：
List<Try<Integer>> result = Lists.newArrayList(1, 2, 3).stream().
map(Try.<Integer, Integer>tryOf(i -> someMethodThrowingAnException(i))).
collect(Collectors.toList());
List result=Lists.newArrayList（1,2,3）.stream（）。
map（Try.tryOf（i->somethodthrowinganexception（i）））。
collect（Collectors.toList（））；
我所做的是允许用户在异常情况下给出他实际想要的值。
所以我有一些东西看起来像这样
public static <T, R> Function<? super T, ? extends R> defaultIfThrows(FunctionThatThrows<? super T, ? extends R> delegate, R defaultValue) {
return x -> {
try {
return delegate.apply(x);
} catch (Throwable throwable) {
return defaultValue;
}
};
}
@FunctionalInterface
public interface FunctionThatThrows<T, R> {
R apply(T t) throws Throwable;
}
实际上，您可以使用一个处理异常的新接口扩展消费者
（和函数
等）——使用Java 8的默认方法
考虑此接口（扩展消费者）：
但是使用这个新接口，您可以使用lambda表达式实例化它，编译器不会抱怨：
final ThrowingConsumer<String> throwingConsumer = aps -> {
// maybe some other code here...
throw new Exception("asdas");
};
list.forEach(throwingConsumer);
这个问题也一直困扰着我；这就是为什么我创造了
有了它，您可以：
final ThrowingFunction<String, Integer> f = yourMethodReferenceHere;
默认行为是，当抛出lambda抛出选中的异常时，抛出一个ThrownByLambdaException
，并将选中的异常作为原因。因此，您可以捕捉到这一点并找到原因
其他功能也可用。公共测试（）{
public void frankTest() {
int pageId= -1;
List<Book> users= null;
try {
//Does Not Compile: Object page=DatabaseConnection.getSpringConnection().queryForObject("SELECT * FROM bookmark_page", (rw, n) -> new Portal(rw.getInt("id"), "", users.parallelStream().filter(uu -> uu.getVbid() == rw.getString("user_id")).findFirst().get(), rw.getString("name")));
//Compiles:
Object page= DatabaseConnection.getSpringConnection().queryForObject("SELECT * FROM bookmark_page", (rw, n) -> {
try {
final Book bk= users.stream().filter(bp -> {
String name= null;
try {
name = rw.getString("name");
} catch (Exception e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
return bp.getTitle().equals(name);
}).limit(1).collect(Collectors.toList()).get(0);
} catch (Exception e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
return new Portal(rw.getInt("id"), "", users.get(0), rw.getString("name"));
} );
} catch (Exception e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
}
int pageId=-1；
列表用户=null；
试一试{
//未编译：Object page=DatabaseConnection.getSpringConnection（）.queryForObject（“从书签页面选择*，（rw，n）->新门户（rw.getInt（“id”），”，users.parallelStream（）.filter（uu->uu.getVbid（）==rw.getString（“user\u id”）.findFirst（）.get（），rw.getString（“name”）；
//汇编：
Object page=DatabaseConnection.getSpringConnection（）.queryForObject（“从书签页面选择*，（rw，n）->{
试一试{
final Book bk=users.stream（）.filter（bp->{
字符串名称=null；
试一试{
name=rw.getString（“name”）；
}捕获（例外e）{
//TODO自动生成的捕捉块
e、 printStackTrace（）；
}
返回bp.getTitle（）.equals（name）；
}).limit（1）.collect（collector.toList（））.get（0）；
}捕获（例外e）{
//TODO自动生成的捕捉块
e、 printStackTrace（）；
}
返回新门户（rw.getInt（“id”），“”，users.get（0），rw.getString（“name”）；
} );
}捕获（例外e）{
//TODO自动生成的捕捉块
e、 printStackTrace（）；
}
}
I h
defaultIfThrows(child -> child.getID(), null)
@FunctionalInterface
public interface ThrowingConsumer<T> extends Consumer<T> {
@Override
default void accept(final T elem) {
try {
acceptThrows(elem);
} catch (final Exception e) {
// Implement your own exception handling logic here..
// For example:
System.out.println("handling an exception...");
// Or ...
throw new RuntimeException(e);
}
}
void acceptThrows(T elem) throws Exception;
}
final List<String> list = Arrays.asList("A", "B", "C");
final Consumer<String> consumer = aps -> {
try {
// maybe some other code here...
throw new Exception("asdas");
} catch (final Exception ex) {
System.out.println("handling an exception...");
}
};
list.forEach(consumer);
final ThrowingConsumer<String> throwingConsumer = aps -> {
// maybe some other code here...
throw new Exception("asdas");
};
list.forEach(throwingConsumer);
list.forEach((ThrowingConsumer<String>) aps -> {
// maybe some other code here...
throw new Exception("asda");
});
list.forEach(Errors.rethrow().wrap(c -> somethingThatThrows(c)));
final ThrowingFunction<String, Integer> f = yourMethodReferenceHere;
myStringStream.map(f) // <-- works
public void frankTest() {
int pageId= -1;
List<Book> users= null;
try {
//Does Not Compile: Object page=DatabaseConnection.getSpringConnection().queryForObject("SELECT * FROM bookmark_page", (rw, n) -> new Portal(rw.getInt("id"), "", users.parallelStream().filter(uu -> uu.getVbid() == rw.getString("user_id")).findFirst().get(), rw.getString("name")));
//Compiles:
Object page= DatabaseConnection.getSpringConnection().queryForObject("SELECT * FROM bookmark_page", (rw, n) -> {
try {
final Book bk= users.stream().filter(bp -> {
String name= null;
try {
name = rw.getString("name");
} catch (Exception e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
return bp.getTitle().equals(name);
}).limit(1).collect(Collectors.toList()).get(0);
} catch (Exception e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
return new Portal(rw.getInt("id"), "", users.get(0), rw.getString("name"));
} );
} catch (Exception e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
}
public Object uncheckedNewInstanceForName (String name) {
try {
return Class.forName(name).newInstance();
}
catch (ClassNotFoundException | InstantiationException | IllegalAccessException e) {
throw new RuntimeException(e);
}
}
@FunctionalInterface
public interface UseInstance<T, X extends Throwable> {
void accept(T instance) throws X;
}
import java.io.FileWriter;
import java.io.IOException;
//lambda expressions and the execute around method (EAM) pattern to
//manage resources
public class FileWriterEAM {
private final FileWriter writer;
private FileWriterEAM(final String fileName) throws IOException {
writer = new FileWriter(fileName);
}
private void close() throws IOException {
System.out.println("close called automatically...");
writer.close();
}
public void writeStuff(final String message) throws IOException {
writer.write(message);
}
//...
public static void use(final String fileName, final UseInstance<FileWriterEAM, IOException> block) throws IOException {
final FileWriterEAM writerEAM = new FileWriterEAM(fileName);
try {
block.accept(writerEAM);
} finally {
writerEAM.close();
}
}
public static void main(final String[] args) throws IOException {
FileWriterEAM.use("eam.txt", writerEAM -> writerEAM.writeStuff("sweet"));
FileWriterEAM.use("eam2.txt", writerEAM -> {
writerEAM.writeStuff("how");
writerEAM.writeStuff("sweet");
});
FileWriterEAM.use("eam3.txt", FileWriterEAM::writeIt);
}
void writeIt() throws IOException{
this.writeStuff("How ");
this.writeStuff("sweet ");
this.writeStuff("it is");
}
}
public final class LambdaExceptionUtil {
@FunctionalInterface
public interface Function_WithExceptions<T, R, E extends Exception> {
R apply(T t) throws E;
}
/**
* .map(rethrowFunction(name -> Class.forName(name))) or .map(rethrowFunction(Class::forName))
*/
public static <T, R, E extends Exception> Function<T, R> rethrowFunction(Function_WithExceptions<T, R, E> function) throws E {
return t -> {
try {
return function.apply(t);
} catch (Exception exception) {
throwActualException(exception);
return null;
}
};
}
@SuppressWarnings("unchecked")
private static <E extends Exception> void throwActualException(Exception exception) throws E {
throw (E) exception;
}
}
public class LambdaExceptionUtilTest {
@Test
public void testFunction() throws MyTestException {
List<Integer> sizes = Stream.of("ciao", "hello").<Integer>map(rethrowFunction(s -> transform(s))).collect(toList());
assertEquals(2, sizes.size());
assertEquals(4, sizes.get(0).intValue());
assertEquals(5, sizes.get(1).intValue());
}
private Integer transform(String value) throws MyTestException {
if(value==null) {
throw new MyTestException();
}
return value.length();
}
private static class MyTestException extends Exception { }
}
// Do this once
ExceptionTranslator et = ET.newConfiguration().done();
...
// if your method returns something
Function<String, Integer> f = (t) -> et.withReturningTranslation(() -> myMethod(t));
// if your method returns nothing
Consumer<String> c = (t) -> et.withTranslation(() -> myMethod(t));
acceptMyMethod(s -> {
try {
Integer i = doSomeOperation(s);
return i;
} catch (IOException e) {
// try catch block because of throws clause
// in functional method, even though doSomeOperation
// might not be throwing any exception at all.
e.printStackTrace();
}
return null;
});
public interface MyAmazingAPI {
Integer myMethod(String s) throws IOException;
}
public interface MyAmazingAPI {
Integer myMethod(String s) throws IOException;
default Optional<Integer> myMethod(String s, Consumer<? super Exception> exceptionConsumer) {
try {
return Optional.ofNullable(this.myMethod(s));
} catch (Exception e) {
if (exceptionConsumer != null) {
exceptionConsumer.accept(e);
} else {
e.printStackTrace();
}
}
return Optional.empty();
}
}
strStream.map(str -> amazingAPIs.myMethod(str, Exception::printStackTrace))
.filter(Optional::isPresent)
.map(Optional::get).collect(toList());
CheckedFunction1<String, Integer> f = this::myMethod;
Try(() -> f.apply("test")) // results in a Success(Integer) or Failure(Throwable)
.map(i -> ...) // only executed on Success
...
Function<String, Integer> standardFn = FluentFunctions.ofChecked(this::myMethod);
FluentFunctions.ofChecked(this::myMethod)
.log(s->log.debug(s),e->log.error(e,e.getMessage())
.try(5,1000)
.apply("my param");
Function<String, Integer> func1 = s -> Unthrow.wrap(() -> myMethod(s));
Function<String, Integer> func2 = s1 -> Unthrow.wrap((s2) -> myMethod(s2), s1);
public static interface CheckedValueSupplier<V> {
public V get () throws Exception;
}
public class CheckedValue<V> {
private final V v;
private final Optional<Exception> opt;
public Value (V v) {
this.v = v;
}
public Value (Exception e) {
this.opt = Optional.of(e);
}
public V get () throws Exception {
if (opt.isPresent()) {
throw opt.get();
}
return v;
}
public Optional<Exception> getException () {
return opt;
}
public static <T> CheckedValue<T> returns (T t) {
return new CheckedValue<T>(t);
}
public static <T> CheckedValue<T> rethrows (Exception e) {
return new CheckedValue<T>(e);
}
public static <V> CheckedValue<V> from (CheckedValueSupplier<V> sup) {
try {
return CheckedValue.returns(sup.get());
} catch (Exception e) {
return Result.rethrows(e);
}
}
public static <V> CheckedValue<V> escalates (CheckedValueSupplier<V> sup) {
try {
return CheckedValue.returns(sup.get());
} catch (Exception e) {
throw new RuntimeException(e);
}
}
}
// Don't use this pattern with FileReader, it's meant to be an
// example. FileReader is a Closeable resource and as such should
// be managed in a try-with-resources block or in another safe
// manner that will make sure it is closed properly.
// This will not compile as the FileReader constructor throws
// an IOException.
Function<String, FileReader> sToFr =
(fn) -> new FileReader(Paths.get(fn).toFile());
// Alternative, this will compile.
Function<String, CheckedValue<FileReader>> sToFr = (fn) -> {
return CheckedValue.from (
() -> new FileReader(Paths.get("/home/" + f).toFile()));
};
// Single record usage
// The call to get() will propagate the checked exception if it exists.
FileReader readMe = pToFr.apply("/home/README").get();
// List of records usage
List<String> paths = ...; //a list of paths to files
Collection<CheckedValue<FileReader>> frs =
paths.stream().map(pToFr).collect(Collectors.toList());
// Find out if creation of a file reader failed.
boolean anyErrors = frs.stream()
.filter(f -> f.getException().isPresent())
.findAny().isPresent();
List<String> lst = Lists.newArrayList();
// won't compile
lst.stream().forEach(e -> throwyMethod(e));
// compiles
lst.stream()
.map(e -> CheckedValueSupplier.from(
() -> {throwyMethod(e); return e;}))
.filter(v -> v.getException().isPresent()); //this example may not actually run due to lazy stream behavior
@FunctionalInterface
public interface FunctionWithException<T, R, E extends Exception> {
R apply(T t) throws E;
}
private FunctionWithException<String, Integer, IOException> myMethod = (str) -> {
if ("abc".equals(str)) {
throw new IOException();
}
return 1;
};
Consumer<E extends Exception>
public interface Lambda {
@FunctionalInterface
public interface CheckedFunction<T> {
T get() throws Exception;
}
public static <T> T handle(CheckedFunction<T> supplier) {
try {
return supplier.get();
} catch (Exception exception) {
throw new RuntimeException(exception);
}
}
}
Lambda.handle(() -> method());
import static Throwing.rethrow;
@Test
public void testRethrow() {
thrown.expect(IOException.class);
thrown.expectMessage("i=3");
Arrays.asList(1, 2, 3).forEach(rethrow(e -> {
int i = e.intValue();
if (i == 3) {
throw new IOException("i=" + i);
}
}));
}
public final class Throwing {
private Throwing() {}
@Nonnull
public static <T> Consumer<T> rethrow(@Nonnull final ThrowingConsumer<T> consumer) {
return consumer;
}
/**
* The compiler sees the signature with the throws T inferred to a RuntimeException type, so it
* allows the unchecked exception to propagate.
*
* http://www.baeldung.com/java-sneaky-throws
*/
@SuppressWarnings("unchecked")
@Nonnull
public static <E extends Throwable> void sneakyThrow(@Nonnull Throwable ex) throws E {
throw (E) ex;
}
}
public class UncheckedExceptions {
/**
* throws {@code exception} as unchecked exception, without wrapping exception.
*
* @return will never return anything, return type is set to {@code exception} only to be able to write <code>throw unchecked(exception)</code>
* @throws T {@code exception} as unchecked exception
*/
@SuppressWarnings("unchecked")
public static <T extends Throwable> T unchecked(Exception exception) throws T {
throw (T) exception;
}
@FunctionalInterface
public interface UncheckedFunction<R> {
R call() throws Exception;
}
/**
* Executes given function,
* catches and rethrows checked exceptions as unchecked exceptions, without wrapping exception.
*
* @return result of function
* @see #unchecked(Exception)
*/
public static <R> R unchecked(UncheckedFunction<R> function) {
try {
return function.call();
} catch (Exception e) {
throw unchecked(e);
}
}
@FunctionalInterface
public interface UncheckedMethod {
void call() throws Exception;
}
/**
* Executes given method,
* catches and rethrows checked exceptions as unchecked exceptions, without wrapping exception.
*
* @see #unchecked(Exception)
*/
public static void unchecked(UncheckedMethod method) {
try {
method.call();
} catch (Exception e) {
throw unchecked(e);
}
}
}
unchecked(() -> new File("hello.txt").createNewFile());
boolean fileWasCreated = unchecked(() -> new File("hello.txt").createNewFile());
myFiles.forEach(unchecked(file -> new File(file.path).createNewFile()));
public class UncheckedUtils {
@FunctionalInterface
public interface ThrowingConsumer<T> {
void accept(T t) throws Exception;
}
@FunctionalInterface
public interface ThrowingSupplier<T> {
T get() throws Exception;
}
@FunctionalInterface
public interface ThrowingRunnable {
void run() throws Exception;
}
public static <T> Consumer<T> unchecked(
ThrowingConsumer<T> throwingConsumer
) {
return i -> {
try {
throwingConsumer.accept(i);
} catch (Exception ex) {
throw new RuntimeException(ex);
}
};
}
public static <T> T unchecked(
ThrowingSupplier<T> throwingSupplier
) {
try {
return throwingSupplier.get();
} catch (Exception ex) {
throw new RuntimeException(ex);
}
}
public static void unchecked(
ThrowingRunnable throwing
) {
try {
throwing.run();
} catch (Exception ex) {
throw new RuntimeException(ex);
}
}
}