Java8 stream中使用检查异常

搬运工 https://stackoverflow.com/questions/27644361/how-can-i-throw-checked-exceptions-from-inside-java-8-streams

题主希望这段代码能够编译通过

 public List<Class> getClasses() throws ClassNotFoundException {

        List<Class> classes =
            Stream.of("java.lang.Object", "java.lang.Integer", "java.lang.String")
                .map(className -> Class.forName(className))
                .collect(Collectors.toList());
        return classes;
    }

image.png
但是lambda表达式中不能有检查异常，必须要try...catch...

public List<Class> getClasses() throws ClassNotFoundException {

        List<Class> classes = Stream.of("java.lang.Object", "java.lang.Integer", "java.lang.String").map(className -> {
            try {
                return Class.forName(className);
            } catch (ClassNotFoundException e) {
                throw new RuntimeException(e);
            }
        }).collect(Collectors.toList());
        return classes;
    }

看起来很不优雅呀。
于是有个大神写了LambdaExceptionUtil利用泛型的擦除机制

import java.util.function.BiConsumer;
import java.util.function.Consumer;
import java.util.function.Function;
import java.util.function.Supplier;


public class LambdaExceptionUtil {
    @FunctionalInterface
    public interface Consumer_WithExceptions<T, E extends Exception> {
        void accept(T t) throws E;
    }

    @FunctionalInterface
    public interface BiConsumer_WithExceptions<T, U, E extends Exception> {
        void accept(T t, U u) throws E;
    }

    @FunctionalInterface
    public interface Function_WithExceptions<T, R, E extends Exception> {
        R apply(T t) throws E;
    }

    @FunctionalInterface
    public interface Supplier_WithExceptions<T, E extends Exception> {
        T get() throws E;
    }

    @FunctionalInterface
    public interface Runnable_WithExceptions<E extends Exception> {
        void run() throws E;
    }

    /**
     * .forEach(rethrowConsumer(name -> System.out.println(Class.forName(name)))); or .forEach(rethrowConsumer(ClassNameUtil::println));
     */
    public static <T, E extends Exception> Consumer<T> rethrowConsumer(Consumer_WithExceptions<T, E> consumer)
        throws E {
        return t -> {
            try {
                consumer.accept(t);
            } catch (Exception exception) {
                throwAsUnchecked(exception);
            }
        };
    }

    public static <T, U, E extends Exception> BiConsumer<T, U> rethrowBiConsumer(
        BiConsumer_WithExceptions<T, U, E> biConsumer) throws E {
        return (t, u) -> {
            try {
                biConsumer.accept(t, u);
            } catch (Exception exception) {
                throwAsUnchecked(exception);
            }
        };
    }

    /**
     * .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) {
                throwAsUnchecked(exception);
                return null;
            }
        };
    }

    /**
     * rethrowSupplier(() -> new StringJoiner(new String(new byte[]{77, 97, 114, 107}, "UTF-8"))),
     */
    public static <T, E extends Exception> Supplier<T> rethrowSupplier(Supplier_WithExceptions<T, E> function)
        throws E {
        return () -> {
            try {
                return function.get();
            } catch (Exception exception) {
                throwAsUnchecked(exception);
                return null;
            }
        };
    }

    /**
     * uncheck(() -> Class.forName("xxx"));
     */
    public static void uncheck(Runnable_WithExceptions t) {
        try {
            t.run();
        } catch (Exception exception) {
            throwAsUnchecked(exception);
        }
    }

    /**
     * uncheck(() -> Class.forName("xxx"));
     */
    public static <R, E extends Exception> R uncheck(Supplier_WithExceptions<R, E> supplier) {
        try {
            return supplier.get();
        } catch (Exception exception) {
            throwAsUnchecked(exception);
            return null;
        }
    }

    /**
     * uncheck(Class::forName, "xxx");
     */
    public static <T, R, E extends Exception> R uncheck(Function_WithExceptions<T, R, E> function, T t) {
        try {
            return function.apply(t);
        } catch (Exception exception) {
            throwAsUnchecked(exception);
            return null;
        }
    }

    @SuppressWarnings("unchecked")
    private static <E extends Throwable> void throwAsUnchecked(Exception exception) throws E {
        throw (E)exception;
    }

}

然后就可以这样写了，检查异常不是只能在lambda表达式用`try...catch...``，也能放在方法签名了。

public List<Class> getClasses() throws ClassNotFoundException {

        List<Class> classes =
            Stream.of("java.lang.Object", "java.lang.Integer", "java.lang.String")
                .map(LambdaExceptionUtil.rethrowFunction(className -> Class.forName(className)))
                .collect(Collectors.toList());
        return classes;
    }