Functional Programming With Java 简明教程

Exception Handling in Lambda Expressions

当函数抛出检查异常时，Lambda 表达式很难编写。请参见以下示例：

import java.net.URLEncoder;
import java.util.Arrays;
import java.util.stream.Collectors;

public class FunctionTester {
   public static void main(String[] args) {
      String url = "www.google.com";
      System.out.println(encodedAddress(url));
   }

   public static String encodedAddress(String... address) {
      return Arrays.stream(address)
         .map(s -> URLEncoder.encode(s, "UTF-8"))
         .collect(Collectors.joining(","));
   }
}

上面的代码无法编译，因为 URLEncode.encode() 抛出 UnsupportedEncodingException，而 encodeAddress() 方法无法抛出该异常。

一种可能的解决方案是将 URLEncoder.encode() 提取到一个单独的方法中，并在那里处理异常。

import java.io.UnsupportedEncodingException;
import java.net.URLEncoder;
import java.util.Arrays;
import java.util.stream.Collectors;

public class FunctionTester {
   public static void main(String[] args) {
      String url = "www.google.com";
      System.out.println(encodedAddress(url));
   }

   public static String encodedString(String s) {
      try {
         URLEncoder.encode(s, "UTF-8");
      }
      catch (UnsupportedEncodingException e) {
         e.printStackTrace();
      }
      return s;
   }

   public static String encodedAddress(String... address) {
      return Arrays.stream(address)
         .map(s -> encodedString(s))
         .collect(Collectors.joining(","));
   }
}

但是以上方法在有这样抛出异常的多重方法时并不适用。查看以下使用函数式接口和包装方法的一般解决方案。

import java.net.URLEncoder;
import java.util.Arrays;
import java.util.function.Function;
import java.util.stream.Collectors;

public class FunctionTester {
   public static void main(String[] args) {
      String url = "www.google.com";
      System.out.println(encodedAddress(url));
   }
   public static String encodedAddress(String... address) {
      return Arrays.stream(address)
         .map(wrapper(s -> URLEncoder.encode(s, "UTF-8")))
         .collect(Collectors.joining(","));
   }

   private static <T, R, E extends Exception> Function<T, R>
   wrapper(FunctionWithThrows<T, R, E> fe) {
      return arg -> {
         try {
            return fe.apply(arg);
         } catch (Exception e) {
            throw new RuntimeException(e);
         }
      };
   }
}

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

Output

www.google.com