Exception Handling in Lambda Expressions


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Lambda expressions are difficult to write when the function throws a checked expression. See the example below −

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(","));
   }
}

The above code fails to compile because URLEncode.encode() throws UnsupportedEncodingException and cannot be thrown by encodeAddress() method.

One possible solution is to extract URLEncoder.encode() into a separate method and handle the exception there.

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(","));
   }   
}

But above approach is not good when we have multiple such methods which throws exception. See the following generalized solution using functional interface and a wrapper method.

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
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