Exception Handling in Lambda Expressions

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

Example - Checked Exception

FunctionTester.java

package com.tutorialspoint;

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.

Example - Checked Exception with try-catch block

FunctionTester.java

package com.tutorialspoint;

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(","));
   }   
}

Output

Run the FunctionTester and verify the output.

www.google.com

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.

Example - Checked Exception with Wrapper Function

FunctionTester.java

package com.tutorialspoint;

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

Run the FunctionTester and verify the output.

www.google.com