Java consumer with exception

Interface Consumer

Type Parameters: T — the type of the input to the operation All Known Subinterfaces: Stream.Builder Functional Interface: This is a functional interface and can therefore be used as the assignment target for a lambda expression or method reference.

Represents an operation that accepts a single input argument and returns no result. Unlike most other functional interfaces, Consumer is expected to operate via side-effects.

This is a functional interface whose functional method is accept(Object) .

Method Summary

Returns a composed Consumer that performs, in sequence, this operation followed by the after operation.

Method Details

accept

andThen

Returns a composed Consumer that performs, in sequence, this operation followed by the after operation. If performing either operation throws an exception, it is relayed to the caller of the composed operation. If performing this operation throws an exception, the after operation will not be performed.

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Exceptions in Java 8 Lambda Expressions

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1. Overview

In Java 8, Lambda Expressions started to facilitate functional programming by providing a concise way to express behavior. However, the Functional Interfaces provided by the JDK don’t deal with exceptions very well – and the code becomes verbose and cumbersome when it comes to handling them.

In this article, we’ll explore some ways to deal with exceptions when writing lambda expressions.

2. Handling Unchecked Exceptions

First, let’s understand the problem with an example.

We have a List and we want to divide a constant, say 50 with every element of this list and print the results:

List integers = Arrays.asList(3, 9, 7, 6, 10, 20); integers.forEach(i -> System.out.println(50 / i));

This expression works but there’s one problem. If any of the elements in the list is 0, then we get an ArithmeticException: / by zero. Let’s fix that by using a traditional try-catch block such that we log any such exception and continue execution for next elements:

List integers = Arrays.asList(3, 9, 7, 0, 10, 20); integers.forEach(i -> < try < System.out.println(50 / i); >catch (ArithmeticException e) < System.err.println( "Arithmetic Exception occured : " + e.getMessage()); >>);

The use of try-catch solves the problem, but the conciseness of a Lambda Expression is lost and it’s no longer a small function as it’s supposed to be.

To deal with this problem, we can write a lambda wrapper for the lambda function. Let’s look at the code to see how it works:

static Consumer lambdaWrapper(Consumer consumer) < return i -> < try < consumer.accept(i); >catch (ArithmeticException e) < System.err.println( "Arithmetic Exception occured : " + e.getMessage()); >>; >

List integers = Arrays.asList(3, 9, 7, 0, 10, 20); integers.forEach(lambdaWrapper(i -> System.out.println(50 / i)));

At first, we wrote a wrapper method that will be responsible for handling the exception and then passed the lambda expression as a parameter to this method.

The wrapper method works as expected but, you may argue that it’s basically removing the try-catch block from lambda expression and moving it to another method and it doesn’t reduce the actual number of lines of code being written.

This is true in this case where the wrapper is specific to a particular use case but we can make use of generics to improve this method and use it for a variety of other scenarios:

static Consumer consumerWrapper(Consumer consumer, Class clazz) < return i -> < try < consumer.accept(i); >catch (Exception ex) < try < E exCast = clazz.cast(ex); System.err.println( "Exception occured : " + exCast.getMessage()); >catch (ClassCastException ccEx) < throw ex; >> >; >

List integers = Arrays.asList(3, 9, 7, 0, 10, 20); integers.forEach( consumerWrapper( i -> System.out.println(50 / i), ArithmeticException.class));

As we can see, this iteration of our wrapper method takes two arguments, the lambda expression and the type of Exception to be caught. This lambda wrapper is capable of handling all data types, not just Integers, and catch any specific type of exception and not the superclass Exception.

Also, notice that we have changed the name of the method from lambdaWrapper to consumerWrapper. It’s because this method only handles lambda expressions for Functional Interface of type Consumer. We can write similar wrapper methods for other Functional Interfaces like Function, BiFunction, BiConsumer and so on.

3. Handling Checked Exceptions

Let’s modify the example from the previous section and instead of printing to the console, let’s write to a file.

static void writeToFile(Integer integer) throws IOException < // logic to write to file which throws IOException >

Note that the above method may throw the IOException.

List integers = Arrays.asList(3, 9, 7, 0, 10, 20); integers.forEach(i -> writeToFile(i));

On compilation, we get the error:

java.lang.Error: Unresolved compilation problem: Unhandled exception type IOException

Because IOException is a checked exception, we must handle it explicitly. We have two options.

First, we may simply throw the exception outside of our method and take care of it somewhere else.

Alternatively, we can handle it inside the method that uses a lambda expression.

Let’s explore both of the options.

3.1. Throwing Checked Exception from Lambda Expressions

Let’s see what happens when we declare the IOException on the main method:

public static void main(String[] args) throws IOException < Listintegers = Arrays.asList(3, 9, 7, 0, 10, 20); integers.forEach(i -> writeToFile(i)); >

Still, we get the same error of unhandled IOException during the compilation.

java.lang.Error: Unresolved compilation problem: Unhandled exception type IOException

This is because lambda expressions are similar to Anonymous Inner Classes.

In our case, writeToFile method is the implementation of Consumer functional interface.

List integers = Arrays.asList(3, 9, 7, 0, 10, 20); integers.forEach(i -> < try < writeToFile(i); >catch (IOException e) < throw new RuntimeException(e); >>); 

static Consumer throwingConsumerWrapper( ThrowingConsumer throwingConsumer) < return i -> < try < throwingConsumer.accept(i); >catch (Exception ex) < throw new RuntimeException(ex); >>; >

List integers = Arrays.asList(3, 9, 7, 0, 10, 20); integers.forEach(throwingConsumerWrapper(i -> writeToFile(i)));

static Consumer handlingConsumerWrapper( ThrowingConsumer throwingConsumer, Class exceptionClass) < return i -> < try < throwingConsumer.accept(i); >catch (Exception ex) < try < E exCast = exceptionClass.cast(ex); System.err.println( "Exception occured : " + exCast.getMessage()); >catch (ClassCastException ccEx) < throw new RuntimeException(ex); >> >; >

List integers = Arrays.asList(3, 9, 7, 0, 10, 20); integers.forEach(handlingConsumerWrapper( i -> writeToFile(i), IOException.class));