Java 8 Lambda function that throws exception?

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I know how to create a reference to a method that has a String parameter and returns an int, it's:

Function<String, Integer>

However, this doesn't work if the function throws an exception, say it's defined as:

Integer myMethod(String s) throws IOException

How would I define this reference?

Java 8 Lambda function that throws exception?, You'll need to do one of the following. If it's your code, then define your own functional interface that declares the checked exception: @FunctionalInterface  The default behavior is that when your throwing lambda throws a checked exception, a ThrownByLambdaException is thrown with the checked exception as the cause. You can therefore capture that and get the cause. Other features are available as well.

You can actually extend Consumer (and Function etc.) with a new interface that handles exceptions -- using Java 8's default methods!

Consider this interface (extends Consumer):

public interface ThrowingConsumer<T> extends Consumer<T> {

    default void accept(final T elem) {
        try {
        } catch (final Exception e) {
            // Implement your own exception handling logic here..
            // For example:
            System.out.println("handling an exception...");
            // Or ...
            throw new RuntimeException(e);

    void acceptThrows(T elem) throws Exception;


Then, for example, if you have a list:

final List<String> list = Arrays.asList("A", "B", "C");

If you want to consume it (eg. with forEach) with some code that throws exceptions, you would traditionally have set up a try/catch block:

final Consumer<String> consumer = aps -> {
    try {
        // maybe some other code here...
        throw new Exception("asdas");
    } catch (final Exception ex) {
        System.out.println("handling an exception...");

But with this new interface, you can instantiate it with a lambda expression and the compiler will not complain:

final ThrowingConsumer<String> throwingConsumer = aps -> {
    // maybe some other code here...
    throw new Exception("asdas");

Or even just cast it to be more succinct!:

list.forEach((ThrowingConsumer<String>) aps -> {
    // maybe some other code here...
    throw new Exception("asda");

Update: Looks like there's a very nice utility library part of Durian called Errors which can be used to solve this problem with a lot more flexibility. For example, in my implementation above I've explicitly defined the error handling policy (System.out... or throw RuntimeException), whereas Durian's Errors allow you to apply a policy on the fly via a large suite of utility methods. Thanks for sharing it, @NedTwigg!.

Sample usage:

list.forEach(Errors.rethrow().wrap(c -> somethingThatThrows(c)));

How to Handle Checked Exceptions With Lambda Expression, And, our private save method also throws checked exceptions, which Since Java 8, an interface can have static methods, let's write one here. 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.

Sneakily Throwing Exceptions in Lambda Expressions, Java's lambda expressions can be problematic for exception handling. But you can use type Java 8 Type Inference. While reading First, we'd need a functional interface that represents a function that throws an Exception:. Java 8 Lambda function that throws exception? Posted by: admin May 7, 2018 Leave a comment. Questions: I know how to create a reference to a method that has a String parameter and returns an int, it’s:

This is not specific to Java 8. You are trying to compile something equivalent to:

interface I {
    void m();
class C implements I {
    public void m() throws Exception {} //can't compile

Java 8 Lambda function that throws exception?, You can try any of the following. set your own functional interface that represents the checked exception: @FunctionalInterface. public interface  A lambda expression can throw an exception but that should be compatible with the exceptions specified in the throws clauses of the abstract method of the functional interface. If a lambda expression body throws a checked exception, the throws clause of the functional interface method must declare the same exception type or its supertype.

Disclaimer: I haven't used Java 8 yet, only read about it.

Function<String, Integer> doesn't throw IOException, so you can't put any code in it that throws IOException. If you're calling a method that expects a Function<String, Integer>, then the lambda that you pass to that method can't throw IOException, period. You can either write a lambda like this (I think this is the lambda syntax, not sure):

(String s) -> {
    try {
        return myMethod(s);
    } catch (IOException ex) {
        throw new RuntimeException(ex);
        // (Or do something else with it...)

Or, if the method you're passing the lambda to is one you wrote yourself, you can define a new functional interface and use that as the parameter type instead of Function<String, Integer>:

public interface FunctionThatThrowsIOException<I, O> {
    O apply(I input) throws IOException;

How to handle an exception using lambda expression in Java?, A lambda expression body can't throw any exceptions that haven't Java 8​Object Oriented ProgrammingProgramming If the lambda expression can throw an exception then the "throws" clause of a functional interface must  This tutorial demonstrates how devs can handle checked exceptions in their projects using Java 8 and lambda expressions for cleaner, more concise code.

Java 8 Functional Interfaces and Checked Exceptions, The Java 8 lambda syntax and functional interfaces have been a The throwsApply method declares a throws clause of type Exception . I wonder if someone could explain the following weirdness to me. I'm using Java 8 update 11. Given this method. private <F,T> T runFun(Function<Optional<F>, T> fun, Optional<F> opt) { return fun.apply(opt) ; } If I first construct a function Object, and pass that in to the method above, things compile.

Exception Handling in Java Lambda Expressions, A lambda expression in Java can throw an exception but that throws an exception, the throws clause of the functional interface method must  • If you are calling a method which literally can never throw the exception that it declares, then you should not include the throws clause. For example: new String(byteArr, "UTF-8") throws UnsupportedEncodingException, but UTF-8 is guaranteed by the Java spec to always be present.

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