How to throw an Exception in a Consumer Java 8

Question

Is there any way to throw an Exception while using a consumer in java 8?

For example:

    private void fooMethod(List<String> list) throws Exception {

    list.forEach(element->{
        if(element.equals("a")) {
            throw new Exception("error!");
        }
    });

}

This gives me a compiler error saying: Unhandled exception type Exception

What is the correct way to throw an exception in this case?

Looks to me like someone already asked this question a few years ago. [Java 8 Lambda function that throws exception?](https://stackoverflow.com/questions/18198176/java-8-lambda-function-that-throws-exception). Did you not Google for _java lambda throw exception_ before posting your question? Have you not read [How much research effort is expected of Stack Overflow users?](https://meta.stackoverflow.com/questions/261592/how-much-research-effort-is-expected-of-stack-overflow-users) — Abra, Mar 10 '20 at 16:03

score 5 · Accepted Answer · edited Mar 10 '20 at 16:02

5

Since Exception and its subclass (other than RuntimeException) are checked Exception and in lambda, you can't throw checked exception. Hence you should use RuntimeException:

private void fooMethod(List<String> list) throws Exception {
    list.forEach(element->{
        if(element.equals("a")) {
            throw new RuntimException("error!");
        }
    });
}

edited Mar 10 '20 at 16:02

Nitin Bisht

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answered Mar 10 '20 at 15:04

Sourav Jha

411
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Jean-Baptiste Yunès · Answer 2 · 2020-03-11T09:56:25.273

3

Streams and related classes are not designed to use checked exceptions. But any RuntimeException can be thrown at any point in code, thus the trick is to throw a runtime whose cause (exception chaining) is the appropriate checked exception :

private void fooMethod(List<String> list) throws Exception {   
    list.forEach(element->{
        if(element.equals("a")) {
            throw new Runtime(new Exception("error!"));
        }
    });  
}

Then in the catching code you just have to get the original checked exception encapsulated :

try {
    fooMethod(someList);
} catch(Throwable e) {
    Exception ex = e.getCause();
}

edited Mar 11 '20 at 09:56

answered Mar 10 '20 at 21:58

Jean-Baptiste Yunès

34,548
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Thank you for your answer, isn't a bad practice to catch a Throwable? – OEH Mar 11 '20 at 09:06
1

Some say it, but in that case it is used as a "generic" example... You have to interpret it and adapt it to your needs. – Jean-Baptiste Yunès Mar 11 '20 at 09:55

E.H. · Answer 3 · 2022-01-27T17:16:51.793

You can do it with apache commons-lang3 library.

https://commons.apache.org/proper/commons-lang/javadocs/api-release/org/apache/commons/lang3/function/Failable.html

<dependency>
    <groupId>org.apache.commons</groupId>
    <artifactId>commons-lang3</artifactId>
    <version>3.12.0</version>
</dependency>

You can write:

import org.apache.commons.lang3.function.Failable;

private void fooMethod(List<String> list) throws Exception {
    list.forEach(Failable.asConsumer(element->{
        if(element.equals("a")) {
            throw new Exception("error!");
        }
    }));
}

How to throw an Exception in a Consumer Java 8

3 Answers3