How to Handle Exceptions in Java 8 Lambda Expressions When Filtering Streams?

Java 8: Lambda-Streams - Filtering by Method that Throws Exceptions

Java 8's lambda expressions offer powerful ways to manipulate and process data. However, when dealing with methods that can throw exceptions, it's important to navigate the limitations of lambda expressions and explore effective solutions.

The Problem:

Consider the following code fragment:

<code class="java">class Bank {
    public Set<String> getActiveAccountNumbers() throws IOException {
        Stream<Account> s = accounts.values().stream();
        s = s.filter(a -> a.isActive());
        Stream<String> ss = s.map(a -> a.getNumber());
        return ss.collect(Collectors.toSet());
    }
}

interface Account {
    boolean isActive() throws IOException;
    String getNumber() throws IOException;
}</code>

This code fails to compile due to the checked exceptions thrown by the isActive and getNumber methods. Simply catching the exceptions in a surrounding try-catch block is not enough, as the exceptions are not properly handled within the lambda expressions.

The Solution:

To overcome this issue, we need to catch the exceptions before they escape the lambda expression. One approach is to use an unchecked exception, as seen in the following modified code fragment:

<code class="java">s = s.filter(a -> {
    try {
        return a.isActive();
    } catch (IOException e) {
        throw new UncheckedIOException(e);
    }
});</code>

Here, we catch the IOException within the lambda and throw an unchecked exception, ensuring that it's handled properly within the lambda expression.

Alternatively, we can use a wrapper method that translates checked exceptions into unchecked ones:

<code class="java">return s.filter(a -> uncheckCall(a::isActive))
        .map(Account::getNumber)
        .collect(toSet());

public static <T> T uncheckCall(Callable<T> callable) {
    try {
        return callable.call();
    } catch (RuntimeException e) {
        throw e;
    } catch (Exception e) {
        throw new RuntimeException(e);
    }
}</code>

This allows us to wrap the lambda expression in a call to uncheckCall, which handles the checked exceptions internally, leaving the lambda expression free of checked exception declarations.

Other Considerations:

It's important to be mindful of the fact that certain exceptions can still escape the lambda expression, even with the suggested solutions. For example, exceptions thrown during the evaluation of the collect operation will still propagate out of the lambda.

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