How to Handle Checked Exceptions in Java 8 Lambda Expressions?

Lambda Functions with Exceptions in Java 8

In Java 8, lambda functions can be used to reference methods with specific signatures. However, when the referenced method declares a checked exception, defining the lambda reference can be challenging.

Problem

Consider the following method which takes a String as an argument and returns an Integer, but may throw an IOException:

Integer myMethod(String s) throws IOException

How do we define a lambda reference to this method?

Solution

There are two main approaches for handling exceptions in lambda references:

1. Custom Functional Interface

If you have control over the code, you can define a custom functional interface that declares the checked exception:

@FunctionalInterface
public interface CheckedFunction<T, R> {
   R apply(T t) throws IOException;
}

This allows you to create references to methods that throw checked exceptions:

void foo (CheckedFunction f) { ... }

2. Wrapping in a Non-Checked Method

If altering the method signature is not an option, you can wrap the checked method in a method that doesn't declare a checked exception:

public Integer myWrappedMethod(String s) {
    try {
        return myMethod(s);
    }
    catch(IOException e) {
        throw new UncheckedIOException(e);
    }
}

You can then reference the wrapped method:

Function<String, Integer> f = (String t) -> myWrappedMethod(t);

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