Why Does This Java 8 Lambda Expression Fail to Compile?

What causes a lambda expression to fail compilation in Java 8, particularly with respect to return types?

(String s1, String s2) -> "hi"

In this article, we analyze the compilation error in Java 8 when using lambda expressions, specifically focusing on the return type discrepancies and functional interfaces.

takeBiConsumer((String s1, String s2) -> {
    System.out.println("hi");
}); // This works correctly, as it returns void.

Causes

• The lambda expression's return type must match the expected return type of the functional interface (void in this case).
• In the second lambda expression, the return type is a String, but the interface method accepts no return value because its return type is void.

Solutions

• Ensure that the lambda expression adheres to the functional interface's defined method signature and return type.
• For the BiConsumer functional interface, use expressions that return void, or simply don't return a value.

Mistake: Using a lambda expression that returns a value for a functional interface that expects void.

Solution: Modify the lambda to ensure it does not return a value.

Mistake: Confusing the expected argument and return types of functional interfaces.

Solution: Review the method signatures of functional interfaces to clarify their expected behaviors.