How Can Methods with Return Types Be Used with Java\'s Consumer Interface?

Understanding Lambda Method References with Return Type Compatibility

Lambda expressions and method references provide alternative ways to define functional interfaces in Java. However, confusion arises when a method reference with a return type appears to match the Consumer interface, despite the Consumer's accept method having a void return type.

Basis for Design

As Brian Goetz explains, this behavior is intentional. It allows developers to adapt methods to functional interfaces in the same way they would call the methods directly. Therefore, value-returning methods can be used with the Consumer interface, ignoring the return value.

Lambda Expressions and Return Types

Lambda expressions can take two forms:

• (args) -> expression: This form evaluates to a value and hence cannot be void compatible. For example, () -> { return ""; } is not void compatible, while () -> {} or () -> { return; } are.
• (args) -> { statements* }: This form contains one or more statements. It is void compatible if no code path attempts to return a value. Exceptions that prevent normal completion also render such lambdas void compatible.

Method References and Statements

Method references are another way to define functional interfaces. However, not all methods can be used as method references with Consumer. Specifically:

• Methods with a return type other than void cannot be used as method references with Consumer.
• Expressions that are also statements can be void compatible. For example, s -> s.toString() is void compatible, as method invocations are considered statements.

Exceptions

The only expression form that is not value compatible is (arg) -> methodReturningVoid(arg). This is because it does not return a value.

Conclusion

The compatibility of method references with Consumer depends on the return type and whether the expression can be used as a statement. Understanding this concept helps clarify how lambdas and method references interact with functional interfaces in Java.

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