Why Can a Method Reference with a Return Value Be Assigned to a Consumer Interface in Java?

Understanding Method References with Consumer Interface

In Java, method references can be used to create instances of functional interfaces. However, in certain scenarios, confusion may arise when the return type of a method reference does not match the accept method of the Consumer interface.

Consider the following code:

Consumer<String> lambda1 = s -> {};
Function<String, String> lambda2 = s -> s;

Consumer<String> lambda3 = LambdaTest::consume; // but s -> s doesn't work!
Function<String, String> lambda4 = LambdaTest::consume;

As you might expect, the assignment of lambda3 to a Consumer should fail because the return type of the consume method (String) doesn't match the expected void type of the accept method.

However, surprisingly, the assignment of lambda3 succeeds. This is because of a design decision that allows adapting a method to a functional interface in the same way that the method can be called. This means that any value-returning method can be assigned to a Consumer, with its return value being ignored.

When it comes to lambda expressions, the rules are slightly more complex. Lambda expressions have two forms:

1. (args) -> expression
2. (args) -> { statements* }

The first form ((args) -> expression) is value compatible only if the expression evaluates to a value. The second form ((args) -> { statements* }) can be void compatible if no code path attempts to return a value.

The expression s -> s is not void compatible because s is not a statement. However, expressions that have side effects, such as method invocations, can be used as statements. This means that expressions like s -> s.toString() and s -> i can be void compatible.

Therefore, in the case of LambdaTest::consume, the method reference is assigned to the Consumer interface because the consume method can be called and its return value can be ignored. This design decision enables adaptability between methods and functional interfaces, even when the return types do not match.

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