Why Do Java Method References with Return Types Seemingly Match the Consumer Interface?

Understanding Method References' Compatibility with the Consumer Interface

Despite the title, "Why does a Java method reference with return type match the Consumer interface?", lambda expressions and method references possess a complex relationship that extends beyond mere type matching. Let's delve into this topic and clarify the intricacies involved.

When defining a lambda expression with a return type, such as lambda1 and lambda2 in the code example, it adheres to the type signature of the functional interface it implements. However, the surprising case occurs with method references.

Consider lambda3 and lambda4, which reference the consume method. Intuitively, we might expect lambda3 to fail due to the mismatch between consume's return type (String) and the expected void return type of the Consumer interface. However, instead of failing, lambda3 succeeds.

The answer lies in the design principle behind method references. They aim to allow the adaptation of methods to functional interfaces, regardless of their return values. In this case, the consume method's return value (String) is seemingly ignored, enabling it to conform to the Consumer interface's void requirements.

Interestingly, the same principle applies to lambda expressions. The lambda form (args) -> expression is value compatible only if the expression evaluates to a non-statement value. However, if the expression is a statement itself (e.g., method invocation, increment/decrement), it becomes void compatible. Thus, consume represents a valid void-compatible form.

As a notable exception, the form (arg) -> methodReturningVoid(arg) is the sole expression that lacks value compatibility. Understanding these complexities enhances our understanding of Java's lambda expressions and method references, enabling us to harness their capabilities effectively.

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