Non-empty methods as a consumer

Hello

I would like to ask a question about the use of methods not void as a consumer in lambda expressions. In blades of lesson 1-4, page 3, it is said that the < T > consumers are "that takes a single value and returns no. result of the operation. So I thought that Java would only accept the void methods, but he accepts something as list.forEach (s-> builder.append (s)); as well. However the method-builder.append () returns StringBuilder. So I guess you meant in the slides that return value is ignored, right? Thank you.

BTW wonderful courses!

Marian

Right, in this case, the return value is ignored.

The idea is that there are some expressions that can be used as statements; See the Java Language Specification, section 14.8 for gory details. Thus, builder.append ("x") is an expression that returns a StringBuilder. This expression can also be used as a statement she with a semicolon:

builder.append("x");

(Probably it only would do this for its side effects.) In this case, the return value is ignored.

It is similar in case a lambda is in a context where the target type is a void method:

void foo(Consumer csb) { ... }

foo(sb -> sb.append("x"));

If the lambda body is an expression that can also be used as a statement, while the expression can be used as a lambda body where the target type abstract method returns void. (Which is covered by JLS 15.27.3.)

Note that only a small subset of the expressions can be used as instructions or return void lambdas: invocations of method, assignments, create instance (e.g., new Foo()) and pre/post increment/decrement (a ++, etc.) expressions. Most of the other expressions (for example, 'a + b') cannot.

Tags: Java

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