BiConsumer and the put () method of Map

[moioi Yang]

var map = new HashMap<String, Integer> ();
BiConsumer<String, Integer> a  =  (k, v) -> map.put (k, v);  

V put (K key, V value)

The put () method has a return type V, but BiConsumer has return type void. I knew it compiles, but why?

[Piet Souris]

hi moioi.

Welcome to the Ranch!

A return value of a method call may always be discarded. So, since map.put is such a call, you can use it as if it were a void method. Just like you can do a single statement: 'map.put(k, v);' where you also discard the return value.

This doesn't work:

Consumer<Integer> cons = a -> 1;

[Paul Clapham]

It's quite common to write code which ignores the value returned by a Java method. You'll often see the Map.put(key, value) method used like that:

Map<String, String> map = new HashMap<>();
String key = "Test";
String value = "No problem";
map.put(key, value);

The compiler doesn't flag that code as an error in any way, and the value returned by the put() method is just ignored and discarded.

It's exactly the same in your example. Methods which return a value can be used in a context which doesn't expect a value, and the return value is ignored.

[moioi Yang]

Thanks for your answers. I didn't realize the method return values could be discarded or ignored in situations like these.

All the examples in the book happen to have the method return types match the interface method return type. So I assumed it's a rule.

[Paul Clapham]

It's a reasonable assumption, especially if you haven't seen examples where the returned value is ignored. And even if you had, it isn't obvious that they can also be ignored in that lambda-expression context. But yeah, there's actually a rule somewhere in the Java language spec which says that methods' return values can be ignored.

[Junilu Lacar]

Paul Clapham wrote:But yeah, there's actually a rule somewhere in the Java language spec which says that methods' return values can be ignored.

That would be here: https://docs.oracle.com/javase/specs/jls/se8/html/jls-15.html#jls-15.1

The Java Language Specification wrote:An expression statement or lambda body that is a method invocation may also invoke a method that produces a result; in this case the value returned by the method is quietly discarded.

Note: emphasis is mine.