Java 8 Function vs Consumer

Question

I can't for the life of me find an explanation of the following:

public static void takesAFunction(Function<String, Void> func) {
    func.apply("Hi I'm running a function");
}

public static void takesAConsumer(Consumer<String> func) {
    func.accept("Hi I'm running a consumer");
}

public static void main(String[] args) throws Exception {
    takesAFunction((String str) -> { System.out.println(str); });
    takesAConsumer((String str) -> { System.out.println(str); });
}

I'm using JDK 1.8.0_66 and the line

takesAFunction((String str) -> { System.out.println(str); });

is marked as an error saying that

The method takesAFunction(Function<String,Void>) in the type MyClass 
is not applicable for the arguments ((String str) -> {})

I can't understand how is

Function<String, Void>

different from

Consumer<String>

when both return nothing and both take in a single String parameter.

Can someone pls shed some light on this cos it's killing.

Thanks in advance!

Similarly, there is a difference between `Runnable` and `Callable` as `Void` is a reference to a class which can only be `null` (Unless you use reflection to create one) — Peter Lawrey, Dec 16 '15 at 16:02

assylias · Accepted Answer · 2015-12-16T16:06:54.990

12

A Function<String, Void> should have the following signature:

Void m(String s);

not to be confused with void m(String s);!

So you need to return a Void value - and the only one available is null:

takesAFunction((String str) -> {
  System.out.println(str);
  return null;
});

compiles as expected.

edited Dec 16 '15 at 16:06

answered Dec 16 '15 at 15:55

assylias

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Oh got it - thanks a ton! – RVP Dec 16 '15 at 15:59

Java 8 Function vs Consumer

1 Answers1