Compilation error while using Method reference in Java 8

Question

I am not understanding the reason, for having the compilation error for the below program. Where am I going wrong? I want to print the value of the string as output using method reference.

public class ConsumerDemo{
 public static void main(String[] args) {
 test("hello", (str)-> str::toUpperCase);
 }
 public static void test(String str, Consumer<String> consumer) {
 consumer.accept(str);
 }
 }

Answer 1

test("hello", String::toUpperCase)

should be the correct syntax.

In order to print the upper case of the input, you can use:

String str = "hello"; // any input value
test(str.toUpperCase(), System.out::println);

• zilcuanu: I want to print after converting to uppercase
• Ousmane D.: Although this compiles the result of String::toUpperCase will be ignored.
• Naman: @Aomine agreed that's why wanted to know what OP was willing to perform with the consumer.

Answer 2

you cannot combine lambda syntax and method reference syntax as such.

You're either looking for:

test("hello", String::toUpperCase);

or:

test("hello", s -> s.toUpperCase());

but this then means the result of String::toUpperCase/s -> s.toUpperCase() is ignored hence, you'll need to perform something more useful. for example:

test("hello", s -> System.out.println(s.toUpperCase()));