CODEWITHSUNDEEP
java
Torarn
Torarn

Reputation: 412

Giving the parameter System.out::println to a method

I forgot a concept that I used and can't remember it.

For the context, I used to have a method like toString() which had a parameter. This parameters allowed me to call my method like myMethod(System.out::println) to print it on the screen, but also to print it in a file with the same syntax.

Does anyone know what can be this parameter? the concept? I think it's kind of a FunctionalInterface but I don't know what it is.

Upvotes: 0

Views: 2006

Answers (3)

pts
pts

Reputation: 87221

Would this work for you?

public class C {
  static void myMethod(java.io.PrintStream p) {
    p.println("Hello!");
  }
  public static void main(String args[]) {
    myMethod(System.out);
  }
}

Unfortunately you can't pass System.out.println directly, because in Java methods (before Java 8) are not first-class objects. All you can do with a method is calling it. As a workaround, you can introduce an interface and create an adapter class.

Upvotes: 0

kajacx
kajacx

Reputation: 12939

This goes well with java 8 functional interface:

@FunctionalInterface
static interface MyPrinter {
    public void println(String line);
}

static void myMethod(MyPrinter mp) {
    mp.println("Hello wolrd");
}

...

public static void main(String[] args) throws IOException {
    myMethod(System.out::println);

    PrintWriter pw = new PrintWriter("myFile.txt");
    myMethod(pw::println);
    pw.close(); //never forget to close file output streams!

    //EDIT: note that you can of course store your MyPrinter in a variable:
    MyPrinter mp = System.err::println;
    myMethod(mp);
}

Upvotes: 0

Cédric Couralet
Cédric Couralet

Reputation: 5223

This is called method reference and applies when you know what method you want to call, provided the method already exist.

From the tutorial :

Because this lambda expression invokes an existing method, you can use a method reference instead of a lambda expression:

Arrays.sort(rosterAsArray, Person::compareByAge);

Available since Java-8.

An exemple using what you want:

    public static void function(String s, Consumer<String> f) {
    f.accept(s);
}

public static void main(String args[]) {
    String test1 = "test";
    String test2 = "test2";
    function(test1, System.out::println);
    function(test2, System.out::println);
    function(test1, System.out::print);
    function(test2, System.out::print);
}

Upvotes: 2

Related Questions