Console.writer().println() vs Console.format() vs System.out.println()

Question

Console.writer().println() is displaying "Enter 3 words" after console.readLine(). Can anyone explain why?

class Class3{
    public static void main(String... args){
        Console console = System.console();
        if(console == null){
            throw new RuntimeException("No console");
        }else{
            //System.out.println("Enter 3 words");
            //console.format("Enter 3 words");
            console.writer().println("Enter 3 words");
            
            List<String> strings = new ArrayList<>();
            for(int i=0;i<3;++i){
                strings.add(console.readLine());
            }
            console.format("You entered %s, %s, %s",
                    strings.get(0),
                    strings.get(1),
                    strings.get(2)
            );
            console.writer().println();  
        } 
    }
}

Output: Output of the application

I tried using System.out.println() and console.format() and both print "Enter 3 words" before the application accepts 3 strings

Answer 1

A quick look at JDK24 source code shows Console as a sealed class with permits ProxyingConsole. The class ProxyingConsole declares writer() to use WrappingWriter extends PrintWriter which constructs super class PrintWriter which sets this.autoFlush = false

This means that when you use console.writer(), it should be followed by console.writer().flush() if you want to guarantee the output is shown when you want it to.

System.out.println() is defined with autoflush so any output is shown for each call.

Thanks for the useful comments. JDKs have different behaviours for autoFlush in Console writer. From some checks on OpenJDK in Windows 11:

JDK	console.writer() autoFlush	System.out autoFlush
14 17 18 19 20	✅ true	✅ true
21 22 23 24 25b18	❌ false	✅ true