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IO class in Java 25

posted Originally published at lnkd.in 1 min read

IO class of Java 25

Java 25 intrroduced a new class called IO in java.lang package, in order to avoid writing longer code when using System.out.println.

I found a difference in the behaviour from the System.out.println

Just try the following main method.

public static void main(String[] args) {
   char[] msg = {'H','e','l','l','o'};
   IO.println(msg);
   System.out.println(msg);
}

I know println in PrintStream handled char[] specially and not just using the toString method.

Can we expect the IO class println method to be consistent with the PrintStream's println?

This is as of 

$ java -version
java version "25" 2025-09-16 LTS
Java(TM) SE Runtime Environment (build 25+37-LTS-3491)
Java HotSpot(TM) 64-Bit Server VM (build 25+37-LTS-3491, mixed mode, sharing)

4 Comments

•

This inconsistency highlights an important design decision in JEP 512. The IO class was specifically created to smooth the learning curve for beginners - part of Oracle's "Paving the On-Ramp" effort to make Java more accessible to new programmers and scripters.

The behavior difference you've identified between IO.println(char[]) and System.out.println(char[]) is likely intentional. The IO class prioritizes simplicity and predictability over the specialized behaviors that PrintStream has accumulated over decades. For beginners writing their first programs, having println always call toString() creates more consistent mental models.

However, this does raise compatibility concerns. During the Amber mailing list discussions mentioned in my briefing, Oracle emphasized that the path from compact source files to full programs should be smooth. A student who starts with IO.println(msg) and later switches to System.out.println(msg) shouldn't encounter surprising behavior changes.

This might be worth raising on the amber-dev mailing list. The Java team has been very responsive to feedback about these onboarding features - they actually removed implicit IO imports based on community input to ensure a smoother upgrade path.

The goal is to let students start simple and grow gracefully without having to rewrite their existing code. Behavioral differences between IO and PrintStream could create exactly the kind of friction JEP 512 aims to eliminate.

1 vote
•

Hi Tom,

Thanks for your very thoughtful comment.

I was trying to look into the reason for this, and wrote the following code to test various possible implementation of the println method of IO class.
I tried writing 5 different implementations in my class:
The code is as follows:

public class TestPrintln {
    public static void println1(Object o) {
        String methodName = Thread.currentThread().getStackTrace()[1].getMethodName();
        System.out.print(methodName+":");
        System.out.println(o);
    }
    public static void println2(Object o) {
        String methodName = Thread.currentThread().getStackTrace()[1].getMethodName();
        System.out.print(methodName+":");
        System.out.println(o.getClass().cast(o));
    }
    public static void println3(Object o) {
        String methodName = Thread.currentThread().getStackTrace()[1].getMethodName();
        System.out.print(methodName+":");
        if (o instanceof char[] ch) {
            System.out.println((char[])ch);
            return;
        }
        System.out.println(o);
    }
    public static void println4(Object o) {
        String methodName = Thread.currentThread().getStackTrace()[1].getMethodName();
        System.out.print(methodName+":");
        if (o instanceof char[] ch) {
            println4(String.valueOf(ch));
            return;
        }
        System.out.println(o);
    }
    public static void println5(Object o) {
        String methodName = Thread.currentThread().getStackTrace()[1].getMethodName();
        System.out.print(methodName+":");
        System.out.println(String.valueOf(o));
    }
    public static void main(String[] args) {
        char[] msg = {'H','e','l','l','o'};
        System.out.println("println:"+msg);
        println1(msg);
        println2(msg);
        println3(msg);
        println4(msg);
        println5(msg);
    }
}

I am surprised to see difference in println2 and println3
The cast using the reflection api is not giving the same result as the compile-time cast.
The first 2 lines in all the println? methods are to help identify the output generated by which method.

1 vote
•

Hello Pravin,

This looks like a legitimate implementation issue that deserves attention from the Java language team. I'd recommend submitting this to:

  • The amber-dev mailing list since JEP 512 falls under Project Amber
  • The OpenJDK bug tracker if you suspect this is an implementation bug
  • The official Java forums for broader community discussion

Your test code demonstrates the issue clearly - the behavioral inconsistency could undermine the "smooth learning path" that JEP 512 is designed to provide. The language designers need to see this.

1 vote
•

Hello Tom,
Thanks for your kind support and your suggestion.
I normally find it very strainous for my eyes when dark mode is not available and so avoided amber-dev.
I hope corporates recognize this need and provide for better accessibliity.
Many of the useful websites provide for dark mode options. like this one and linkedIn.
Thank you once again.

1 vote

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