Setting Up Java Locally

Why work locally

You can write contest code in an online editor, and for your first contest that is fine. But local setup pays off quickly: you can test against sample input without retyping it, run a program repeatedly with different inputs, and use a debugger. During a four-hour contest, saved minutes accumulate.

If you cannot install software on the computer you use — a school machine with locked-down permissions is common — the online option is a genuine fallback, not a compromise. Skip to the end of this lesson.

Installing a JDK

You need a Java Development Kit, version 17 or newer. Most online judges run a recent long-term-support release, and anything from 17 up behaves the same for contest purposes.

javac --version
java --version

Compiling and running

A contest submission is a single file. Keep the class name and the file name matching.

// File: Main.java
import java.io.*;

public class Main {
    public static void main(String[] args) throws IOException {
        BufferedReader in = new BufferedReader(new InputStreamReader(System.in));
        int n = Integer.parseInt(in.readLine().trim());
        System.out.println(n * 2);
    }
}

Compile and run:

javac Main.java
java Main

Newer JDKs also let you skip the compile step for single-file programs, which is convenient while iterating:

java Main.java

Testing against sample input

This is the part that saves real time. Put the sample input in a file instead of typing it every run.

Save the problem’s sample input as in.txt, then redirect it into your program:

java Main < in.txt

To compare your output against the expected output, save that as expected.txt and diff them:

java Main < in.txt > out.txt
Compare-Object (Get-Content out.txt) (Get-Content expected.txt)

java Main < in.txt > out.txt
diff out.txt expected.txt

Using an IDE

An IDE is optional but helpful, mainly for the debugger and for catching typos as you write. IntelliJ IDEA Community Edition and VS Code with the Java extension pack are both free and both let you configure sample input redirection in the run settings.

If you already use Android Studio for FTC, you already have IntelliJ’s editor — the same shortcuts apply.

If you cannot install anything

Online judges and editors let you compile and run Java in a browser. Paste the sample input into the input pane and run. It is slower than a local setup and you lose the debugger, but every problem in this curriculum can be solved this way.

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