How to Code in Java Using AI

In short: say the Java version and build tool, give the method signature and rules, ask for JUnit 5 tests, run them.

Why Java with AI

Java is used in big systems at banks, telecom and e-commerce companies like Flipkart and Amazon. It needs a lot of boilerplate: classes, getters, tests. AI writes that part fast, so you focus on the logic.

What you need

• JDK 21 (LTS): Eclipse Temurin or another OpenJDK build. Check with java -version.
• Build tool: Maven or Gradle. They download JUnit for you.
• IDE + assistant: IntelliJ with GitHub Copilot, or VS Code with Copilot or Claude Code.
• JUnit 5: the standard Java test library.

How it flows

Install and check

1. Install JDK 21 (for example Eclipse Temurin 21 from adoptium.net).
2. Check: java -version and javac -version show 21.
3. Install Maven, or let IntelliJ create a Maven project for you (New Project → Maven).
4. Add JUnit 5 to pom.xml (ask the AI: “add JUnit 5 test dependency to this pom.xml”, then check the version on Maven Central).

Step 1 — the prompt

Step 2 — the code (tested here)

// Check password strength. Built with an AI assistant, then tested.
public class PasswordCheck {
    public static boolean isStrong(String pw) {
        if (pw == null || pw.length() < 8) return false;
        boolean upper = false, lower = false, digit = false;
        for (char c : pw.toCharArray()) {
            if (Character.isUpperCase(c)) upper = true;
            else if (Character.isLowerCase(c)) lower = true;
            else if (Character.isDigit(c)) digit = true;
        }
        return upper && lower && digit;
    }

    public static void main(String[] args) {
        System.out.println(isStrong("Mumbai2026"));  // true
        System.out.println(isStrong("password"));    // false
    }
}

Step 3 — compile and run

javac PasswordCheck.java
java PasswordCheck
# -> true
# -> false

Step 4 — ask for JUnit 5 tests

import org.junit.jupiter.api.Test;
import static org.junit.jupiter.api.Assertions.*;

class PasswordCheckTest {
    @Test void strongPassword() { assertTrue(PasswordCheck.isStrong("Mumbai2026")); }
    @Test void tooShort()       { assertFalse(PasswordCheck.isStrong("Ab1")); }
    @Test void noDigit()        { assertFalse(PasswordCheck.isStrong("MumbaiCity")); }
    @Test void nullIsWeak()     { assertFalse(PasswordCheck.isStrong(null)); }
}

mvn test
# -> Tests run: 4, Failures: 0

Settings and config

1. Project rules: in .github/copilot-instructions.md or CLAUDE.md write “Java 21, Maven, JUnit 5, no new dependencies without asking”.
2. IntelliJ: File → Project Structure → SDK = 21, so the AI's modern code (records, switch expressions) compiles.
3. Format: Code → Reformat Code after accepting AI code, so style stays the same.

Good Java prompts (reuse these)

1. “Turn this class into a Java record.”
2. “Add null checks and throw IllegalArgumentException with a clear message.”
3. “Replace this for loop with a stream, same behaviour, and keep the tests passing.”
4. “Explain this stack trace and fix it:” + paste only the top lines.

Watch out (common AI mistakes in Java)

Quick quiz

Save tokens and money

Paste one class, not the whole project or target/ folder. Paste only the top 10 lines of a stack trace. Let IntelliJ generate getters and constructors for free (Alt+Insert) instead of asking the AI.

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