Initialize a map entry only when absent.

Java: Use computeIfAbsent when creating a collection for a new key.

import java.util.*;
import java.util.stream.*;
import java.util.regex.*;
import java.nio.file.*;
import java.nio.charset.StandardCharsets;
import java.time.*;
import java.time.format.DateTimeFormatter;
import java.time.temporal.*;
import java.math.*;
import java.io.*;
import java.net.*;
import java.security.MessageDigest;
import java.util.concurrent.*;

public class Main {
    public static void main(String[] args) throws Exception {
        var groups = new TreeMap<String, List<Integer>>();
        groups.computeIfAbsent("A", key -> new ArrayList<>()).add(7);
        System.out.println(groups);
    }
}

Expected output EXAMPLE

{A=[7]}

Details

Save as Main.java, compile with javac Main.java, and run java Main. Uses the Java 21 standard library. Use computeIfAbsent when creating a collection for a new key.

✓ Verified

Tested on Ubuntu 24.04.5 LTS · 2026-11-02 18:01:54 UTC · openjdk 21.0.12.1 2026-08-18 · openjdk-21-jdk-headless 21.0.12.1+1-1~24.04.4

Test inputs, results and environment

The exact published code ran in a dedicated Linux test container with prepared inputs. Unprivileged snippet process, external networking disabled, read-only root filesystem, and a fresh temporary working directory. Fixtures and output records are isolated from the application database. Testing covers this example and its stated checks, not every input, operating system, or runtime version. The Linux container shares its host's kernel; this is not a native Windows test.

Architecture / host kernel: x86_64 / 6.18.54.1-microsoft-standard-WSL2
Snippet identity / locale: uid=65534, gid=65534 (unprivileged) / C.UTF-8
Interpreter invocation: javac Main.java && java Main
Container image ID: sha256:32dc2d998158b6554c99b73b533f423696a0f3e31e8cbea9db10b5eee9a2fadd
Code SHA-256: 57e785fd4d9163a2b3bb74b2e12273b13dcaca3649e2263e53227ff1b8cd7ad5
Dependencies: openjdk-21-jdk-headless 21.0.12.1+1-1~24.04.4

Test input setup

These are the prepared inputs used by the test harness. See Details above for the prerequisites and how to run this example.

# No input files required.

Observed standard output

{A=[7]}

Exit code: 0. Standard error: empty. Validation: Exact stdout comparison. System-dependent values above are one recorded run.