Java 25 (LTS) reference · Map and HashMap Methods

Java Map putIfAbsent() Method

Add a key-value pair only if the key is missing (or mapped to null). Returns the existing value otherwise.

Java 25 (LTS) Since Java 8

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Syntax

map.putIfAbsent(key, value)

Parameters

Parameter	Description
`key` K	The key.
`value` V	Value to add if absent.

Return value

The existing value, or null if it was added.

Examples

Defaults that don't overwrite

import java.util.*;

public class Main {
    public static void main(String[] args) {
        Map<String, String> prefs = new HashMap<>(Map.of("theme", "dark"));
        prefs.putIfAbsent("theme", "light");
        prefs.putIfAbsent("lang", "en");
        System.out.println(new TreeMap<>(prefs));
    }
}

Output

{lang=en, theme=dark}

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Remember the first occurrence

import java.util.*;

public class Main {
    public static void main(String[] args) {
        Map<String, Integer> first = new HashMap<>();
        for (String visitor : List.of("ana", "ben", "ana")) first.putIfAbsent(visitor, first.size() + 1);
        System.out.println(new TreeMap<>(first));
    }
}