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Avoid Adding Null Values Using computeIfAbsent() and computeIfPresent() in Maps

High
maintainabilitybug detection

What is it?​

The misuse of computeIfAbsent() and computeIfPresent() methods on Java Maps to add null values can lead to unexpected behavior. These methods were designed to simplify conditional additions to a Map but do not support adding null values directly.

Why apply it?​

Using these methods improperly can result in the absence of expected entries or unintended Map entry removals, leading to bugs and maintenance issues in the code.

How to fix it?​

Use traditional approaches with containsKey() checks and conditional put() calls to handle null values explicitly, ensuring that Map entries are added as intended.

Examples​

Example 1:​

Negative​

This negative example uses computeIfAbsent() with a lambda returning null, which will not insert the key-value pair in the map.

import java.util.HashMap;
import java.util.Map;

public class Example {
    public static void main(String[] args) {
        Map<String, String> map = new HashMap<>();
        
        String key = "testKey";
        
        map.computeIfAbsent(key, k -> null);  /* Noncompliant */
        
        System.out.println("Map size: " + map.size()); // Outputs: Map size: 0
    }
}

Example 2:​

Positive​

This positive example shows the use of containsKey before adding null to a map explicitly with put().

import java.util.HashMap;
import java.util.Map;

public class Example {
    public static void main(String[] args) {
        Map<String, String> map = new HashMap<>();
        
        String key = "testKey";
        
        if (!map.containsKey(key)) {
            map.put(key, null);  /* Compliant */
        }
        System.out.println("Map size: " + map.size()); // Outputs: Map size: 1
    }
}

Negative​

This negative example uses computeIfPresent() improperly, which will remove the entry if the lambda returns null.

import java.util.HashMap;
import java.util.Map;

public class Example {
    public static void main(String[] args) {
        Map<String, String> map = new HashMap<>();
        map.put("testKey", "value");
        
        String key = "testKey";
        
        map.computeIfPresent(key, (k, v) -> null);  /* Noncompliant */
        
        System.out.println("Map contains key: " + map.containsKey(key)); // Outputs: false
    }
}

Example 3:​

Positive​

This positive example checks if a key is present before replacing its value to null, using put() safely.

import java.util.HashMap;
import java.util.Map;

public class Example {
    public static void main(String[] args) {
        Map<String, String> map = new HashMap<>();
        map.put("testKey", "value");
        
        String key = "testKey";
        
        if (map.containsKey(key)) {
            map.put(key, null);  /* Compliant */
        }
        System.out.println("Map contains key with null value: " + (map.get(key) == null)); // Outputs: true
    }
}