'Comparator' class not declared 'Serializable'

Reports classes that implement java.lang.Comparator, but do not implement java.io.Serializable.

If a non-serializable comparator is used to construct an ordered collection such as a java.util.TreeMap or java.util.TreeSet, then the collection will also be non-serializable. This can result in unexpected and difficult-to-diagnose bugs.

Since subclasses of java.lang.Comparator are often stateless, simply marking them serializable is a small cost to avoid such issues.

Example:

  class Foo implements Comparator { // warning
      @Override
      public int compare(Object o1, Object o2) {
          /* ... */
      }
  }

After the quick-fix is applied:

  class Foo implements Comparator, Serializable { // no warning here
      @Override
      public int compare(Object o1, Object o2) {
          /* ... */
      }
  }

Locating this inspection

By ID

Can be used to locate inspection in e.g. Qodana configuration files, where you can quickly enable or disable it, or adjust its settings.

ComparatorNotSerializable

Via Settings dialog

Path to the inspection settings via IntelliJ Platform IDE Settings dialog, when you need to adjust inspection settings directly from your IDE.

Settings or Preferences | Editor | Inspections | Java | Serialization issues

Suppressing Inspection

You can suppress this inspection by placing the following comment marker before the code fragment where you no longer want messages from this inspection to appear:

//noinspection ComparatorNotSerializable

More detailed instructions as well as other ways and options that you have can be found in the product documentation:

IntelliJ IDEA 2025.2

Inspection Details
By default bundled with:	IntelliJ IDEA 2025.2, Qodana for JVM 2025.2,

Last modified: 18 September 2025