Collection declared by class, not interface

Reports declarations of Collection variables made by using the collection class as a type, rather than an appropriate interface. The warning is not issued if weakening the variable type will cause a compilation error.

Example:

  // Warning: concrete collection class ArrayList used.
  int getTotalLength(ArrayList<String> list) {
    return list.stream().mapToInt(String::length).sum();
  }

  // No warning, as trimToSize() method is not
  // available in the List interface
  void addData(ArrayList<String> data) {
    data.add("Hello");
    data.add("World");
    data.trimToSize();
  }

A quick-fix is suggested to use the appropriate collection interface (e.g. Collection, Set, or List).

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.

CollectionDeclaredAsConcreteClass

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 | Abstraction issues

Inspection options

Here you can find the description of settings available for the Collection declared by class, not interface inspection, and the reference of their default values.

Ignore local variables: Default value:
Not selected
Ignore 'private' fields and methods: Default value:
Not selected

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 CollectionDeclaredAsConcreteClass

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