Local classes¶

Basics¶

Local classes are classes declared in a code block, such as a method, a for loop, or an if statement. When declaring a local class, we omit information about the access modifier. Local class objects have access to the method fields of the parent classes. The next example shows the use of a local class directly in the main method:

public static void main(String[] args) {
  final List<String> names = List.of("Kate", "Maggie", "Peggie");
  final List<String> surnames = List.of("Jones", "Smith", "Budden");
  final int someVariable = 3; // variable to represent the access of local class variables

  class Name { // creating a local class definition, without an access modifier
    private final String firstName;
    private final String lastName;

    public Name(final String firstName, final String lastName) {
      this.firstName = firstName;
      this.lastName = lastName;
    }

    public String getReadableName() {
      System.out.println("Hey I can use outer variable " + someVariable);
      return firstName + " " + lastName;
    }
  }

  for (int idx = 0; idx < names.size(); idx++) {
    final Name name = new Name(names.get(idx), surnames.get(idx)); // use of local class
    System.out.println(name.getReadableName());
  }
}

NOTE: Interfaces cannot be declared in a code block.

However, local classes are not as flexible as "regular" [classes] (../javaBasics/object_classes.md). Well:

cannot define static methods
cannot contain static fields
but they can contain static constants, i.e. those with the static final modifier

The example below shows the limits of local classes:

class InnerClassExample {
  private static final String APP_NAME = "DummyApp"; // OK
  private static String INCORRECT_FIELD = "IAmMissingFinal"; // Compilation error, no final modifier

  public void printAppName() {
    System.out.println(APP_NAME);
  }

  public static void shouldNotBeDeclaredHere() {} // Compile error, static method in local class
}