Can we define a class inside a Java interface?

Yes, you can define a class inside an interface. In general, if the methods of the interface use this class and if we are not using it anywhere else we will declare a class within an interface.

Example

interface Library {
   void issueBook(Book b);
   void retrieveBook(Book b);
   public class Book {
      int bookId;
      String bookName;
      int issueDate;
      int returnDate;
   }
}
public class Sample implements Library {
   public void issueBook(Book b) {
      System.out.println("Book Issued");
   }
   public void retrieveBook(Book b) {
      System.out.println("Book Retrieved");
   }
   public static void main(String args[]) {
      Sample obj = new Sample();
      obj.issueBook(new Library.Book());
      obj.retrieveBook(new Library.Book());
   }
}

Output

Hello welcome to tutorialspoint

If we need to provide a default implementation of the interface, we will define a class inside an interface as:

Example

interface Library {
   void issueBook(Book b);
   void retrieveBook(Book b);
   public class Book implements Library {
      int bookId;
      String bookName;
      int issueDate;
      int returnDate;
      public void issueBook(Book b) {
         System.out.println("book issued");
      }
      public void retrieveBook(Book b) {
         System.out.println("book retrieved");
      }
   }
}
public class Sample {
   public void demo() {
      System.out.println("Hello welcome to tutorialspoint");
   }
   public static void main(String args[]) {
      Sample obj = new Sample();
      obj.demo();
   }
}