OCP 17 Exam - chapter 7 notes - part 2 (sealed and records)

Posted Mar 29, 2025

By Rafał Garbowski

views 2 min read

Sealed classes and interfaces

sealed classes allow us to implement “enum” on class level, so it can be used in switch statement:

  
      Sealed v = new Chapter7_SealedClasses().new MyClass();
      // note: use "java --enable-preview --source 17" to enable patterns in switch statements
      switch(v) {
        // case SubClass -> System.out.println("Subclass"); error: type pattern expected
        case SubClass s -> System.out.println("Subclass");
        default ->  System.out.println("default"); // without default, there is error: the switch statement does not cover all possible input values
        case MyClass m -> System.out.println("MyClass");
      }

“sealed class must have subclasses”, following is not permitted:

  
    sealed class Sealed { }

class that extend sealed class must have one of the following modifiers: final, sealed, non-sealed

  
    sealed class Sealed permits SubClass { }
    class SubClass extends Sealed { } // error: sealed, non-sealed or final modifiers expected

class mentioned in permits must extend sealed class (review Q.30)

  
    sealed class Sealed permits SubClass { }
    final class SubClass { } // DOES NOT COMPILE: subclass Chapter7_SealedClasses.SubClass must extend sealed class

permits is not required for classes in the same file! (review Q.30)

sealed class can be indirectly extended when extending non-sealed subclass (review Q.14)

Records

record cannot declare static field with the same name as instance field

  
record SameName(String name) { static int name = 0; } // DOES NOT COMPILE: record component name is already defined in record SameName

we can define compact constructor in order to validate parameters w/o need to repeating all parameters in signature

  
  record CompactConstructor(int i, String s) { 
    CompactConstructor {  
     i=2; s="b"; // can modify implicit constructor params
      // this.i = 3; // but not record field - DOES NOT COMPILE: error: cannot assign a value to final variable i
    }
  }

overloaded constructor must first call default one (review Q.21)

  
  record CustomConstructor(int i, String s) { 
    CustomConstructor() {  
      this(23, "custom"); // must invoke default all args constructor, otherwise DOES NOT COMPILE: error: constructor is not canonical, so its first statement must invoke another constructor of class CustomConstructor
    }
  }

Nested classes

Types:
Inner class: A non-static type defined at the member level of a class
Static nested class: A static type defined at the member level of a class
Local class: A class defined within a method body
Anonymous class: A special case of a local class that does not have a name