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

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

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

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
  

Nested classes

exam may trick us by extending final class, even using anonymous class! (review Q.15)

Polymorphism

encapsulation allows both getters and setters as the goal is to only restrict direct access to variable (review Q.12)

Playground code

https://github.com/RG9/rg-playground-ocp17/blob/main/Chapter7/Chapter7_SealedClasses.java