public abstract interface Run { //abstract implicit modifiers
public abstract void run(); //public abstract implicit modifiers
}Concise form:
public interface Run {
void run();
}An interface cannot be marked as final
//does not compile
public final interface Walk {}Interface variables are implicitly public, static, final.
public interface Weight {
int max_weight = 5;
public static final int max_height = 50;
String color; //does not compile as it's not initialized
} Class which implements two interfaces having the same(signature) default method:
@Override
public void go() {
Run.super.go();
//or
Walk.super.go();
}Abstract class default methods
interface House {
default void work(){} //default method
}
interface Office {
void work(); //abstract method
}
class HomeOffice implements Office, House {
//I am forced to implement work()!
@Override
public void work() {}
}A private interface method cannot be called in a method outside the interface declaration.
interface InterfaceWithAPrivateMethod {
private void message();
void hello();
}
class InterfaceWithAPrivateMethodImpl implements InterfaceWithAPrivateMethod {
@Override
public void hello(){
//does not compile!
//message();
}
}@Override annotation informs the compiler that the element is meant to override an element declared
in a superclass and/or interface. While it is not required to use this annotation when overriding a method, it helps
to prevent errors.
First, if a class or interface inherits two interfaces containing default methods with the same signature, it must override the method with its own implementation.
Inherit two default methods with the same signature
A default method can invoke any other type of method within the interface:
- public static methods
- private static methods
- public instance methods
- private static methods
- other default methods
DefaultMethodCallingOtherMethods
interface Certifications {
//public by default
public static void ocp17() {}
}
class MyCertifications implements Certifications {
public static void main(String[] args) {
Certifications.ocp17(); //valid
MyCertifications myCertifications = new MyCertifications();
//I cannot invoke a static method of the interface from an instance of the class which implements the interface!
myCertifications.ocp17(); //DOES NOT COMPILE!
}
}A sealed class requires at least one subclass to extend.
- A sealed class needs to be declared in the same package or named module as their direct subclasses.
- Direct subclasses of a sealed classes must be either
finalorsealedornon-sealed. - The permits class is optional if the classes (sealed and subclasses) are declared in the same file.
The permits clause is optional if the subclass is nested or declared in the same file.
//same file
public sealed class Snake {}
final class Cobra extends Snake {}An interface can be declared sealed. The permits list can apply to a class that implements the interface
or an interface that extend the interface.
Then a sealed interface can be extended only by:
sealedinterfacesnon-sealedinterfaces.
// Sealed interface
public sealed interface Pet permits Cat, Dog, Rabbit {}
// Classes permitted to implement sealed interface Pet
public final class Cat implements Pet {}
public final class Dog implements Pet {}
// Interface permitted to extend sealed interface pet
public non-sealed interface Rabbit extends Pet {}public sealed class Snake permits Cobra, Viper {}
//separated files, but same pacakge
final class Cobra extends Snake {}
non-sealed class Viper extends Snake {}//same file
//no permits needed
sealed class HumanBeing {}
sealed class Male extends HumanBeing {}
non-sealed class EuropeanMale extends HumanBeing {}
final class AsianMale extends HumanBeing {}A subclass (Male) of a sealed class (HumanBeing) must be marked either final or sealed or non-sealed.
Named Module: which allow sealed classes and their direct subclasses in different packages, provided they are in the same named module.
sealed class Pet {
public final class Dog extends Pet {}
}final class AFinal {
public static void main(String[] args) {
//I can't override a final class
//new AFinal() {}; //does not compile
}
}public record Person(String firstName, String lastName) {
//this is a compact constructor
public Person { //no parenthesis!!!
if (firstName.isEmpty() || lastName.isEmpty()) {
throw new IllegalArgumentException("invalid");
}
}
}A compact constructor cannot set an instance variable through this but just with the normal assignment operator.
public record Name(String name) {
public Name {
name = "Enrico"; //this works
//this.name = "Enrico"; //this does not compile
}
}A compact constructor must have the same access modifiers as the record itself.
//does not compile
public record Name(String name) {
Name {
name="John";
}
}public record Person(String firstName, String lastName) {
//this is an overloaded constructor
public Person() {
//he first line must be a call to another constructor,
this("Enrico", "Giurin");
}
}record Game() {
//does not compile
final int score = 10;
}It does not compile because records cannot include instance variables not listed in the declaration of the record, as it could break immutability.
If an enum contains an abstract method, then every enum value must include an override of this abstract method.
public enum Gender {
MALE {
@Override
public String description() {
return "male";
}
}, FEMALE {
@Override
public String description() {
return "female";
}
};
public abstract String description();
}enum constructors are implicitly private!
public SeasonWithValues(String description) { //DOES NOT COMPILE!
this.description = description;
}It is illegal to access (not final) static member from enum constructor or instance initializer.
enum Api {
UPDATE("update the data"), READ("read the data");
private String description;
private static String version = "2.0";
private static final String firstVersion = "1.0";
Api(String description) {
this.description = description;
System.out.println(firstVersion); //this is fine
//It is illegal to access (not final) static member 'version' from enum constructor or instance initializer
//( JLS 8.9.2 )
//System.out.println(version); //does not compile
}
}all the enum implements Comparable.
int compare = CardinalPoints.NORTH.compareTo(CardinalPoints.EAST);
//so they can be used in a TreeSet
TreeSet<CardinalPoints> set = new TreeSet<>();
set.add(CardinalPoints.SOUTH);
set.add(CardinalPoints.NORTH);
set.add(CardinalPoints.WEST);
System.out.println(set); //[NORTH, WEST, SOUTH]Reference: Outer.this.field.
class University {
int id;
class Department {
int id;
class Professor {
int id;
void printID() {
System.out.println("University id: " + University.this.id);
//System.out.println("University id: "+this.id); //equivalent
System.out.println("Department id: " + University.Department.this.id);
// System.out.println("Department id: "+Department.this.id); //equivalent
System.out.println("professor id: " + University.Department.Professor.this.id);
//System.out.println("professor id: "+Professor.this.id);//equivalent
}
}
}
}A local class can access only final and effectively final local variables.
public void sortArray() {
int size = 0;
class NestedLocal {
public void sort() {
//size is not effectively fianl, as it's changed in the last line
int c = size; //does not compile!
}
}
//here I modify size, so I make it not effectively final
size = 0;
}