Modificatori Java


Modificatori

Ormai hai abbastanza familiarità con la publicparola chiave che appare in quasi tutti i nostri esempi:

public class Main

La publicparola chiave è un modificatore di accesso , il che significa che viene utilizzata per impostare il livello di accesso per classi, attributi, metodi e costruttori.

Dividiamo i modificatori in due gruppi:

  • Modificatori di accesso : controlla il livello di accesso
  • Modificatori di non accesso: non controllano il livello di accesso, ma forniscono altre funzionalità

Modificatori di accesso

Per le classi , puoi utilizzare uno publico l' altro predefinito :

Modifier Description Try it
public The class is accessible by any other class
default The class is only accessible by classes in the same package. This is used when you don't specify a modifier. You will learn more about packages in the Packages chapter

Per attributi, metodi e costruttori è possibile utilizzare uno dei seguenti:

Modifier Description Try it
public The code is accessible for all classes
private The code is only accessible within the declared class
default The code is only accessible in the same package. This is used when you don't specify a modifier. You will learn more about packages in the Packages chapter
protected The code is accessible in the same package and subclasses. You will learn more about subclasses and superclasses in the Inheritance chapter

Modificatori di non accesso

Per le classi , puoi utilizzare finalo abstract:

Modifier Description Try it
final The class cannot be inherited by other classes (You will learn more about inheritance in the Inheritance chapter)
abstract The class cannot be used to create objects (To access an abstract class, it must be inherited from another class. You will learn more about inheritance and abstraction in the Inheritance and Abstraction chapters)

Per attributi e metodi , puoi utilizzare uno dei seguenti:

Modifier Description
final Attributes and methods cannot be overridden/modified
static Attributes and methods belongs to the class, rather than an object
abstract Can only be used in an abstract class, and can only be used on methods. The method does not have a body, for example abstract void run();. The body is provided by the subclass (inherited from). You will learn more about inheritance and abstraction in the Inheritance and Abstraction chapters
transient Attributes and methods are skipped when serializing the object containing them
synchronized Methods can only be accessed by one thread at a time
volatile The value of an attribute is not cached thread-locally, and is always read from the "main memory"


Finale

Se non desideri la possibilità di sovrascrivere i valori degli attributi esistenti, dichiara gli attributi come final:

Esempio

public class Main {
  final int x = 10;
  final double PI = 3.14;

  public static void main(String[] args) {
    Main myObj = new Main();
    myObj.x = 50; // will generate an error: cannot assign a value to a final variable
    myObj.PI = 25; // will generate an error: cannot assign a value to a final variable
    System.out.println(myObj.x);
  }
}


Statico

Un staticmetodo significa che è possibile accedervi senza creare un oggetto della classe, a differenza di public:

Esempio

Un esempio per dimostrare le differenze tra statice publicmetodi:

public class Main {
  // Static method
  static void myStaticMethod() {
    System.out.println("Static methods can be called without creating objects");
  }

  // Public method
  public void myPublicMethod() {
    System.out.println("Public methods must be called by creating objects");
  }

  // Main method
  public static void main(String[ ] args) {
    myStaticMethod(); // Call the static method
    // myPublicMethod(); This would output an error

    Main myObj = new Main(); // Create an object of Main
    myObj.myPublicMethod(); // Call the public method
  }
}


Astratto

Un abstractmetodo appartiene a una abstractclasse e non ha un corpo. Il corpo è fornito dalla sottoclasse:

Esempio

// Code from filename: Main.java
// abstract class
abstract class Main {   public String fname = "John";   public int age = 24;   public abstract void study(); // abstract method } // Subclass (inherit from Main) class Student extends Main {   public int graduationYear = 2018;   public void study() { // the body of the abstract method is provided here     System.out.println("Studying all day long");   } } // End code from filename: Main.java // Code from filename: Second.java class Second {   public static void main(String[] args) {     // create an object of the Student class (which inherits attributes and methods from Main)     Student myObj = new Student();     System.out.println("Name: " + myObj.fname);     System.out.println("Age: " + myObj.age);     System.out.println("Graduation Year: " + myObj.graduationYear);     myObj.study(); // call abstract method
  } }