Modificatori Java
Modificatori
Ormai hai abbastanza familiarità con la public
parola chiave che appare in quasi tutti i nostri esempi:
public class Main
La public
parola 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 public
o 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 final
o 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 static
metodo significa che è possibile accedervi senza creare un oggetto della classe, a differenza di public
:
Esempio
Un esempio per dimostrare le differenze tra static
e public
metodi:
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 abstract
metodo appartiene a una abstract
classe 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
}
}