更新時間:2022-08-12 11:21:18 來源:動力節(jié)點(diǎn) 瀏覽1484次
Java修飾符有哪些?動力節(jié)點(diǎn)小編來為大家解答。
到目前為止,您已經(jīng)public對我們幾乎所有示例中出現(xiàn)的關(guān)鍵字非常熟悉:
public class Main
public關(guān)鍵字是訪問修飾符,意味著它用于設(shè)置類、屬性、方法和構(gòu)造函數(shù)的訪問級別。
我們將修飾符分為兩組:
訪問修飾符- 控制訪問級別
非訪問修飾符- 不控制訪問級別,但提供其他功能
對于classes,您可以使用public或者default:
Modifier | Description |
---|---|
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 |
對于屬性、方法和構(gòu)造函數(shù),您可以使用以下之一:
Modifier | Description |
---|---|
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 |
對于classes,您可以使用final或abstract:
Modifier | Description |
---|---|
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) |
對于attributes 和 methods,您可以使用以下之一:
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" |
如果您不想覆蓋現(xiàn)有屬性值,請將屬性聲明為final:
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);
}
}
方法static意味著可以在不創(chuàng)建類的對象的情況下訪問它,不像public:
static一個例子來演示和方法之間的差異public:
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
}
}
abstract方法屬于一個類abstract,它沒有主體。主體由子類提供:
// 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
}
}
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