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一文告訴你Java修飾符有哪些

更新時間:2022-08-12 11:21:18 來源:動力節點 瀏覽1476次

Java修飾符有哪些?動力節點小編來為大家解答。

修飾符

到目前為止,您已經public對我們幾乎所有示例中出現的關鍵字非常熟悉:

public class Main

public關鍵字是訪問修飾符,意味著它用于設置類、屬性、方法和構造函數的訪問級別。

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

訪問修飾符

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、方法和構造函數,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

非訪問修飾符

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"

最后

如果您不想覆蓋現有屬性值,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);
  }
}

靜止的

方法static意味著可以在不創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
  }
}

抽象的

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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